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ees3dc
Joined: Jun 17, 2020 Posts: 1 Location: UK
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Posted: Wed Jun 17, 2020 9:44 am Post subject:
Why Can't a VCA just be an opamp |
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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 |
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blue hell
Site Admin
Joined: Apr 03, 2004 Posts: 24085 Location: The Netherlands, Enschede
Audio files: 278
G2 patch files: 320
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Posted: Wed Jun 17, 2020 12:05 pm Post subject:
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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. _________________ Jan
also .. could someone please turn down the thermostat a bit.
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PHOBoS
Joined: Jan 14, 2010 Posts: 5603 Location: Moon Base
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JovianPyx
Joined: Nov 20, 2007 Posts: 1988 Location: West Red Spot, Jupiter
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Posted: Thu Jun 18, 2020 8:26 am Post subject:
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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. _________________ FPGA, dsPIC and Fatman Synth Stuff
Time flies like a banana. Fruit flies when you're having fun. BTW, Do these genes make my ass look fat? corruptio optimi pessima
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Ricko
Joined: Dec 25, 2007 Posts: 251 Location: Sydney, Australia
Audio files: 27
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Posted: Thu Jun 18, 2020 8:22 pm Post subject:
Re: Why Can't a VCA just be an opamp |
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ees3dc wrote: | 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 |
An opamp is not really "voltage controlled". The supply voltages just provide limits, not multiplication of gain.
You could take a rail-to-rail opamp and use the supply voltages to dynamically limit the inputs, I guess, but that is not the function needed for VCAs.
Cheers,
Rick
(IIRC OTAs are usually not voltage controlled either, but current controlled. No big deal.) |
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AlanP
Joined: Mar 11, 2014 Posts: 746 Location: New Zealand
Audio files: 41
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Posted: Thu Jun 18, 2020 10:20 pm Post subject:
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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. |
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Electric Druid
Joined: Mar 13, 2012 Posts: 44 Location: UK
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Posted: Thu Jul 09, 2020 5:27 pm Post subject:
Re: Why Can't a VCA just be an opamp |
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ees3dc wrote: | a standard opamp already is voltage controlled.
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No, it isn't. Not in the sense that a voltage-controlled filter is voltage controlled.
An OTA on the other hand has an "Iabc" current input that *can* control the gain, and it's not that hard to turn voltages into currents, so the OTA can be turned into a proper VCA. That's not a trick you can pull off with a basic op-amp.
If it was, we'd have tried it before now! _________________ Electric Druid Synth and Pedal DIY website |
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