[sdiy] RE: [AH] Parametric EQ

Tom Wiltshire tom at electricdruid.net
Sun Feb 21 20:21:49 CET 2010

On 21 Feb 2010, at 17:58, David G. Dixon wrote:

>> What did you do about the resonance response of the SSM2164? I found
>> that getting a reasonably constant resonance out of it w.r.t cutoff
>> was quite difficult. Did you bother with any compensation? In the
>> end, I think I decided I didn't care that much (it sounded so *fat*
>> and *good* anyway...yuummmm!) and I've just got a couple of back-to-
>> back 4V7 zeners on the LPF output amp. This isn't shown on this
>> earlier circuit diagram, but they're on IC2.2, in parallel with C4.
>> http://www.electricdruid.com/VCResonator.jpg
> One thing that strikes me about this schematic is that the Res CV  
> will have
> an exponential effect on the degree of resonance feedback.  Of  
> course, an
> exponential response is desired on the variable resistors and the  
> output
> level.  However, I would have thought that a linear response would  
> be more
> appropriate on the Res CV.  Of course, that would require two 2164  
> VCAs (a
> la Mike Irwin), thus raising the total number to five, which is pretty
> darned inconvenient unless the output one is sacrificed, or moved to a
> dedicated four-input mixer.  Also, that might not improve the  
> problem of not
> achieving constant Q with constant Res CV, but then again it might,  
> and it
> would definitely make it easier to "dial in" a certain Q with a pot.

If I was to try and linearize the resonance response, I'd probably do  
it using a lookup table in the software ahead of the CV DAC, much  
like I recently did with the pan law for my stereo VCA (also SSM2164).
I intended the circuit to be used as a "body-modelling filter"; e.g.  
for modelling formants and resonances in instrument bodies, rather  
than for going "BEEEOOOWWW!" every note. If you're not moving the  
cutoff, the variable-Q-with-frequency isn't much of a problem, and  
neither is the exponential Q response. In practice, the frequency- 
dependent element of the Q is the worst of the two. I don't find it  
hard to get the setting I want once cutoff is set.


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