Posted a while back about a project I am trying to concoct- an intercom for
my front door- and have made some progress. Unfortunately I hit a speed
bump when I added transistor Q4. Now it only gives me noise at the output,
and lots and lots of that. Capacitors are all 100uF 35V, which I am
thinking may be the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it with a 2N5296
from my junkbox, but that just doubled the volume of the noisy output. If
anyone sees something I should but don't, please post. The only thing I can
think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave

On Wed, 23 May 2012 15:40:17 -0500, "Dave" wrote:

Posted a while back about a project I am trying to concoct- an intercom for
my front door- and have made some progress. Unfortunately I hit a speed
bump when I added transistor Q4. Now it only gives me noise at the output,
and lots and lots of that. Capacitors are all 100uF 35V, which I am
thinking may be the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it with a 2N5296
from my junkbox, but that just doubled the volume of the noisy output. If
anyone sees something I should but don't, please post. The only thing I can
think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that last
stage. (In fact, calculate all your stage biases.)

How much voltage gain do you need?

What is the load impedance? Is it a speaker?

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

"Dave" wrote in message
netamerica...
Posted a while back about a project I am trying to concoct- an intercom
for my front door- and have made some progress. Unfortunately I hit a
speed bump when I added transistor Q4. Now it only gives me noise at the
output, and lots and lots of that. Capacitors are all 100uF 35V, which I
am thinking may be the problem (maybe the last couple need to be 50 or
75V?) Originally thought I might be overdriving Q4, so I replaced it with
a 2N5296 from my junkbox, but that just doubled the volume of the noisy
output. If anyone sees something I should but don't, please post. The
only thing I can think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...


Multiplying gain like that won't get you where you want to go. You're
"noise" could well be hiss from high frequency feedback (you did decouple
the supply rail didn't you?!).

You also need input and output stages with appropriate impedance - to save
duplication, most intercoms have the speakers double as microphones - you
not only need the final output stage capable of driving the low impedance of
a speaker, best matching is had with a low input impedance input stage
(common base input stage).

As a matter of preference, I'd buy a £1.99p pocket radio and strip out the
RF/IF stages and make the minimal required additions to the AF stage &
speaker driver.

"Jim Thompson" wrote in
message ...
On Wed, 23 May 2012 15:40:17 -0500, "Dave" wrote:

Posted a while back about a project I am trying to concoct- an intercom
for
my front door- and have made some progress. Unfortunately I hit a speed
bump when I added transistor Q4. Now it only gives me noise at the output,
and lots and lots of that. Capacitors are all 100uF 35V, which I am
thinking may be the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it with a
2N5296
from my junkbox, but that just doubled the volume of the noisy output. If
anyone sees something I should but don't, please post. The only thing I
can
think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that last
stage. (In fact, calculate all your stage biases.)

How much voltage gain do you need?

What is the load impedance? Is it a speaker?

...Jim Thompson
--

Bias *current*? When I think of biasing I think of voltage. Do you mean
collector current? I'm thinking it's around 180 mA. And after th first
three stages, I'm thinking voltage gain isn't lacking. Load is an 8 Ohm
speaker, which *seems* to be functioning fine. Only problem is the buzzing
that fills the air when I turn it on (which makes me think I have a
capacitor breaking down). Don't mean to be critical, but these seem like
very generic questions. What, specifically, are you thinking?

Thanks,

Dave.

"Ian Field" wrote in message
...

"Dave" wrote in message
netamerica...
Posted a while back about a project I am trying to concoct- an intercom
for my front door- and have made some progress. Unfortunately I hit a
speed bump when I added transistor Q4. Now it only gives me noise at the
output, and lots and lots of that. Capacitors are all 100uF 35V, which I
am thinking may be the problem (maybe the last couple need to be 50 or
75V?) Originally thought I might be overdriving Q4, so I replaced it with
a 2N5296 from my junkbox, but that just doubled the volume of the noisy
output. If anyone sees something I should but don't, please post. The
only thing I can think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...


Multiplying gain like that won't get you where you want to go. You're
"noise" could well be hiss from high frequency feedback (you did decouple
the supply rail didn't you?!).

You also need input and output stages with appropriate impedance - to save
duplication, most intercoms have the speakers double as microphones - you
not only need the final output stage capable of driving the low impedance
of a speaker, best matching is had with a low input impedance input stage
(common base input stage).

As a matter of preference, I'd buy a £1.99p pocket radio and strip out the
RF/IF stages and make the minimal required additions to the AF stage &
speaker driver.


Hey Ian,

I don't *think* the noise is from high frequency feedback, as it is a
low-pitch buzzing, which makes me think of a capacitor being overloaded.
And (REALLY hate to ask this) How would I *decouple* the supply rail? Power
is taken from a wall-wart and run through an LM317T before feeding into the
circuit. Sorry, I really am making this up as I go along. Don't mean to be
painfully ignorant, just am not anything like an EE. The input impedance,
as I calulate it, matches prety well with the 1.5KOhms of the microphone,
and I *thought* the output impedance was similarly matched to the 8-Ohm
speaker.

I thought about the portable radio path, but decided against it for the sake
of simplicity. And it worked fine untill I added Q4.

Thanks,

Dave

On Wed, 23 May 2012 17:36:23 -0500, "Dave" wrote:


"Jim Thompson" wrote in
message ...
On Wed, 23 May 2012 15:40:17 -0500, "Dave" wrote:

Posted a while back about a project I am trying to concoct- an intercom
for
my front door- and have made some progress. Unfortunately I hit a speed
bump when I added transistor Q4. Now it only gives me noise at the output,
and lots and lots of that. Capacitors are all 100uF 35V, which I am
thinking may be the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it with a
2N5296
from my junkbox, but that just doubled the volume of the noisy output. If
anyone sees something I should but don't, please post. The only thing I
can
think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that last
stage. (In fact, calculate all your stage biases.)

How much voltage gain do you need?

What is the load impedance? Is it a speaker?

...Jim Thompson
--

Bias *current*? When I think of biasing I think of voltage. Do you mean
collector current? I'm thinking it's around 180 mA. And after th first
three stages, I'm thinking voltage gain isn't lacking. Load is an 8 Ohm
speaker, which *seems* to be functioning fine. Only problem is the buzzing
that fills the air when I turn it on (which makes me think I have a
capacitor breaking down). Don't mean to be critical, but these seem like
very generic questions. What, specifically, are you thinking?

Thanks,

Dave.



I guess you don't need any help... you already know it all.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

"Jim Thompson" wrote in
message ...
On Wed, 23 May 2012 17:36:23 -0500, "Dave" wrote:


"Jim Thompson" wrote
in
message ...
On Wed, 23 May 2012 15:40:17 -0500, "Dave" wrote:

Posted a while back about a project I am trying to concoct- an intercom
for
my front door- and have made some progress. Unfortunately I hit a speed
bump when I added transistor Q4. Now it only gives me noise at the
output,
and lots and lots of that. Capacitors are all 100uF 35V, which I am
thinking may be the problem (maybe the last couple need to be 50 or
75V?)
Originally thought I might be overdriving Q4, so I replaced it with a
2N5296
from my junkbox, but that just doubled the volume of the noisy output.
If
anyone sees something I should but don't, please post. The only thing I
can
think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that last
stage. (In fact, calculate all your stage biases.)

How much voltage gain do you need?

What is the load impedance? Is it a speaker?

...Jim Thompson
--

Bias *current*? When I think of biasing I think of voltage. Do you mean
collector current? I'm thinking it's around 180 mA. And after th first
three stages, I'm thinking voltage gain isn't lacking. Load is an 8 Ohm
speaker, which *seems* to be functioning fine. Only problem is the buzzing
that fills the air when I turn it on (which makes me think I have a
capacitor breaking down). Don't mean to be critical, but these seem like
very generic questions. What, specifically, are you thinking?

Thanks,

Dave.



I guess you don't need any help... you already know it all.

...Jim Thompson
--

Sorry. But if you weren't giving me complete and utter bull****, you
wouldn't be so offended when I pointed it out.

Dave

"Dave" wrote in message
rica...

"Ian Field" wrote in message
...

"Dave" wrote in message
netamerica...
Posted a while back about a project I am trying to concoct- an intercom
for my front door- and have made some progress. Unfortunately I hit a
speed bump when I added transistor Q4. Now it only gives me noise at the
output, and lots and lots of that. Capacitors are all 100uF 35V, which
I am thinking may be the problem (maybe the last couple need to be 50 or
75V?) Originally thought I might be overdriving Q4, so I replaced it
with a 2N5296 from my junkbox, but that just doubled the volume of the
noisy output. If anyone sees something I should but don't, please post.
The only thing I can think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...


