[sdiy] Simple VCA?

[mike ruberto]

I think I have seen something similar in a schematic for an old audio
compressor.

The simplest VCa out there is the Yamaha single transistor design. I
think you can find details about it on Mr. Haible's website.

Using an FET optocoupler is also a simple VCA solution.

Mike

On Sat, Apr 12, 2008 at 1:18 PM, Roy J. Tellason <rtellason at verizon.net> wrote:
> I've got this "Modern Electronic Circuits Reference Manual" I acquired
>  recently,  not all that modern since it was published in 1980,  but I started
>  browsing it anyhow...
>
>  There's one circuit I ran across that I'm wondering about,  never saw it
>  actually used anywhere and I don't know how well this would work,  so I
>  figured I'd toss it out for you guys to pick apart.
>
>  It's a very simple circuit,  really.  An op amp inverting amplifier,  one
>  resistor going from the non-inverting input to ground,  one input resistor,
>  and one feedback resistor.  And,  a JFET connected between the two input
>  terminals with the gate lead labeled as being the gain input.
>
>  The caption for this reads "VOLTAGE-CONTROLLED GAIN -- 2N5457 FET acts as
>  voltage-vaiable resistor between differential input terminals of opamp.
>  Resistance variation is linear with voltage over several decades of
>  resistance, to give excellent electronic gain control. Values of resistors
>  depend on opamp used."  --"FET Databook", National Semiconductor, 1977, p.
>  6-26 -- 6-36.
>
>  Comments on this?  Could that be done that simply?  Or what am I missing here?
>
>  --
>  Member of the toughest, meanest, deadliest, most unrelenting -- and
>  ablest -- form of life in this section of space,  a critter that can
>  be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
>  -
>  Information is more dangerous than cannon to a society ruled by lies. --James
>  M Dakin
>
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