[sdiy] CEM3372 passband gain trick?

[Tom Wiltshire]

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?

Tom


On 12 Aug 2012, at 09:15, Olivier Gillet wrote:

>> 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
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