Why Can't a VCA just be an opamp

[ees3dc]

Why am I seeing transconductance amplifiers being used for VCAs? Ie the CA3280 (that is now obsolete) - I'm looking at the Prophet 5 voice circuit. Since it says voltage control, a standard opamp already is voltage controlled.

Thanks

[blue hell]

A transconductance amplifier is a multiplier, in that it multiplies one signal by another (or put differently, one signal determines the amplification factor of the other, and when that one signal is a voltage .. it is a VCA).

An opamp can multiply a signal by a constant value only.

Now when you'd change that constant an opamp could do the trick too .. but it is not so easy to do that in a precise way. An OTA otoh is pretty precise in what it does.

[PHOBoS]

I am curious how you would like to do that since you'd have to control the gain.

[JovianPyx]

The OTA is the best way to make a VCA. It provides lower distortion than other methods. One way to make a non-OTA VCA is to make a voltage divider using an FET and use the gate voltage to modulate the amplitude. An opamp would then provide gain after the divider. Another might be to put a FET into the feedback loop of an opamp serving as an amplifer. These methods that are less than optimal often came about because the better methods were already patented. These alternative methods aren't necessarily bad though, sometimes the right kind of distortion sounds good which then becomes part of the device's character.

[Ricko]

[AlanP]

Another possibility would be a vactrol in the feedback resistor position.

But to answer the OP on a broader level, it's because ain't nothing useful ever come easy.

[Electric Druid]