[sdiy] why +/- 15V

[René Schmitz]

jhaible wrote:
> Bringing the level up (and down again) between filter stages
> helps to avoid leakage thru parasitic capacitance.
> 
> Theoretically, You can go from one filter stage to the
> next with 20mV signals.

The Moog filter is an example.

But relatively large caps there, so the parasitics shouldn't matter.

> In practice, doing this with 3080's or 13600's will result
> in reduced attenuation a few octaves above the cutoff
> frequency. (24dB slope alright, but then it reaches a
> level of maximum attenuation somewhere, and this
> is vastly different with direct coupled OTAs compared
> with the usual 100k/200R attenuation and gain.

Never tried that.

> I suspect that this was an issue in single-chip VCFs
> that used direct coupling, too. "Old ones"
> like 2040 and 3320 have divider coupling;
> 2nd generation SSMs and CEMs (numbers not at hand)
> sometimes were directly coupled. Maybe that's
> why they sound less impressive.

Interesting.

Cheers,
  René


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