Multiplying gain like that won't get you where you want to go. You're
"noise" could well be hiss from high frequency feedback (you did decouple
the supply rail didn't you?!).

You also need input and output stages with appropriate impedance - to
save duplication, most intercoms have the speakers double as
microphones - you not only need the final output stage capable of driving
the low impedance of a speaker, best matching is had with a low input
impedance input stage (common base input stage).

As a matter of preference, I'd buy a £1.99p pocket radio and strip out
the RF/IF stages and make the minimal required additions to the AF stage
& speaker driver.


Hey Ian,

I don't *think* the noise is from high frequency feedback, as it is a
low-pitch buzzing, which makes me think of a capacitor being overloaded.
And (REALLY hate to ask this) How would I *decouple* the supply rail?
Power is taken from a wall-wart and run through an LM317T before feeding
into the circuit. Sorry, I really am making this up as I go along. Don't
mean to be painfully ignorant, just am not anything like an EE. The input
impedance, as I calulate it, matches prety well with the 1.5KOhms of the
microphone, and I *thought* the output impedance was similarly matched to
the 8-Ohm speaker.

I thought about the portable radio path, but decided against it for the
sake of simplicity. And it worked fine untill I added Q4.

Thanks,

Dave



Just had a thought that may save some gnashing of teeth. Decoupling the
supply rail is accomplished through the addition of small value caps from
the supply rail to ground as it circles the board/circuit, isn't it? If so,
I did not do that. Now that it comes up, I may make some adjustments to the
schematic/circuit. Thanks for that question/observation. Since I'm here,
what else did I miss? I really am just making this up as I go along...

Thanks,

Dave

On Wed, 23 May 2012 18:49:23 -0500, "Dave" wrote:


"Jim Thompson" wrote in
message ...
On Wed, 23 May 2012 17:36:23 -0500, "Dave" wrote:


"Jim Thompson" wrote
in
message ...
On Wed, 23 May 2012 15:40:17 -0500, "Dave" wrote:

Posted a while back about a project I am trying to concoct- an intercom
for
my front door- and have made some progress. Unfortunately I hit a speed
bump when I added transistor Q4. Now it only gives me noise at the
output,
and lots and lots of that. Capacitors are all 100uF 35V, which I am
thinking may be the problem (maybe the last couple need to be 50 or
75V?)
Originally thought I might be overdriving Q4, so I replaced it with a
2N5296
from my junkbox, but that just doubled the volume of the noisy output.
If
anyone sees something I should but don't, please post. The only thing I
can
think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that last
stage. (In fact, calculate all your stage biases.)

How much voltage gain do you need?

What is the load impedance? Is it a speaker?

...Jim Thompson
--

Bias *current*? When I think of biasing I think of voltage. Do you mean
collector current? I'm thinking it's around 180 mA. And after th first
three stages, I'm thinking voltage gain isn't lacking. Load is an 8 Ohm
speaker, which *seems* to be functioning fine. Only problem is the buzzing
that fills the air when I turn it on (which makes me think I have a
capacitor breaking down). Don't mean to be critical, but these seem like
very generic questions. What, specifically, are you thinking?

Thanks,

Dave.



I guess you don't need any help... you already know it all.

...Jim Thompson
--

Sorry. But if you weren't giving me complete and utter bull****, you
wouldn't be so offended when I pointed it out.

Dave


Ask anyone but a Larkin boot-licker... you don't know WTF you're
doing. You're just pasting crap together. That's why you failed the
simple bias question. Why can't you make it work?

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

On 2012-05-23, Dave wrote:

I don't *think* the noise is from high frequency feedback, as it is a
low-pitch buzzing, which makes me think of a capacitor being overloaded.
And (REALLY hate to ask this) How would I *decouple* the supply rail? Power
is taken from a wall-wart and run through an LM317T before feeding into the
circuit. Sorry, I really am making this up as I go along. Don't mean to be
painfully ignorant, just am not anything like an EE. The input impedance,
as I calulate it, matches prety well with the 1.5KOhms of the microphone,
and I *thought* the output impedance was similarly matched to the 8-Ohm
speaker.

I thought about the portable radio path, but decided against it for the sake
of simplicity. And it worked fine untill I added Q4.

you seem to have a simulator there (looks a bit like lt-spice)
replace V2 with a more realistic model of your power supply
(internal resitance, ripple)

Your circuit appears to have heaps of gain, especially at high
frequencies, (HF like CB radio - add a few microvolts of that
to your simulated input)

Put some resistors in series with emitter capacitors to make
the AC gain more predictable... do you really need ten million times
amplification?

If you just want to build an intercom and don't want to learn design
by doing try this one, it looks sane to me.

http://www.zen22142.zen.co.uk/Circui...io/doorint.htm

--
š‚šƒ 100% natural

"Dave" wrote in message
netamerica...

"Dave" wrote in message
rica...

"Ian Field" wrote in message
...

"Dave" wrote in message
netamerica...
Posted a while back about a project I am trying to concoct- an intercom
for my front door- and have made some progress. Unfortunately I hit a
speed bump when I added transistor Q4. Now it only gives me noise at
the output, and lots and lots of that. Capacitors are all 100uF 35V,
which I am thinking may be the problem (maybe the last couple need to
be 50 or 75V?) Originally thought I might be overdriving Q4, so I
replaced it with a 2N5296 from my junkbox, but that just doubled the
volume of the noisy output. If anyone sees something I should but
don't, please post. The only thing I can think of is upping the voltage
on C8 and C9.

Any help is *greatly* appreciated...


Multiplying gain like that won't get you where you want to go. You're
"noise" could well be hiss from high frequency feedback (you did
decouple the supply rail didn't you?!).

You also need input and output stages with appropriate impedance - to
save duplication, most intercoms have the speakers double as
microphones - you not only need the final output stage capable of
driving the low impedance of a speaker, best matching is had with a low
input impedance input stage (common base input stage).

As a matter of preference, I'd buy a £1.99p pocket radio and strip out
the RF/IF stages and make the minimal required additions to the AF stage
& speaker driver.


Hey Ian,

I don't *think* the noise is from high frequency feedback, as it is a
low-pitch buzzing, which makes me think of a capacitor being overloaded.
And (REALLY hate to ask this) How would I *decouple* the supply rail?
Power is taken from a wall-wart and run through an LM317T before feeding
into the circuit. Sorry, I really am making this up as I go along.
Don't mean to be painfully ignorant, just am not anything like an EE.
The input impedance, as I calulate it, matches prety well with the
1.5KOhms of the microphone, and I *thought* the output impedance was
similarly matched to the 8-Ohm speaker.

I thought about the portable radio path, but decided against it for the
sake of simplicity. And it worked fine untill I added Q4.

Thanks,

Dave



Just had a thought that may save some gnashing of teeth. Decoupling the
supply rail is accomplished through the addition of small value caps from
the supply rail to ground as it circles the board/circuit, isn't it? If
so, I did not do that. Now that it comes up, I may make some adjustments
to the schematic/circuit. Thanks for that question/observation. Since
I'm here, what else did I miss?

I've already told you.

"Ian Field" wrote in message
...

"Dave" wrote in message
netamerica...

"Dave" wrote in message
rica...

"Ian Field" wrote in message
...

"Dave" wrote in message
netamerica...
Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4. Now it
only gives me noise at the output, and lots and lots of that.
Capacitors are all 100uF 35V, which I am thinking may be the problem
(maybe the last couple need to be 50 or 75V?) Originally thought I
might be overdriving Q4, so I replaced it with a 2N5296 from my
junkbox, but that just doubled the volume of the noisy output. If
anyone sees something I should but don't, please post. The only thing
I can think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...


Multiplying gain like that won't get you where you want to go. You're
"noise" could well be hiss from high frequency feedback (you did
decouple the supply rail didn't you?!).

You also need input and output stages with appropriate impedance - to
save duplication, most intercoms have the speakers double as
microphones - you not only need the final output stage capable of
driving the low impedance of a speaker, best matching is had with a low
input impedance input stage (common base input stage).

As a matter of preference, I'd buy a £1.99p pocket radio and strip out
the RF/IF stages and make the minimal required additions to the AF
stage & speaker driver.


Hey Ian,

I don't *think* the noise is from high frequency feedback, as it is a
low-pitch buzzing, which makes me think of a capacitor being overloaded.
And (REALLY hate to ask this) How would I *decouple* the supply rail?
Power is taken from a wall-wart and run through an LM317T before feeding
into the circuit. Sorry, I really am making this up as I go along.
Don't mean to be painfully ignorant, just am not anything like an EE.
The input impedance, as I calulate it, matches prety well with the
1.5KOhms of the microphone, and I *thought* the output impedance was
similarly matched to the 8-Ohm speaker.

