AW: Re: AW: Transistor matching

[Haible_Juergen#Tel2743]

> BTW, how's your VCA project advancing? :)

It works great. I have done extensive tests on the weekend,
and I doubt I will ever use a 3080 for a synth's main VCA again.

It is quiet. I connected it to a 2040 filter with low cutoff setting:
No noise. Opening the master faders on the desk all the way
up: still almost no noise. In comparison, my OB-8 produces
a lot of noise with the filter closed and the VCA open.
(I have a late OB-8 model with CA3080 VCA.)
I don't think that it has a better SNR than a special VCA chip
like SSM2024 or CEM3330, but then again it's just a few
cheap transistors, DM 0.16 each! And I won't have to rely
on some special chip being still available in 10years.

Thumping is minimal. I could trimm it out almost perfectly
(with a 1ms envelope). Again, better than most 3080's
or 2024's I have offset-trimmed.

It is quite nonlinear. Well, that's what the Minimoog VCA is,
and that's what I wanted as well. Not your first choice
for general audio signal processing, but great for a synth
voice.

The scanning between four inputs works as expected.
I have refined the technique I have used in my Interpolating
Scanner module, and now I only need 6 transistors, a
handful of resistors and one opamp to make the triangle-
shaped current distribution for four differential pairs.
So expect a polysynth that has a Filter Mode knob like
the Obie SEM, but this time it will change continuously
from 2-pole-LP (8 o'clock position) to 2-pole-Notch
(10 o'clock) to 2-pole-HP (12 o'clock) to 2-pole-BP
(2 o'clock) to 4-pole-LP (4 o'clock).
Under voltage control ! Of course there are some artifacts
in between (the 2-pole-NF actually being an artifact between
HP and LP, right from the original Obie design), but nevertheless
I think it is a useful feature. And the notches won't gap too
deep between the other modes.

JH.