AW: [sdiy] ota's as gates.

[René Schmitz]

Hi Nils and List,

> I took a look at it.. Looks like a good solution for me. I don't mind the
> extra tranny.

:-)

> Btw, I'm somewhat shocked by the simplicity of your VCA2. How good does it
> perform?

Its haveing the same soft clipping as a 3080 would have. Which I
personally find desireable in a audio processing VCA. (THere is nothing
worse than harsh clipping.) So distortion wise there is nothing outstanding.
In my recent incarnations I added a second trimmer at the base of the
transistors, then the offset can be minimized as well as feedthrough.
Also one should use a monolithic dual like a 2SC1583 instead of the
BC550s. That helps to lower the drift, and to keep the feedthough/offset
trimming with changing temperature.
With these changes I am now totally happy with it! Thumping is
practically inaudible.

The only thing that might be an issue, is that there is a dead band of
about 0-600mV where the VCA is closed. It would cut out the tail of a
decay for example. Shifting the base somewhat negative would help there.
I never bothered to do that since I have a pot which I use to manually
open the VCA, so I just set it to the right spot.

A circuit derrived from it was a contestant in the "VCA shootout" which
Jörgen Bergfors did a couple of years ago. That didn't make a good
figure there. (Which is IMO due to the fact that he drove the diffamp
with less current (higher CV summing resistors) and used a TL071 instead
of the 5534. Both of these changes decrease the SNR.) His criticism with
respect to offsets and feedthough was what led me to develop the changes
above. I guess I should be updating the diagram some time....

Cheers,
  René