I thought about the portable radio path, but decided against it for the
sake of simplicity. And it worked fine untill I added Q4.

Thanks,

Dave



Just had a thought that may save some gnashing of teeth. Decoupling the
supply rail is accomplished through the addition of small value caps from
the supply rail to ground as it circles the board/circuit, isn't it? If
so, I did not do that. Now that it comes up, I may make some adjustments
to the schematic/circuit. Thanks for that question/observation. Since
I'm here, what else did I miss?

I've already told you.


Okay, well, thanks for the reminder. On the subject of decoupling the supply
rail, I had to add a large capacitor at the power supply going from +14V to
ground to eliminate noise from the transformer, and it works wonderfully.
Would this qualify for decoupling the supply rail?

Also, I've been thinking about your comment that multiplying gain the way I
am trying to do won't work. Sounds like I am indeed overdriving Q4, which
should be removed. Comments on this?

More later.I'm still thinking about the other stuff you offered...

Thanks.

Dave

On Thu, 24 May 2012 21:21:41 -0500, "Dave" wrote:

[snip]

I know it offends your (and Ian Field's) sensibilities, but you might
try returning to fundamentals and do a little thinking and math.

Oooops, I forgot, Your mantra is, when you can't run the math, run
your mouth.

So forget what I suggested. There is no hope for you (or Ian) :-)

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

"Dave" schreef in bericht
netamerica...
Posted a while back about a project I am trying to concoct- an intercom
for my front door- and have made some progress. Unfortunately I hit a
speed bump when I added transistor Q4. Now it only gives me noise at the
output, and lots and lots of that. Capacitors are all 100uF 35V, which I
am thinking may be the problem (maybe the last couple need to be 50 or
75V?) Originally thought I might be overdriving Q4, so I replaced it with
a 2N5296 from my junkbox, but that just doubled the volume of the noisy
output. If anyone sees something I should but don't, please post. The
only thing I can think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave



So if you are not able to do the basic math you can at least run the sim to
get an idea about what might have gone wrong.

petrus bitbyter

"Jim Thompson" wrote in
message ...
On Thu, 24 May 2012 21:21:41 -0500, "Dave" wrote:

[snip]

I know it offends your (and Ian Field's) sensibilities, but you might
try returning to fundamentals and do a little thinking and math.

Oooops, I forgot, Your mantra is, when you can't run the math, run
your mouth.


No one runs their mouth anything like you - and yours is rarely on topic.

"Dave" wrote in message
netamerica...

"Ian Field" wrote in message
...

"Dave" wrote in message
netamerica...

"Dave" wrote in message
rica...

"Ian Field" wrote in message
...

"Dave" wrote in message
netamerica...
Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4. Now it
only gives me noise at the output, and lots and lots of that.
Capacitors are all 100uF 35V, which I am thinking may be the problem
(maybe the last couple need to be 50 or 75V?) Originally thought I
might be overdriving Q4, so I replaced it with a 2N5296 from my
junkbox, but that just doubled the volume of the noisy output. If
anyone sees something I should but don't, please post. The only thing
I can think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...


Multiplying gain like that won't get you where you want to go. You're
"noise" could well be hiss from high frequency feedback (you did
decouple the supply rail didn't you?!).

You also need input and output stages with appropriate impedance - to
save duplication, most intercoms have the speakers double as
microphones - you not only need the final output stage capable of
driving the low impedance of a speaker, best matching is had with a
low input impedance input stage (common base input stage).

As a matter of preference, I'd buy a £1.99p pocket radio and strip out
the RF/IF stages and make the minimal required additions to the AF
stage & speaker driver.


Hey Ian,

I don't *think* the noise is from high frequency feedback, as it is a
low-pitch buzzing, which makes me think of a capacitor being
overloaded. And (REALLY hate to ask this) How would I *decouple* the
supply rail? Power is taken from a wall-wart and run through an LM317T
before feeding into the circuit. Sorry, I really am making this up as
I go along. Don't mean to be painfully ignorant, just am not anything
like an EE. The input impedance, as I calulate it, matches prety well
with the 1.5KOhms of the microphone, and I *thought* the output
impedance was similarly matched to the 8-Ohm speaker.

I thought about the portable radio path, but decided against it for the
sake of simplicity. And it worked fine untill I added Q4.

Thanks,

Dave



Just had a thought that may save some gnashing of teeth. Decoupling the
supply rail is accomplished through the addition of small value caps
from the supply rail to ground as it circles the board/circuit, isn't
it? If so, I did not do that. Now that it comes up, I may make some
adjustments to the schematic/circuit. Thanks for that
question/observation. Since I'm here, what else did I miss?

I've already told you.


Okay, well, thanks for the reminder. On the subject of decoupling the
supply rail, I had to add a large capacitor at the power supply going from
+14V to ground to eliminate noise from the transformer, and it works
wonderfully. Would this qualify for decoupling the supply rail?

Also, I've been thinking about your comment that multiplying gain the way
I am trying to do won't work. Sounds like I am indeed overdriving Q4,
which should be removed. Comments on this?


The last stage isn't going to put much power into a speaker - the matching
will be terrible.

You'd stand a better chance with a high impedance telephone earpiece - they
can be found as high as about 140 Ohms.

That would open other possibilities - such as copying the simple automatic
bias scheme from the Philips EE1003 or EE20 construction kit intercom.

All their gain stages have no emitter resistor to reduce the signal
headroom, the simple biassing just returns the base-bias to the collector,
as current rises collector voltage falls forcing base current to do the
same - this impacts on AC gain as well as DC gain, but you can split the
base reasistor and decouple that to get more AC gain.

The collection of user manuals might give you some useful study material:

http://ee.old.no/library/

Once you actually get the final stage to put some useful power into the
load, you will need to upgrade the supply decoupling to stop the amplifier
"motorboating" - the simple method is adequately illustrated in any of the
amplifier projects in the EE kit books.

On Wed, 23 May 2012 15:40:17 -0500, "Dave" wrote:

Posted a while back about a project I am trying to concoct- an intercom for
my front door- and have made some progress. Unfortunately I hit a speed
bump when I added transistor Q4. Now it only gives me noise at the output,
and lots and lots of that. Capacitors are all 100uF 35V, which I am
thinking may be the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it with a 2N5296
from my junkbox, but that just doubled the volume of the noisy output. If
anyone sees something I should but don't, please post. The only thing I can
think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...

Can you post the circuit list, please?

--
JF

On Fri, 25 May 2012 09:44:12 -0500, John Fields
wrote:

On Wed, 23 May 2012 15:40:17 -0500, "Dave" wrote:

Posted a while back about a project I am trying to concoct- an intercom for
my front door- and have made some progress. Unfortunately I hit a speed
bump when I added transistor Q4. Now it only gives me noise at the output,
and lots and lots of that. Capacitors are all 100uF 35V, which I am
thinking may be the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it with a 2N5296
from my junkbox, but that just doubled the volume of the noisy output. If
anyone sees something I should but don't, please post. The only thing I can
think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...

Can you post the circuit list, please?

You're late to the game. Track thru the message-ID's. Dave is one of
Ian Field's buddies. See how he responded to my offer of help.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

"Jim Thompson" wrote in
message ...
On Fri, 25 May 2012 09:44:12 -0500, John Fields
wrote:

On Wed, 23 May 2012 15:40:17 -0500, "Dave" wrote:

Posted a while back about a project I am trying to concoct- an intercom
for
my front door- and have made some progress. Unfortunately I hit a speed
bump when I added transistor Q4. Now it only gives me noise at the
output,
and lots and lots of that. Capacitors are all 100uF 35V, which I am
thinking may be the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it with a
2N5296
from my junkbox, but that just doubled the volume of the noisy output.
If
anyone sees something I should but don't, please post. The only thing I
can
think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...

Can you post the circuit list, please?

You're late to the game. Track thru the message-ID's. Dave is one of
Ian Field's buddies. See how he responded to my offer of help.

...Jim Thompson
--


You're not paranoid at all - everyone *IS* out to get you!

"Jasen Betts" wrote in message
...
On 2012-05-23, Dave wrote:

I don't *think* the noise is from high frequency feedback, as it is a
low-pitch buzzing, which makes me think of a capacitor being overloaded.
And (REALLY hate to ask this) How would I *decouple* the supply rail?
Power
is taken from a wall-wart and run through an LM317T before feeding into
the
circuit. Sorry, I really am making this up as I go along. Don't mean to
be
painfully ignorant, just am not anything like an EE. The input
impedance,
as I calulate it, matches prety well with the 1.5KOhms of the microphone,
and I *thought* the output impedance was similarly matched to the 8-Ohm
speaker.

