[sdiy] CEM3372 passband gain trick?

Magnus Danielson magnus at rubidium.dyndns.org
Sun Aug 12 15:49:17 CEST 2012

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.


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