[sdiy] Experimental VCA RFC

[Jerry Gray-Eskue]

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