[sdiy] RE: [AH] Parametric EQ
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.
> 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
> 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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