[sdiy] CEM3372 passband gain trick?

[Magnus Danielson]

On 08/12/2012 03:49 PM, Magnus Danielson wrote:
> On 08/12/2012 11:15 AM, Tom Wiltshire wrote:
>> 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?
>
> I was just about to comment about this. The classical Moog feedback the
> full output signal goes into the resonance damper and then the passband
> signal gets subtracted from the input signal and hence dampen when the
> resonance is raised. The CEM3372 actually removes the (mostly in phase)
> pass-band from the feedback signal, and what remains is the bandpass
> signal which then spikes the cut-off frequency.
>
> No big magic, just clever and understanding what is going on.
>
> Oh, a note about the inversion. At the cut-off frequency, each of the 4
> filter sections have 45 degrees phase-shift, totalling in 180 degrees,
> so by inverting that signal and then feedback it will increase the
> response at the cut-off, but the pass band that experience close to 0
> degrees shift, will be suppressed with the 180 degrees version of
> itself. Removing the passband prior to feedback damping removes the
> passband effect, as noted above.

Oh, and notice that it is trivial to simulate the other behaviour, by 
also feed the resonance CV into the input (no distorsion) or output (can 
distort for high Qs) VCAs as it feels fit.

Cheers,
Magnus