"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