[sdiy] VCA distortion - bad

[Michael Ruberto]

I guess I should have posted this before but I don't feel right posting 
schematics from other people. here is the schematic I am following:
http://www.sunrisecomputersolutions.com/samples/VCA-Hutchins.gif

~M

>From: Scott Bernardi <sbernardi at attbi.com>
>To: Synth-DIY list <synth-diy at dropmix.xs4all.nl>
>Subject: Re: [sdiy] VCA distortion - bad
>Date: Sat, 14 Jun 2003 21:46:38 -0700
>
>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
>
>
>

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