[sdiy] AC coupling caps on MS20 clone

[Mattias Rickardsson]

Did you just look & listen to an applied waveform, or did you VC
modulate it with a quick envelope or LFO?
CV bleedthrough is what I'd suspect to be the main problem here.

/mr

2010/8/6, David G. Dixon <dixon at interchange.ubc.ca>:
> Concerning AC coupling caps on my Korgasmatron 2164 MS-20 VCF clone, I
> realized that I could simply add a DC bias to the input signal on the
> simulation to see how the hypothetical circuit would handle it.  Here's what
> I found:
>
> Adding a small DC bias to the input signal (e.g., 2V, or raising a 10Vp-p
> square wave input to +7V/-3V) has no discernible effect on the output.
>
> Adding a large DC bias (e.g., 8V, or raising that same square wave input to
> +13V/+3V) causes the higher resonance peaks from the HP filter to clip
> slightly on every other half cycle.  Neither results in a DC bias on the HP
> output.
>
> Hence, I see no pressing need to apply an AC coupling cap on the input.
>
> Adding a DC bias to the first LP filter summer (i.e., the first VCA) causes
> the LP output to retain that same DC bias (albeit inverted).  However, since
> the HP output evidently will never have a DC bias, I see no need to put an
> AC coupling cap between the two filters.
>
> Ergo, I see no need for AC coupling caps in this filter circuit.
>
> I suppose my final step will be to simulate the original (OTA-based) circuit
> and see what effect the AC coupling caps have there (unless someone here has
> already done so, and can save me the trouble...?)
>
>
>> When are AC coupling caps necessary on filters?  Should they be on the
>> input
>> and the output?  Should they be on all outputs of multimode filters?
>> Should
>> they be switchable?  What's the best value and type?
>>
>> I haven't used them on my lowpass filters, but I'm unsure how to handle
>> this
>> aspect of my 2164-based MS20 VCF clone design.  Any suggestions would be
>> most appreciated!
>
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