I thought about the portable radio path, but decided against it for the
sake
of simplicity. And it worked fine untill I added Q4.

you seem to have a simulator there (looks a bit like lt-spice)
replace V2 with a more realistic model of your power supply
(internal resitance, ripple)

Your circuit appears to have heaps of gain, especially at high
frequencies, (HF like CB radio - add a few microvolts of that
to your simulated input)

Put some resistors in series with emitter capacitors to make
the AC gain more predictable... do you really need ten million times
amplification?

If you just want to build an intercom and don't want to learn design
by doing try this one, it looks sane to me.

http://www.zen22142.zen.co.uk/Circui...io/doorint.htm

--
?? 100% natural

Hey, Jasen. THANK YOU for that schematic. That is exactly what I started
out looking for. Only problem is, wife didn't want something on eye level
outside the front door. Hmm. Will have to figure out what to do about
that. But this is perfect. Thank you again.

And, btw, the circuit was simulated in LT-Spice, like you thought. Will
have to upload the netlist, for whoever it was wanted that.

Take it easy...

Dave

"flipper" wrote in message
...
On Thu, 24 May 2012 21:21:41 -0500, "Dave" wrote:


Okay, well, thanks for the reminder. On the subject of decoupling the
supply
rail, I had to add a large capacitor at the power supply going from +14V
to
ground to eliminate noise from the transformer, and it works wonderfully.
Would this qualify for decoupling the supply rail?

Also, I've been thinking about your comment that multiplying gain the way
I
am trying to do won't work. Sounds like I am indeed overdriving Q4, which
should be removed. Comments on this?

More later.I'm still thinking about the other stuff you offered...

Thanks.

Dave


I think you should start thinking in terms of functional stages rather
than just 'one big thing'. In particular, looking at your existing
layout, you have a microphone preamp and a power stage, assuming that
last section is intended to drive a speaker. I'd suggest there may be
a third section which matches the two together and perchance, includes
a volume control with, perhaps, something to cater to two way
communication. The requirements are different for each.

If the last stage is intended to drive a speaker then it isn't going
to work, at least not very well. There is simply too much load for the
'plain old' resistor to pull up. The standard common emitter
amplifier, like you've repeated everywhere, depends on a high
impedance input, relative to it's own load, in the following stage in
order to work well and to do that for an 8 ohm speaker would take a
ridiculously low load resistor and huge collector currents. It could
be done with an audio transformer but that's expensive, which is why
no one does it anymore with solid state.

In short, you need to build a conventional push pull power stage.

Your 100uF bypass caps are too small and by the time you go through 3
stages audio response is down 3 dB at 600 Hz (and even worse with the
100uF into a speaker). Rather than increase them, however, you might
as well increase all the resistor values because there's no reason why
the gain stages need to pull 8-9mA. A tenth of that is plenty.

You're also operating them all max gain open loop and distortion is
rather high that way, say 3-10% at 1V out (before we get to the
speaker driver).

That's another reason to break it down into functional sections
because you don't want to try wrapping negative feedback around a
large number of gain stages, especially when cap coupled. So the more
practical approach is to make a decent microphone preamp, with NFB, or
not, around that stage. Make a decent power amp, with NFB, or not,
around that stage. And then glue the two together with volume controls
and whatever else is needed. That doesn't mean the sections
necessarily need be 'complicated' but they need to be considered in
light of their own requirements.

Its' easier with ICs, of course, because someone has already stuck a
gaggle of transistors inside the plastic.

Thank you, flipper, for this incredibly informative post. Will have to try
to follow your lead, and make this work in stages. Although I may abandon
*my* circuit for the one Jasen posted. Still, thanks much for your input.
It is most welcome.

Dave

"Ian Field" wrote in message
...

"Dave" wrote in message
netamerica...

"Ian Field" wrote in message
...

"Dave" wrote in message
netamerica...

"Dave" wrote in message
rica...

"Ian Field" wrote in message
...

"Dave" wrote in message
netamerica...
Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4. Now it
only gives me noise at the output, and lots and lots of that.
Capacitors are all 100uF 35V, which I am thinking may be the problem
(maybe the last couple need to be 50 or 75V?) Originally thought I
might be overdriving Q4, so I replaced it with a 2N5296 from my
junkbox, but that just doubled the volume of the noisy output. If
anyone sees something I should but don't, please post. The only
thing I can think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...


Multiplying gain like that won't get you where you want to go. You're
"noise" could well be hiss from high frequency feedback (you did
decouple the supply rail didn't you?!).

You also need input and output stages with appropriate impedance - to
save duplication, most intercoms have the speakers double as
microphones - you not only need the final output stage capable of
driving the low impedance of a speaker, best matching is had with a
low input impedance input stage (common base input stage).

As a matter of preference, I'd buy a £1.99p pocket radio and strip
out the RF/IF stages and make the minimal required additions to the
AF stage & speaker driver.


Hey Ian,

I don't *think* the noise is from high frequency feedback, as it is a
low-pitch buzzing, which makes me think of a capacitor being
overloaded. And (REALLY hate to ask this) How would I *decouple* the
supply rail? Power is taken from a wall-wart and run through an LM317T
before feeding into the circuit. Sorry, I really am making this up as
I go along. Don't mean to be painfully ignorant, just am not anything
like an EE. The input impedance, as I calulate it, matches prety well
with the 1.5KOhms of the microphone, and I *thought* the output
impedance was similarly matched to the 8-Ohm speaker.

I thought about the portable radio path, but decided against it for
the sake of simplicity. And it worked fine untill I added Q4.

Thanks,

Dave



Just had a thought that may save some gnashing of teeth. Decoupling
the supply rail is accomplished through the addition of small value
caps from the supply rail to ground as it circles the board/circuit,
isn't it? If so, I did not do that. Now that it comes up, I may make
some adjustments to the schematic/circuit. Thanks for that
question/observation. Since I'm here, what else did I miss?

I've already told you.


Okay, well, thanks for the reminder. On the subject of decoupling the
supply rail, I had to add a large capacitor at the power supply going
from +14V to ground to eliminate noise from the transformer, and it works
wonderfully. Would this qualify for decoupling the supply rail?

Also, I've been thinking about your comment that multiplying gain the way
I am trying to do won't work. Sounds like I am indeed overdriving Q4,
which should be removed. Comments on this?


The last stage isn't going to put much power into a speaker - the matching
will be terrible.

You'd stand a better chance with a high impedance telephone earpiece -
they can be found as high as about 140 Ohms.

That would open other possibilities - such as copying the simple automatic
bias scheme from the Philips EE1003 or EE20 construction kit intercom.

All their gain stages have no emitter resistor to reduce the signal
headroom, the simple biassing just returns the base-bias to the collector,
as current rises collector voltage falls forcing base current to do the
same - this impacts on AC gain as well as DC gain, but you can split the
base reasistor and decouple that to get more AC gain.

The collection of user manuals might give you some useful study material:

http://ee.old.no/library/

Once you actually get the final stage to put some useful power into the
load, you will need to upgrade the supply decoupling to stop the amplifier
"motorboating" - the simple method is adequately illustrated in any of the
amplifier projects in the EE kit books.


Oookay. Thank you for that link. May learn something yet. And for the
additional input. I do appreciate your patience.

Dave

"Dave" wrote in message
netamerica...

"Ian Field" wrote in message
...

"Dave" wrote in message
netamerica...

"Ian Field" wrote in message
...

"Dave" wrote in message
netamerica...

"Dave" wrote in message
rica...

"Ian Field" wrote in message
...

"Dave" wrote in message
netamerica...
Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4. Now it
only gives me noise at the output, and lots and lots of that.
Capacitors are all 100uF 35V, which I am thinking may be the
problem (maybe the last couple need to be 50 or 75V?) Originally
thought I might be overdriving Q4, so I replaced it with a 2N5296
from my junkbox, but that just doubled the volume of the noisy
output. If anyone sees something I should but don't, please post.
The only thing I can think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...


Multiplying gain like that won't get you where you want to go.
You're "noise" could well be hiss from high frequency feedback (you
did decouple the supply rail didn't you?!).

You also need input and output stages with appropriate impedance -
to save duplication, most intercoms have the speakers double as
microphones - you not only need the final output stage capable of
driving the low impedance of a speaker, best matching is had with a
low input impedance input stage (common base input stage).

As a matter of preference, I'd buy a £1.99p pocket radio and strip
out the RF/IF stages and make the minimal required additions to the
AF stage & speaker driver.


