There are two places affected by the "Chorus disable" signal. One is
bottom right, in the output of the chorus section.
The other is near the top middle diagram, near the op-amp section
formed by pins 1, 2 and 3 of U15.
Now here the schematic looks wrong. I'm almost certain that the
output of this op-amp should be connected be connected to R135 and C79,
which should form the basic feedback of this inverting stage.
But the connections seem to cross there without a junction! If
that's the case, then in one position of the U16 switch (the top one),
the output of the op-amp would be disconnected, leaving it without
any feedback, oops.
If we assume that there is a junction there, then it looks like
the circuit simply alters the gain based on whether the chorus
is enabled or disabled.
The gain is normally unity, because the input is arriving over a 20K
input resistor and the feedback resistance is also 20K. The switch
will put another 47K resistance in parallel with the 20K, which
will reduce the feedback resistance to 14K. So this will
drop the voltage gain from 1 to 14/20, or 0.7. A 0.7 voltage gain
corresponds to a cut of 20 * log10(0.7) = -3dB, which is noticeable.
So indeed, it does look as if there is a bit of a volume cut happening
here, by design, maybe to make room for the pitch-shifted chorus
But, it looks like the purpose of that volume cut is to make
room for mixing in the chorus signal. So I suspect that there should
not simply be a -3dB drop in volume. The dry signal drops by -3dB,
but at the same time, the chorus should be mixed in, and at the lowest
depth, it is mostly in phase with the dry signal.
When I get home, I will see whether I can hear a volume cut
on my MP-1 when I engage the chorus from 0 to 1. Though I haven't
ever used the chorus much, I don't seem to remember anything like that
from the few times that I played with it.
Is your chorus actually working?
And did this start happening after some op-amp replacements, or
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