[sdiy] OTA to voltage-control resonance?

[Jerry Gray-Eskue]

If you put it in the feed back loop I believe you will need the "Floating
Voltage Controlled Resistor" shown in figure 10 of the National LM13700 data
sheet.

My guess is that it will behave very close to a variable resistor and sound
much the same if not indistinguishable. Note that this only gives you One
resistor, so if you are using Both ends of the pot you are a resistor short,
if the pot was acting as a variable resistor only its good.

The value of the Voltage Controlled Resistor is given as Rx=2R/GmRA but
there is little other data on its performance. I suspect that in a low
current feed back loop you will not be pushing any limits. There may be some
weird toward the extremes like very low or high values of Rx.

I think the main reason the circuit is not widely used is that the more
common voltage control techniques work as well or better and require less
complexity.

- Jerry

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of cheater cheater
Sent: Sunday, August 30, 2009 4:28 PM
To: synth-diy
Subject: [sdiy] OTA to voltage-control resonance?


Hi guys,
I've been looking at some monosynths and one thing has me wondering:
if I have a potentiometer somewhere in the filter's resonance
feedback, can I safely replace it with an OTA based 'voltage
controlled resistor' without changing the sound?

I understand the best answer could be 'depends' - so, what does it depend
on?

Do OTAs work like buffers, similarly to op-amps?

Thanks
D.
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