[sdiy] RE: [AH] Parametric EQ

[Tom Wiltshire]

On 21 Feb 2010, at 13:29, Neil Johnson wrote:

> Tom Wiltshire 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
>
> Trouble with that arrangement is that it relies on zeners being  
> perfect diodes, i.e. no current for V below 4V7 (in this case).   
> However, there _is_ current flow, albeit tiny compared to the 'on'  
> current, so at low frequencies, where the current flowing into/out  
> of the SSM2164 gain cell is equally tiny you end up with a DC  
> current path in parallel with the integrator capacitor.
>
> The solution I used in my 3-pole filter is to configure the zeners  
> as the bottom leg of a potential divider from the output of the LP  
> integrator back to the input.  See here:
>
> http://www.milton.arachsys.com/nj71/pdf/cesyg/vcf-sch.pdf
>
> In this case focus on IC3A, with R10 and D1,D2 form the potential  
> divider.  This keeps the main signal current path clean.


Interesting. Does this provide an audible improvement over putting  
the zeners across the op-amp, Neil? It sounds like it ought to, but  
do you hear it in practice?
Presumably this DC current path in parallel with C4 will make the  
integrator into a slightly leaky integrator? What's the effect of  
having a small current drain to ground instead?

I'm a fan of low-component count solutions (hence no frequency  
dependent funkiness like you have) but just moving the zeners I can  
cope with!

Thanks,
Tom