[sdiy] VCA distortion - bad

[Scott Bernardi]

I think Walter Jung was the original author of this topology. You basically have
the bottom current mirror in the feedback loop of the biasing opamp, and the opamp
output drives the negative rail of the OTA, which will go to some negative voltage
that will support the current mirror - but just. So it's like driving the OTA with
a small negative supply rather than the -15v. This would have an effect on the
OTA"s negative output compliance, which is why you have to drive it into a virtual
ground rather than just a buffered resistive load.
Is there a resistor in series with the biasing opamp output and the OTA negative
rail?
What does the biasing opamp output do when the OTA clips/is overdriven?

harrybissell wrote:

> This is with the topology where the negative rail of the OTA is varied with
> the CV. I saw this in a Craig Anderton design (and also in Electronotes as you
> mentioned)... but I have never tried it.
>
> It certainly saves having to use a PNP current source to drive the Iabc pin
> (referenced to the negative rail) but I always wondered if it would affect the
> headroom.
>
> So anyone tried this and vouch for it ???
>
> H^) harry
>
> Michael Ruberto wrote:
>
> > I was wondering if it's possible that the op-amps I am using might have too
> > hot an output. in the original schematic Bernie specifies a LF353 and I am
> > using a TL072. could there be enough of a difference in the current sinking
> > ability between these devices to cause the overdrive?
> >
> > ~M
> >
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--
Scott Bernardi
sbernardi at attbi.com