[sdiy] CEM3372 passband gain trick?

[Olivier Gillet]

> Couldn't it be as simple as adding the res CV to the VCA?

In my first version of a 4-pole VCF + VCA board, I did it this way and
this could cause unwanted clipping if the self-oscillation tone got
close to the rails (and was further amplified by the VCA).

Another way to do it which is straightforward if you have implemented
VC-resonance through a gain cell controlling the amount of 4th stage
output fed back into the input: feed the filter input signal into this
gain cell. This is equivalent to increasingly adding an extra amount
of the input signal at the input of the filter when resonance is
increased. The transfer function would be:

(1 + 4k) / (4k + (1 + s) ^ 4)

Where k is the resonance control and goes from 0 to 1.

Seen for example in the JP-8:
http://fa.utfs.org/diy/roland_filters/JP8.jpg (25A is the OTA
responsible for CV-resonance).

You can switch the compensation on/off by disconnecting this ; and you
can insert a slight high-pass filter there (or use undersized
AC-coupling cap) to prevent the compensation to affect the lowest
frequencies to get a resonance with a more pronounced "nasal"
character.

Olivier