[sdiy] CEM3372 passband gain trick?

[Lanterman, Aaron]

On Aug 12, 2012, at 9:49 AM, 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.

As an aside, there's an ARP 4-pole VCF patent out there -- I forget which one -- that has an error in the diagrams, which is a source of confusion. The diagram shows positive resonance feedback when it should be negative (as Magnus noted).

- Aaron