Minimoog VCA (!)

[Haible_Juergen#Tel2743]

Hi!

While everybody is talking about the Minimoog *filter*,
the Minimoog VCA is rarely mentioned.
In search of a good VCA section for my polyphonic,
I came to look a little deeper into this circuit, and I find
it rather interesting!
It's actually 2 VCAs and a fixed gain amplifier in series,
and that with the signal only going thru 3 differential pair
stages. No need for current mirrors like in OTAs.
Only a *little* bit of *local* feedback in the last stage.
Quite nonlinear, the whole stuff. Distortion is higher
during the peak of the loudness envelope - very unlike
your normal 3080-VCA, where distortion is created
just in the input stage which sees a rather fixed level.

I consider using a similar scheme for my JH-4 PolySynth.
I will also need 2 VCA stages. The first stage is to mix
filter outputs: HP, BP and LP from a state variable, and
a 4pole from a ladder or OTA design. The second stage
is for Loudness contour and panorama modulation.
So what I plan is this:

Four differential pairs work on the same pair of collector
resistors. Current distribution on the emitters will do the
mixing of the 4 filter outputs.
The Value of these collector resistors is low (approx.
100 OHMs for 500uA emitter current), to give the
right voltage swing for the next stage.
The second stage are two differential pairs which are tied
in parallel at their inputs, but work to separate collector
resistor pairs. Current distribution (for panorama modulation)
is controlled by another differential pair, which feeds the
emitters of the second stage.
The third stage will be slightly degenerated differential pairs,
like in the minimoog.

I am currently veroboarding this. First results are encouraging.
I hope I know more after the weekend.

JH.