Hey Ian,

I don't *think* the noise is from high frequency feedback, as it is a
low-pitch buzzing, which makes me think of a capacitor being
overloaded. And (REALLY hate to ask this) How would I *decouple* the
supply rail? Power is taken from a wall-wart and run through an
LM317T before feeding into the circuit. Sorry, I really am making
this up as I go along. Don't mean to be painfully ignorant, just am
not anything like an EE. The input impedance, as I calulate it,
matches prety well with the 1.5KOhms of the microphone, and I
*thought* the output impedance was similarly matched to the 8-Ohm
speaker.

I thought about the portable radio path, but decided against it for
the sake of simplicity. And it worked fine untill I added Q4.

Thanks,

Dave



Just had a thought that may save some gnashing of teeth. Decoupling
the supply rail is accomplished through the addition of small value
caps from the supply rail to ground as it circles the board/circuit,
isn't it? If so, I did not do that. Now that it comes up, I may make
some adjustments to the schematic/circuit. Thanks for that
question/observation. Since I'm here, what else did I miss?

I've already told you.


Okay, well, thanks for the reminder. On the subject of decoupling the
supply rail, I had to add a large capacitor at the power supply going
from +14V to ground to eliminate noise from the transformer, and it
works wonderfully. Would this qualify for decoupling the supply rail?

Also, I've been thinking about your comment that multiplying gain the
way I am trying to do won't work. Sounds like I am indeed overdriving
Q4, which should be removed. Comments on this?


The last stage isn't going to put much power into a speaker - the
matching will be terrible.

You'd stand a better chance with a high impedance telephone earpiece -
they can be found as high as about 140 Ohms.

That would open other possibilities - such as copying the simple
automatic bias scheme from the Philips EE1003 or EE20 construction kit
intercom.

All their gain stages have no emitter resistor to reduce the signal
headroom, the simple biassing just returns the base-bias to the
collector, as current rises collector voltage falls forcing base current
to do the same - this impacts on AC gain as well as DC gain, but you can
split the base reasistor and decouple that to get more AC gain.

The collection of user manuals might give you some useful study material:

http://ee.old.no/library/

Once you actually get the final stage to put some useful power into the
load, you will need to upgrade the supply decoupling to stop the
amplifier "motorboating" - the simple method is adequately illustrated in
any of the amplifier projects in the EE kit books.


Oookay. Thank you for that link. May learn something yet. And for the
additional input. I do appreciate your patience.

Dave


The English manuals don't have annotated schematics, but the other languages
do, so you might have to download 2 manuals to make sense of the examples
(there is an intercom among the projects).

There are template cards for the projects which are easier to read, but only
for 3 of the kits and take some exploring the links to find.

"Dave" wrote in message
netamerica...

"Jasen Betts" wrote in message
...
On 2012-05-23, Dave wrote:

I don't *think* the noise is from high frequency feedback, as it is a
low-pitch buzzing, which makes me think of a capacitor being overloaded.
And (REALLY hate to ask this) How would I *decouple* the supply rail?
Power
is taken from a wall-wart and run through an LM317T before feeding into
the
circuit. Sorry, I really am making this up as I go along. Don't mean
to be
painfully ignorant, just am not anything like an EE. The input
impedance,
as I calulate it, matches prety well with the 1.5KOhms of the
microphone,
and I *thought* the output impedance was similarly matched to the 8-Ohm
speaker.

I thought about the portable radio path, but decided against it for the
sake
of simplicity. And it worked fine untill I added Q4.

you seem to have a simulator there (looks a bit like lt-spice)
replace V2 with a more realistic model of your power supply
(internal resitance, ripple)

Your circuit appears to have heaps of gain, especially at high
frequencies, (HF like CB radio - add a few microvolts of that
to your simulated input)

Put some resistors in series with emitter capacitors to make
the AC gain more predictable... do you really need ten million times
amplification?

If you just want to build an intercom and don't want to learn design
by doing try this one, it looks sane to me.

http://www.zen22142.zen.co.uk/Circui...io/doorint.htm

--
?? 100% natural

Hey, Jasen. THANK YOU for that schematic. That is exactly what I started
out looking for. Only problem is, wife didn't want something on eye level
outside the front door. Hmm. Will have to figure out what to do about
that. But this is perfect. Thank you again.

And, btw, the circuit was simulated in LT-Spice, like you thought. Will
have to upload the netlist, for whoever it was wanted that.


That circuit looks like a winner - but I'd make a couple of changes.

First of all the emitter current is flowing through the speaker as it serves
as a mic (Ie=Ic+Ib), I'd put a resistor there and use a small electrolytic
for coupling.

Secondly I'd move the 270k resistor to the collector to make the bias
automatically self compensating. For a one off; its no big deal to build the
stage with a high resistance pot in place of the 270k and adjust for 1/2 Vcc
on the collector - then measure the pot and replace it with the nearest
preferred value.

Jim Thompson wrote:
On Wed, 23 May 2012 15:40:17 -0500, "Dave" wrote:

Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4. Now it
only gives me noise at the output, and lots and lots of that.
Capacitors are all 100uF 35V, which I am thinking may be the
problem (maybe the last couple need to be 50 or 75V?) Originally
thought I might be overdriving Q4, so I replaced it with a 2N5296
from my junkbox, but that just doubled the volume of the noisy
output. If anyone sees something I should but don't, please post.
The only thing I can think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that last
stage. (In fact, calculate all your stage biases.)

In my head, it's 14ma. That can't be right. The calc concurs. Is that a
little bit too much?

Aren't the emitter caps about 10 times as big as needed?

Ian did notice that he was just throwing gain at the problem though.


--

Reply in group, but if emailing add one more
zero, and remove the last word.

On Tue, 5 Jun 2012 18:28:22 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Wed, 23 May 2012 15:40:17 -0500, "Dave" wrote:

Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4. Now it
only gives me noise at the output, and lots and lots of that.
Capacitors are all 100uF 35V, which I am thinking may be the
problem (maybe the last couple need to be 50 or 75V?) Originally
thought I might be overdriving Q4, so I replaced it with a 2N5296
from my junkbox, but that just doubled the volume of the noisy
output. If anyone sees something I should but don't, please post.
The only thing I can think of is upping the voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that last
stage. (In fact, calculate all your stage biases.)

In my head, it's 14ma. That can't be right. The calc concurs. Is that a
little bit too much?

Aren't the emitter caps about 10 times as big as needed?

Ian did notice that he was just throwing gain at the problem though.

Yep, I was stunned... Ian said something cogent. But his buddy, Dave,
is beyond all hope... rude little POS.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

Jim Thompson wrote:
On Tue, 5 Jun 2012 18:28:22 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Wed, 23 May 2012 15:40:17 -0500, "Dave"
wrote:

Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4.
Now it only gives me noise at the output, and lots and lots of
that. Capacitors are all 100uF 35V, which I am thinking may be
the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it
with a 2N5296 from my junkbox, but that just doubled the volume
of the noisy output. If anyone sees something I should but
don't, please post. The only thing I can think of is upping the
voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that
last stage. (In fact, calculate all your stage biases.)

In my head, it's 14ma. That can't be right. The calc concurs. Is
that a little bit too much?

Aren't the emitter caps about 10 times as big as needed?

Ian did notice that he was just throwing gain at the problem though.

Yep, I was stunned... Ian said something cogent. But his buddy, Dave,
is beyond all hope... rude little POS.

I don't have AoE in reach, but I don't think they mention bias current in
their approach to design. They approach it with the rule of thumb that the
input impedance should be 10x the output impedance of the previous stage.
Similar. (The input impedance of the OP's stages are only slightly higher
than the previous output impedance.)

I'd like to know if you use any rules of thumb for this sort of thing. Or
is everything just optimized by multi-variable calculus?


--

Reply in group, but if emailing add one more
zero, and remove the last word.

On Tue, 5 Jun 2012 22:12:30 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Tue, 5 Jun 2012 18:28:22 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Wed, 23 May 2012 15:40:17 -0500, "Dave"
wrote:

Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4.
Now it only gives me noise at the output, and lots and lots of
that. Capacitors are all 100uF 35V, which I am thinking may be
the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it
with a 2N5296 from my junkbox, but that just doubled the volume
of the noisy output. If anyone sees something I should but
don't, please post. The only thing I can think of is upping the
voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that
last stage. (In fact, calculate all your stage biases.)

In my head, it's 14ma. That can't be right. The calc concurs. Is
that a little bit too much?

Aren't the emitter caps about 10 times as big as needed?

Ian did notice that he was just throwing gain at the problem though.

