VCAs, and Noise reduction for NE571

[Haible Juergen]

	>From there it was only a hop step
	>and jump to using an OTA instead of the one built into the 571 and
the
	>whole thing was redundant. 

I fully agree with that. Once built a compressor around the 570 (or 571),
and found no way to get a fast attack / slow decay operation.

	>Believe me a 3080 is a whole lot quieter than
	>the 571.

Are you sure this is not just because of the shitty internal opamp ?
There is a recommendation to replace it with an external opamp and
only use the variable gain cell. This is mainly to get a reasonable
slew rate, but probably also to improove the SNR. Don't know, it's
too long since I've played with these.

But as you wrote earlier, the real application is companding, not single
ended, noise reduction, where the internal noise is masked by the 
noise of the channel anyway.

On a related topic: I'm really surprised how easy it is to build OTA-less
VCAs that are good for synth applications. I never really measure the
SNR of this stuff (too lazy), but I'm content if it has no thumping, no
nasty distortion above 3%, and if it's dead silent when CV is zero.

Latest thing I've tried is a diode ring of the Minikorg style: no more than
the diode ring and one (!) npn transistor to get the control current, and
a second npn to compensate for insertion loss and to serve as buffer amp.
Get a bunch of 1N4148's on a belt and measure the "resistance" with
a DVM ("resistance" actually being the voltage drop @ whichever current
the DVM sends into the device under test ...), and select four ones that
are less than 1mV apart from each other. Don't touch the diodes with
your fingers (touch the paper belt instead) to avoid temperature drift
during the measurement.
As I said, I have not made SNR measurments, it just does its job
nicely as a synth VCA.

JH.