[sdiy] Experimental VCA RFC

[Jerry Gray-Eskue]

For what it's worth I did a quick FFT on the signal out in PSpice and got
this:

http://myweb.cableone.net/ankh/FFT.jpg

And a zoom in of the same plot:

http://myweb.cableone.net/ankh/FFTzoom.jpg


I do not have a good feel for how well this will translate to actual
hardware or if this is a good indicator of distortion levels.


-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Jerry Gray-Eskue
Sent: Friday, March 18, 2011 10:54 AM
To: 'René Schmitz'
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Experimental VCA RFC

There may be some distortion at very low output levels, in the millivolts
range. Due to using the Optos and R20 as a voltage divider there is no
intrinsic offset. Higher levels seemed OK, it is something to look into.

The C-E reverse is 7 volts for these, as they are zero volt centered and
working at about +-5 volts (10VPP) its good. Much more that 15VPP could be a
problem.

A servo loop may work, I have not done much with them. 

Part of the reason I went with the setup I did is the ratio of the Feedback
R to the Thermistor R value allows the slope of the temperature correction
to changed allowing various values and types of Thermistors to be used by
matching the Feedback R value. This may also be true of servo loops and if I
was more into them I might have gone that way. 

-----Original Message-----
From: René Schmitz [mailto:uzs159 at uni-bonn.de] 
Sent: Friday, March 18, 2011 10:30 AM
To: Jerry Gray-Eskue
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Experimental VCA RFC


Hi Jerry,

Interesting concept. How does the output signal look like waveform-wise. 
I.e. how much distortion? Offsets?

I would have thought that such a circuit could work as a switch/gate but 
not for proportional control.

What you should be aware is how much reverse voltage the C-E of your 
opto can stand. Sometimes only 5-6V. Which would be your maximum amplitude.

As others have pointed out you can linearize the control by a servo loop.

Cheers,
  René

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