[sdiy] Simple VCA?

[Scott Gravenhorst]

tim102 at tstinchcombe.freeserve.co.uk wrote:
>> Does it look like this ASCIImatic:
>
>Yep, that is as per the circuit in the Teledyne book: JFET is n-channel, is
>shown with no resistor on the gate, but has a '-' (minus) on it.

I would expect using a negative CV.  I'd bet a MOSFET works too.

Interesting that there's no CV input resistor.

So it's like this then:
                                 FB ?
 
                        +------\/\/\/\---+
             510R       |                |
                        |  |\            |
     IN *--/\/\/\/---+--+--|-\           |
                     |     |  \          |
                     |     |   \         |   
                   |-+     |    \        |    OUT
                   |       |     >-------+----->
 - CV *----------->|-+     |    /
                     |     |   /
                     |     |  /
                     +-----|+/
                     |     |/ 
                     |
                     /
                     \ 510R
                     /
                     \
                     /
                     |
                     |
                    GND

When I imagine this without the FET, a 500K FB resistor would give a gain of 1000. 
Input impedance seems very low (510R).  My sense is that this VCA probably works better
with smaller signal voltages.  I'm wondering if JFET input opamps would work, don't see
why not.  A dual could provide a high impedance buffer/inverter for it.  Intriguing
simplicity though.

>Tim
>>[old dwg]

-- ScottG

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-- Scott Gravenhorst
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