"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.