Yep, I was stunned... Ian said something cogent. But his buddy, Dave,
is beyond all hope... rude little POS.

I don't have AoE in reach, but I don't think they mention bias current in
their approach to design. They approach it with the rule of thumb that the
input impedance should be 10x the output impedance of the previous stage.
Similar. (The input impedance of the OP's stages are only slightly higher
than the previous output impedance.)

I'd like to know if you use any rules of thumb for this sort of thing. Or
is everything just optimized by multi-variable calculus?

I don't know if there are "rules of thumb"... maybe just calculate
rather than doing a NOLA white-trash guess ?:-)

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

On Tue, 5 Jun 2012 22:12:30 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Tue, 5 Jun 2012 18:28:22 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Wed, 23 May 2012 15:40:17 -0500, "Dave"
wrote:

Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4.
Now it only gives me noise at the output, and lots and lots of
that. Capacitors are all 100uF 35V, which I am thinking may be
the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it
with a 2N5296 from my junkbox, but that just doubled the volume
of the noisy output. If anyone sees something I should but
don't, please post. The only thing I can think of is upping the
voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that
last stage. (In fact, calculate all your stage biases.)

In my head, it's 14ma. That can't be right. The calc concurs. Is
that a little bit too much?

Aren't the emitter caps about 10 times as big as needed?

Ian did notice that he was just throwing gain at the problem though.

Yep, I was stunned... Ian said something cogent. But his buddy, Dave,
is beyond all hope... rude little POS.

I don't have AoE in reach, but I don't think they mention bias current in
their approach to design. They approach it with the rule of thumb that the
input impedance should be 10x the output impedance of the previous stage.
Similar. (The input impedance of the OP's stages are only slightly higher
than the previous output impedance.)

I'd like to know if you use any rules of thumb for this sort of thing. Or
is everything just optimized by multi-variable calculus?

The 10X impedance is a good rule-of-thumb for general hacking.

I tend to be an over-calculator, so I know every interaction.

I am blessed with an ability to "walk" circuits in my head and see
their functions (and faults) in a matter of seconds.

****es lots of people off, when I instantly say, "Can't be" :-)

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

On Tue, 5 Jun 2012 22:12:30 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Tue, 5 Jun 2012 18:28:22 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Wed, 23 May 2012 15:40:17 -0500, "Dave"
wrote:

Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4.
Now it only gives me noise at the output, and lots and lots of
that. Capacitors are all 100uF 35V, which I am thinking may be
the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it
with a 2N5296 from my junkbox, but that just doubled the volume
of the noisy output. If anyone sees something I should but
don't, please post. The only thing I can think of is upping the
voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that
last stage. (In fact, calculate all your stage biases.)

In my head, it's 14ma. That can't be right. The calc concurs. Is
that a little bit too much?

Aren't the emitter caps about 10 times as big as needed?

Ian did notice that he was just throwing gain at the problem though.

Yep, I was stunned... Ian said something cogent. But his buddy, Dave,
is beyond all hope... rude little POS.

I don't have AoE in reach, but I don't think they mention bias current in
their approach to design. They approach it with the rule of thumb that the
input impedance should be 10x the output impedance of the previous stage.
Similar. (The input impedance of the OP's stages are only slightly higher
than the previous output impedance.)

I'd like to know if you use any rules of thumb for this sort of thing. Or
is everything just optimized by multi-variable calculus?


There's really no reason for that. The input impedance of a bipolar
transistor amp could be less than the output impedance of the previous
stage and you'd still get lots of gain. You'd be essentially
multiplying betas in successive stages.

In a simple chain of common-emitter amp stages, all biased at the same
current, without emitter degeneration, the 10:1 ratio isn't very
reasonable.


--

John Larkin Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation

On Wed, 06 Jun 2012 16:49:55 -0700, John Larkin
wrote:

On Tue, 5 Jun 2012 22:12:30 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Tue, 5 Jun 2012 18:28:22 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Wed, 23 May 2012 15:40:17 -0500, "Dave"
wrote:

Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4.
Now it only gives me noise at the output, and lots and lots of
that. Capacitors are all 100uF 35V, which I am thinking may be
the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it
with a 2N5296 from my junkbox, but that just doubled the volume
of the noisy output. If anyone sees something I should but
don't, please post. The only thing I can think of is upping the
voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that
last stage. (In fact, calculate all your stage biases.)

In my head, it's 14ma. That can't be right. The calc concurs. Is
that a little bit too much?

Aren't the emitter caps about 10 times as big as needed?

Ian did notice that he was just throwing gain at the problem though.

Yep, I was stunned... Ian said something cogent. But his buddy, Dave,
is beyond all hope... rude little POS.

I don't have AoE in reach, but I don't think they mention bias current in
their approach to design. They approach it with the rule of thumb that the
input impedance should be 10x the output impedance of the previous stage.
Similar. (The input impedance of the OP's stages are only slightly higher
than the previous output impedance.)

I'd like to know if you use any rules of thumb for this sort of thing. Or
is everything just optimized by multi-variable calculus?


There's really no reason for that. The input impedance of a bipolar
transistor amp could be less than the output impedance of the previous
stage and you'd still get lots of gain. You'd be essentially
multiplying betas in successive stages.

In a simple chain of common-emitter amp stages, all biased at the same
current, without emitter degeneration, the 10:1 ratio isn't very
reasonable.

Yup. And getting gain based on "multiplying betas" is an
unpredictable way to "design".

I'd probably go with an MC1552-style feedback triple to get
predictable voltage gain, but beef up the output to handle the speaker
load... although why not just roll a little power amp from the get-go?

I don't think "Dave" could cope with the usual bi-directional intercom
schemes.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

On Tue, 5 Jun 2012 22:12:30 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Tue, 5 Jun 2012 18:28:22 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Wed, 23 May 2012 15:40:17 -0500, "Dave"
wrote:

Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4.
Now it only gives me noise at the output, and lots and lots of
that. Capacitors are all 100uF 35V, which I am thinking may be
the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it
with a 2N5296 from my junkbox, but that just doubled the volume
of the noisy output. If anyone sees something I should but
don't, please post. The only thing I can think of is upping the
voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that
last stage. (In fact, calculate all your stage biases.)

In my head, it's 14ma. That can't be right. The calc concurs. Is
that a little bit too much?

Aren't the emitter caps about 10 times as big as needed?

Ian did notice that he was just throwing gain at the problem though.

Yep, I was stunned... Ian said something cogent. But his buddy, Dave,
is beyond all hope... rude little POS.

I don't have AoE in reach, but I don't think they mention bias current in
their approach to design. They approach it with the rule of thumb that the
input impedance should be 10x the output impedance of the previous stage.
Similar. (The input impedance of the OP's stages are only slightly higher
than the previous output impedance.)

I'd like to know if you use any rules of thumb for this sort of thing. Or
is everything just optimized by multi-variable calculus?

Well, i know of two ways to design multistage transistor amplifiers:
One starts at the beginning and starts with the required/desired input
impedance. That sets up the bias network and then the rest falls out
rather naturally. If the output voltage/current/impedance does not work
yet add another stage.
The second starts at the output requirements and proceeds to the input
requirements.
If there is noticeable excess gain in the system you can retune the stages
(with an eye toward reducing cost) or add global feedback. Global
feedback after three stages is rather risky.

?-)

"Jim Thompson" wrote in
message ...
On Tue, 5 Jun 2012 22:12:30 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Tue, 5 Jun 2012 18:28:22 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Wed, 23 May 2012 15:40:17 -0500, "Dave"
wrote:

Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4.
Now it only gives me noise at the output, and lots and lots of
that. Capacitors are all 100uF 35V, which I am thinking may be
the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it
with a 2N5296 from my junkbox, but that just doubled the volume
of the noisy output. If anyone sees something I should but
don't, please post. The only thing I can think of is upping the
voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that
last stage. (In fact, calculate all your stage biases.)

In my head, it's 14ma. That can't be right. The calc concurs. Is
that a little bit too much?

Aren't the emitter caps about 10 times as big as needed?

Ian did notice that he was just throwing gain at the problem though.

Yep, I was stunned... Ian said something cogent. But his buddy, Dave,
is beyond all hope... rude little POS.

I don't have AoE in reach, but I don't think they mention bias current in
their approach to design. They approach it with the rule of thumb that
the
input impedance should be 10x the output impedance of the previous stage.
Similar. (The input impedance of the OP's stages are only slightly higher
than the previous output impedance.)

I'd like to know if you use any rules of thumb for this sort of thing. Or
is everything just optimized by multi-variable calculus?

