"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