[sdiy] Experimental VCA RFC

[Jerry Gray-Eskue]

Remember that the base to emitter ( if I have this straight at the moment )
can act as a diode in a NPN transistor, Adding the resistor, I did actually
see a difference in the output signal level at very low CV in voltages in
the PSpice plots.

-----Original Message-----
From: Paul Maddox [mailto:paul.Maddox.mail-list at Synth.net] 
Sent: Friday, March 18, 2011 10:12 AM
To: Jerry Gray-Eskue
Cc: Harry Bissell; synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Experimental VCA RFC

Jerry,

  The resistor isn't connected to anything though. I would normally
have expected to see two resistors, going from each base to ground.
But you have the resistor between the bases....

Paul

On 18 March 2011 15:04, Jerry Gray-Eskue <jerryge at cableone.net> wrote:
> If you look closely you will see the transistor outputs face opposite
directions, one handles the positive half the other the negative half of the
signal.
>
> The resistor pulls down the base a bit and gives a better turn off of the
transistors.
>
> I will take a look at that app note.
>
> -Jerry
>
> -----Original Message-----
> From: Harry Bissell [mailto:harrybissell at wowway.com]
> Sent: Friday, March 18, 2011 9:21 AM
> To: Jerry Gray-Eskue
> Cc: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] Experimental VCA RFC
>
> 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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