[sdiy] CEM3372 passband gain trick?

[Olivier Gillet]

> The "block and connection diagram" actually shows a connection from the output of the two VCA mixers back to the Res VCA, just like the Jupiter 8 schematic Olivier posted.
> Could someone explain to me why they've used a differential mixer for this job (in both cases it's shown that way)? Is there an inversion somewhere in the resonance signal path that they're compensating for?

No, but to arrive at the "canonical" transfer function of the 4-pole
LP with resonance (1 / (4k + (1 + s) ^ 4)), you need to subtract the
output of the 4th stage from the input, not add it.

And to arrive at the compensated transfer function ((1 + 4k) / (4k +
(1 + s) ^ 4)), you need to add an additional fraction of the input
signal to the input of the filter.

So it's actually useful to have a gain cell with differential inputs
for this function.

Olivier