I don't know if there are "rules of thumb"... maybe just calculate
rather than doing a NOLA white-trash guess ?:-)

...Jim Thompson


Does it really make you feel good to talk that way about other people,
calling someone you don't know a POS, and white-trash? Personally, I think
it says a lot more about you than it does about me.

And no, I don't know anything about the calculations involved in anything as
simple as an audio amp. Like I said, I'm making this up as I go along. I
don't know anything. I'm just trying to learn.

Dave

"Dave" wrote in message
netamerica...

"Jim Thompson" wrote
in message ...
On Tue, 5 Jun 2012 22:12:30 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Tue, 5 Jun 2012 18:28:22 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Wed, 23 May 2012 15:40:17 -0500, "Dave"
wrote:

Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4.
Now it only gives me noise at the output, and lots and lots of
that. Capacitors are all 100uF 35V, which I am thinking may be
the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it
with a 2N5296 from my junkbox, but that just doubled the volume
of the noisy output. If anyone sees something I should but
don't, please post. The only thing I can think of is upping the
voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that
last stage. (In fact, calculate all your stage biases.)

In my head, it's 14ma. That can't be right. The calc concurs. Is
that a little bit too much?

Aren't the emitter caps about 10 times as big as needed?

Ian did notice that he was just throwing gain at the problem though.

Yep, I was stunned... Ian said something cogent. But his buddy, Dave,
is beyond all hope... rude little POS.

I don't have AoE in reach, but I don't think they mention bias current in
their approach to design. They approach it with the rule of thumb that
the
input impedance should be 10x the output impedance of the previous stage.
Similar. (The input impedance of the OP's stages are only slightly
higher
than the previous output impedance.)

I'd like to know if you use any rules of thumb for this sort of thing.
Or
is everything just optimized by multi-variable calculus?

I don't know if there are "rules of thumb"... maybe just calculate
rather than doing a NOLA white-trash guess ?:-)

...Jim Thompson


Does it really make you feel good to talk that way about other people,
calling someone you don't know a POS, and white-trash? Personally, I
think it says a lot more about you than it does about me.


He seems to be ripping on a lot of people lately - and that's about as close
as he gets to posting on topic!

On Thu, 7 Jun 2012 15:32:20 -0500, "Dave" wrote:


"Jim Thompson" wrote in
message ...
On Tue, 5 Jun 2012 22:12:30 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Tue, 5 Jun 2012 18:28:22 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Wed, 23 May 2012 15:40:17 -0500, "Dave"
wrote:

Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4.
Now it only gives me noise at the output, and lots and lots of
that. Capacitors are all 100uF 35V, which I am thinking may be
the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it
with a 2N5296 from my junkbox, but that just doubled the volume
of the noisy output. If anyone sees something I should but
don't, please post. The only thing I can think of is upping the
voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that
last stage. (In fact, calculate all your stage biases.)

In my head, it's 14ma. That can't be right. The calc concurs. Is
that a little bit too much?

Aren't the emitter caps about 10 times as big as needed?

Ian did notice that he was just throwing gain at the problem though.

Yep, I was stunned... Ian said something cogent. But his buddy, Dave,
is beyond all hope... rude little POS.

I don't have AoE in reach, but I don't think they mention bias current in
their approach to design. They approach it with the rule of thumb that
the
input impedance should be 10x the output impedance of the previous stage.
Similar. (The input impedance of the OP's stages are only slightly higher
than the previous output impedance.)

I'd like to know if you use any rules of thumb for this sort of thing. Or
is everything just optimized by multi-variable calculus?

I don't know if there are "rules of thumb"... maybe just calculate
rather than doing a NOLA white-trash guess ?:-)

...Jim Thompson


Does it really make you feel good to talk that way about other people,
calling someone you don't know a POS, and white-trash? Personally, I think
it says a lot more about you than it does about me.

And no, I don't know anything about the calculations involved in anything as
simple as an audio amp. Like I said, I'm making this up as I go along. I
don't know anything. I'm just trying to learn.

Dave


I tried to help, suggesting you calculate the bias currents. Rather
than asking what I meant, if you're "just trying to learn", you
smart-mouthed.

And I don't respond well to smart-mouthed brats... it's enough of a
problem to deal with Larkin ;-)

HOWEVER... If you are really "just trying to learn", calculate the
bias as I suggested... why is it so high? And how much gain do you
really need? Put some numbers on things, and I'll try to point you in
the right direction.

For really, I'll help... but take the chip off your shoulder.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

On Thu, 07 Jun 2012 13:42:13 -0700, Jim Thompson
wrote:

On Thu, 7 Jun 2012 15:32:20 -0500, "Dave" wrote:


"Jim Thompson" wrote in
message ...
On Tue, 5 Jun 2012 22:12:30 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Tue, 5 Jun 2012 18:28:22 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Wed, 23 May 2012 15:40:17 -0500, "Dave"
wrote:

Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4.
Now it only gives me noise at the output, and lots and lots of
that. Capacitors are all 100uF 35V, which I am thinking may be
the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it
with a 2N5296 from my junkbox, but that just doubled the volume
of the noisy output. If anyone sees something I should but
don't, please post. The only thing I can think of is upping the
voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that
last stage. (In fact, calculate all your stage biases.)

In my head, it's 14ma. That can't be right. The calc concurs. Is
that a little bit too much?

Aren't the emitter caps about 10 times as big as needed?

Ian did notice that he was just throwing gain at the problem though.

Yep, I was stunned... Ian said something cogent. But his buddy, Dave,
is beyond all hope... rude little POS.

I don't have AoE in reach, but I don't think they mention bias current in
their approach to design. They approach it with the rule of thumb that
the
input impedance should be 10x the output impedance of the previous stage.
Similar. (The input impedance of the OP's stages are only slightly higher
than the previous output impedance.)

I'd like to know if you use any rules of thumb for this sort of thing. Or
is everything just optimized by multi-variable calculus?

I don't know if there are "rules of thumb"... maybe just calculate
rather than doing a NOLA white-trash guess ?:-)

...Jim Thompson


Does it really make you feel good to talk that way about other people,
calling someone you don't know a POS, and white-trash? Personally, I think
it says a lot more about you than it does about me.

And no, I don't know anything about the calculations involved in anything as
simple as an audio amp. Like I said, I'm making this up as I go along. I
don't know anything. I'm just trying to learn.

Dave


I tried to help, suggesting you calculate the bias currents. Rather
than asking what I meant, if you're "just trying to learn", you
smart-mouthed.

And I don't respond well to smart-mouthed brats... it's enough of a
problem to deal with Larkin ;-)

HOWEVER... If you are really "just trying to learn", calculate the
bias as I suggested... why is it so high? And how much gain do you
really need? Put some numbers on things, and I'll try to point you in
the right direction.

For really, I'll help... but take the chip off your shoulder.

...Jim Thompson

Here's an "intercom" I found at the first Google listing...

http://www.rcrowley.com/comclone/Schematic.htm

Maybe start with something simpler?

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

"Jim Thompson" wrote in
message ...
On Thu, 7 Jun 2012 15:32:20 -0500, "Dave" wrote:


"Jim Thompson" wrote
in
message ...
On Tue, 5 Jun 2012 22:12:30 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Tue, 5 Jun 2012 18:28:22 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Wed, 23 May 2012 15:40:17 -0500, "Dave"
wrote:

Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4.
Now it only gives me noise at the output, and lots and lots of
that. Capacitors are all 100uF 35V, which I am thinking may be
the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it
with a 2N5296 from my junkbox, but that just doubled the volume
of the noisy output. If anyone sees something I should but
don't, please post. The only thing I can think of is upping the
voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that
last stage. (In fact, calculate all your stage biases.)

In my head, it's 14ma. That can't be right. The calc concurs. Is
that a little bit too much?

Aren't the emitter caps about 10 times as big as needed?

Ian did notice that he was just throwing gain at the problem though.

Yep, I was stunned... Ian said something cogent. But his buddy, Dave,
is beyond all hope... rude little POS.

I don't have AoE in reach, but I don't think they mention bias current
in
their approach to design. They approach it with the rule of thumb that
the
input impedance should be 10x the output impedance of the previous
stage.
Similar. (The input impedance of the OP's stages are only slightly
higher
than the previous output impedance.)

I'd like to know if you use any rules of thumb for this sort of thing.
Or
is everything just optimized by multi-variable calculus?

I don't know if there are "rules of thumb"... maybe just calculate
rather than doing a NOLA white-trash guess ?:-)

...Jim Thompson


Does it really make you feel good to talk that way about other people,
calling someone you don't know a POS, and white-trash? Personally, I
think
it says a lot more about you than it does about me.

