Been tracking down some extraneous noise problems
in the 38s. Found one that can be fixed that'll
improve the noise floor significantly.
Here's a quick-n-dirty (perhaps non-optimal) fix.
Take out C38, the 22uf/16volt electrolitic. Re-
connect it across R25, being careful to observe
polarity. (+ve end goes to +8v side of R25, -ve
end goes to R26). This may cause audio
oscillations. If so, cut pin 8 of U5 away from
the +8v bus (on the bottom of the board) and add
a jumper from pin 8 to raw +12v . Don't have to do
this if audio remains stable. And before you apply
power, check that you haven't accidentally shorted
out the 7808 with this wire jumper, else you'll
blow up the 74HC240. +12v should only go to the
op-amp, most other chips can't take this voltage.
Ok, here's the explanation, if you're
interested:
The audio op-amp stage has high gain,
something over 1000. And its input comes from
the NE602 output, pin 4. This pin has an internal
resistor tied to +8v line of about 1500 ohms.
Turns out that the 7808 is injecting noise into
the NE602's Vcc line; that's wideband audio-type
noise at a mighty low driving impedance. All the
filtering capacitance on this Vcc line only takes
out high-frequency noise, but can't bypass the lower
audio frequencies because of this low source
resistance.
When U5's non-inverting inputs (pins 5, 3)
are bypassed to ground, the op-amp amplifies the
noise voltage to give a high noise level at its
output. When U5's non-inverting inputs are bypassed
to Vcc, the noise becomes a common-mode signal and
is mostly rejected by the op-amp's excellent
common-mode performance.
Can anyone come up with a better fix?
Perhaps this noise source can be rejected even more.
Here's a test you can do that'll convince you that
the 7808 is the noisy culprit:
Bypass U3 pin 4 to ground with about 20uf. Wince,
while your headphones pop. There's no noise now;
the op-amp is really very quiet. Now bypass the
NE602's input (U3) pin 1. Noise doesn't go away. Noise
source is either a really noisy NE602, or a noisy
Vcc line (its the Vcc line).
Glen VE3DNL leinwebe@mcmail.mcmaster.ca
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