[sdiy] CEM3372 passband gain trick?

Dave Manley dlmanley at sonic.net
Mon Aug 13 17:15:48 CEST 2012

Is there a published mod to the minimoog filter to change this?


-------- Original Message --------
 From: Magnus Danielson <magnus at rubidium.dyndns.org>
 Sent: Sun, Aug 12, 2012 06:49 AM
 To: synth-diy at dropmix.xs4all.nl
 Subject: Re: [sdiy] CEM3372 passband gain trick?

>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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