And no, I don't know anything about the calculations involved in anything
as
simple as an audio amp. Like I said, I'm making this up as I go along. I
don't know anything. I'm just trying to learn.

Dave


I tried to help, suggesting you calculate the bias currents. Rather
than asking what I meant, if you're "just trying to learn", you
smart-mouthed.

And I don't respond well to smart-mouthed brats... it's enough of a
problem to deal with Larkin ;-)


You declared war on JL by being a spitefull old fart.

Take care now - you're way past fighting on 2 fronts.

On Thu, 7 Jun 2012 21:54:21 +0100, "Ian Field"
wrote:


"Jim Thompson" wrote in
message ...
On Thu, 7 Jun 2012 15:32:20 -0500, "Dave" wrote:


"Jim Thompson" wrote
in
message ...
On Tue, 5 Jun 2012 22:12:30 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Tue, 5 Jun 2012 18:28:22 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Wed, 23 May 2012 15:40:17 -0500, "Dave"
wrote:

Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4.
Now it only gives me noise at the output, and lots and lots of
that. Capacitors are all 100uF 35V, which I am thinking may be
the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it
with a 2N5296 from my junkbox, but that just doubled the volume
of the noisy output. If anyone sees something I should but
don't, please post. The only thing I can think of is upping the
voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that
last stage. (In fact, calculate all your stage biases.)

In my head, it's 14ma. That can't be right. The calc concurs. Is
that a little bit too much?

Aren't the emitter caps about 10 times as big as needed?

Ian did notice that he was just throwing gain at the problem though.

Yep, I was stunned... Ian said something cogent. But his buddy, Dave,
is beyond all hope... rude little POS.

I don't have AoE in reach, but I don't think they mention bias current
in
their approach to design. They approach it with the rule of thumb that
the
input impedance should be 10x the output impedance of the previous
stage.
Similar. (The input impedance of the OP's stages are only slightly
higher
than the previous output impedance.)

I'd like to know if you use any rules of thumb for this sort of thing.
Or
is everything just optimized by multi-variable calculus?

I don't know if there are "rules of thumb"... maybe just calculate
rather than doing a NOLA white-trash guess ?:-)

...Jim Thompson


Does it really make you feel good to talk that way about other people,
calling someone you don't know a POS, and white-trash? Personally, I
think
it says a lot more about you than it does about me.

And no, I don't know anything about the calculations involved in anything
as
simple as an audio amp. Like I said, I'm making this up as I go along. I
don't know anything. I'm just trying to learn.

Dave


I tried to help, suggesting you calculate the bias currents. Rather
than asking what I meant, if you're "just trying to learn", you
smart-mouthed.

And I don't respond well to smart-mouthed brats... it's enough of a
problem to deal with Larkin ;-)


You declared war on JL by being a spitefull old fart.

Take care now - you're way past fighting on 2 fronts.


Look at his posts. Nothing but boasting about how smart he is, how
many things he's done, how many great ideas he has. But no content, no
help, nothing but the occasional Spice graphs from who-knows-what
secret circuits. He apparently expects all deference due to a
self-declared "Master Circuit Designer" and gets all hen-squawkey when
he doesn't get it. He doesn't seem to be interested in electronics at
all.

He doesn't "try to help", he just tries to inflate his ego. It's not
working. Sad excuse for "war."


--

John Larkin Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation

On Wed, 06 Jun 2012 15:31:57 -0700, Jim Thompson
wrote:

On Tue, 5 Jun 2012 22:12:30 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Tue, 5 Jun 2012 18:28:22 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Wed, 23 May 2012 15:40:17 -0500, "Dave"
wrote:

Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4.
Now it only gives me noise at the output, and lots and lots of
that. Capacitors are all 100uF 35V, which I am thinking may be
the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it
with a 2N5296 from my junkbox, but that just doubled the volume
of the noisy output. If anyone sees something I should but
don't, please post. The only thing I can think of is upping the
voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that
last stage. (In fact, calculate all your stage biases.)

In my head, it's 14ma. That can't be right. The calc concurs. Is
that a little bit too much?

Aren't the emitter caps about 10 times as big as needed?

Ian did notice that he was just throwing gain at the problem though.

Yep, I was stunned... Ian said something cogent. But his buddy, Dave,
is beyond all hope... rude little POS.

I don't have AoE in reach, but I don't think they mention bias current in
their approach to design. They approach it with the rule of thumb that the
input impedance should be 10x the output impedance of the previous stage.
Similar. (The input impedance of the OP's stages are only slightly higher
than the previous output impedance.)

I'd like to know if you use any rules of thumb for this sort of thing. Or
is everything just optimized by multi-variable calculus?

The 10X impedance is a good rule-of-thumb for general hacking.

I tend to be an over-calculator, so I know every interaction.

I am blessed with an ability to "walk" circuits in my head and see
their functions (and faults) in a matter of seconds.

****es lots of people off, when I instantly say, "Can't be" :-)

...Jim Thompson

Gosh, you are wonderful. Tell us more about how smart you are, how you
know everything, and how much you enjoy ****ing off people who aren't
as smart as you are. Are who aren't the superb marksman (at 30 feet)
that you are.


--

John Larkin Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation

On Thu, 07 Jun 2012 16:45:20 -0700, John Larkin
wrote:

On Thu, 7 Jun 2012 21:54:21 +0100, "Ian Field"
wrote:


"Jim Thompson" wrote in
message ...
On Thu, 7 Jun 2012 15:32:20 -0500, "Dave" wrote:


"Jim Thompson" wrote
in
message ...
On Tue, 5 Jun 2012 22:12:30 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Tue, 5 Jun 2012 18:28:22 -0400, "Tom Del Rosso"
wrote:


Jim Thompson wrote:
On Wed, 23 May 2012 15:40:17 -0500, "Dave"
wrote:

Posted a while back about a project I am trying to concoct- an
intercom for my front door- and have made some progress.
Unfortunately I hit a speed bump when I added transistor Q4.
Now it only gives me noise at the output, and lots and lots of
that. Capacitors are all 100uF 35V, which I am thinking may be
the problem (maybe the last couple need to be 50 or 75V?)
Originally thought I might be overdriving Q4, so I replaced it
with a 2N5296 from my junkbox, but that just doubled the volume
of the noisy output. If anyone sees something I should but
don't, please post. The only thing I can think of is upping the
voltage on C8 and C9.

Any help is *greatly* appreciated...

Dave


Back up and do a little math. Calculate the bias current in that
last stage. (In fact, calculate all your stage biases.)

In my head, it's 14ma. That can't be right. The calc concurs. Is
that a little bit too much?

Aren't the emitter caps about 10 times as big as needed?

Ian did notice that he was just throwing gain at the problem though.

Yep, I was stunned... Ian said something cogent. But his buddy, Dave,
is beyond all hope... rude little POS.

I don't have AoE in reach, but I don't think they mention bias current
in
their approach to design. They approach it with the rule of thumb that
the
input impedance should be 10x the output impedance of the previous
stage.
Similar. (The input impedance of the OP's stages are only slightly
higher
than the previous output impedance.)

I'd like to know if you use any rules of thumb for this sort of thing.
Or
is everything just optimized by multi-variable calculus?

I don't know if there are "rules of thumb"... maybe just calculate
rather than doing a NOLA white-trash guess ?:-)

...Jim Thompson


Does it really make you feel good to talk that way about other people,
calling someone you don't know a POS, and white-trash? Personally, I
think
it says a lot more about you than it does about me.

And no, I don't know anything about the calculations involved in anything
as
simple as an audio amp. Like I said, I'm making this up as I go along. I
don't know anything. I'm just trying to learn.

Dave


I tried to help, suggesting you calculate the bias currents. Rather
than asking what I meant, if you're "just trying to learn", you
smart-mouthed.

And I don't respond well to smart-mouthed brats... it's enough of a
problem to deal with Larkin ;-)


You declared war on JL by being a spitefull old fart.

Take care now - you're way past fighting on 2 fronts.


Look at his posts. Nothing but boasting about how smart he is, how
many things he's done, how many great ideas he has. But no content, no
help, nothing but the occasional Spice graphs from who-knows-what
secret circuits. He apparently expects all deference due to a
self-declared "Master Circuit Designer" and gets all hen-squawkey when
he doesn't get it. He doesn't seem to be interested in electronics at
all.

He doesn't "try to help", he just tries to inflate his ego. It's not
working. Sad excuse for "war."

So sayeth John "Dog Turd" Larkin ;-)

...Jim Thompson
--
| James E.Thompson, CTO | mens |
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| Voice480)460-2350 Fax: Available upon request | Brass Rat |
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I love to cook with wine. Sometimes I even put it in the food.