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.


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