"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