[sdiy] Experimental VCA RFC

[Paul Maddox]

Harry,

  Looking at the way the VCA input is wired, one opto handles +ve
going voltages, the other handles negative going voltages (I recall
Roland or Korg Did something similar).

  I'm interested to see how this works out..

Paul

On 18 March 2011 14:20, Harry Bissell <harrybissell at wowway.com> wrote:
> Why are there two optos in parallel ?
>
> What is the function of the resistor from base to base... ?
>
> Most of the thermal issues are probably from the LED.  I'd try using one opto with transistor feedback to
> the LED drive opamp, and slave the second opto LED from that circuit  (see National Semi AN-20 Analog Multiplier
> for the idea using photoconductive cells).  Then use the second transistor for the audio path...
>
> H^) harry
>
>
>
>
>
> ----- Original Message -----
> From: Jerry Gray-Eskue <jerryge at cableone.net>
> To: synth-diy at dropmix.xs4all.nl
> Sent: Fri, 18 Mar 2011 09:08:06 -0400 (EDT)
> Subject: [sdiy] Experimental VCA RFC
>
> Request for Comments:
>
> This is a link to a VCA design I have been working on using H11A1 Optos as
> voltage controlled resistors.
> This includes a PSpice schematic for the core, and two PSpice plots at 20
> and 60 degrees C.
> The biggest design challenge is keeping the circuit from being effected by
> temperature changes, U3D uses the Thermistor NTC3 for this purpose.
> U1C is just the signal in buffer, U4A,U5 and U6 are the control /
> attenuation circuit. R15 sets the current per voltage in for the LEDs of the
> Optos. U4B is a just a spare at the moment.
> The Schematic shows H11A2s which seem to share the same PSpice model with
> the H11A1.
>
> In the plots the Green line is the envelope voltage, the red line is signal
> out.
> The signal in not shown is 1000hz at +-5v
>
>
> http://myweb.cableone.net/ankh/VCA.htm
>
> Jerry Gray-Eskue
>
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> --
> Harry Bissell & Nora Abdullah 4eva
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