[sdiy] why +/- 15V

[jhaible]

> I doubt that a little, because in the filters and VCAs the signals are
> brought to low levels for the differential pairs (think ladder filter or
> VCA). So that wouldn't have gained anything.

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.

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.

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.

JH.