Darrell Bellerive wrote:
I played a bit with this mod this evening. I can duplicate the
results, but
I'd like to understand a bit more about this circuit.
What is the purpose of RP5 pins 7/8 and pins 9/10? This grounds the
audio
signal via 940 ohms of resistance. Is this just to provide a DC bias
for the
electrolytic capacitors or does it have a role to play with the audio?
Hi Darrell,
There's some subtlety to this circuit. Among the various goals was to
minimize component count.
C32, on pin 8 of U9, is the AF amp's supply decoupling capacitor. It
would normally be returned to ground, as shown in any LM380 datasheet.
However, we played a trick here, using pin 8 as the sidetone injection
point. In other words, we're treating pin 8 as a low-gain, single-ended
third input to the amplifier.
But C32 still needs a ground return path so it can perform its supply
decoupling function reasonably well. L1, which is part of the sidetone
shaping circuit, serves the ground-return function at low audio
frequencies, but as F increases, the Z of the inductor does, too.
This is where RP5 comes in. We had a couple of RP5 sections left over,
so we put them in series to form about a 1K resistor that establishes a
minimum ground-return impedance, ensuring stability of the AF amp's
internal biasing. Of course this resistor also reduces the Q of L1, but
not enough to impact its shaping function.
We could have injected the sidetone at the amplifier's differential
inputs, but doing this right (without AF artifacts) requires
differential drive, e.g. more parts.
73,
Wayne
N6KR
---
http://www.elecraft.com
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