Old school VCA

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Skullandcircuits
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Old school VCA

Post by Skullandcircuits » Thu Oct 31, 2019 3:47 am

I've build an old school VCA, mainly because it's simple and doesn't use any expensive, fragile or hard to get parts. Something you can throw together with what most of us have lying around anyway. I'm always looking for a bit of feedback, so what you think? Could it be done even simpler?

Here's the link

ixtern
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Post by ixtern » Fri Nov 01, 2019 6:24 am

Not so old school as it uses TL072 :)
Simpler it may be done but I would suggest rather adding and op amp or two for the audio input mixer (4 inputs or more) and it would be quite useful module then.
In complex setups mix modules is what is always needed in number. More than VCAs.

jorg
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Post by jorg » Fri Nov 01, 2019 8:32 am

Brilliantly simple!

ixtern - no extra op amps needed for an input mixer. Just add some more 200k input resistors. :-)

For those who really want low offset (low thump) you could sub a BCM847 for the transistor pair.

ixtern
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Post by ixtern » Fri Nov 01, 2019 11:27 am

jorg wrote:Brilliantly simple!

ixtern - no extra op amps needed for an input mixer. Just add some more 200k input resistors. :-)
But an extra pots for set input levels are needed also. Maybe channel separation would be enough (200 to 1) without an op amp.

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devinw1
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Post by devinw1 » Fri Nov 01, 2019 12:18 pm

A nice old school VCA is using the CA3080 and the VCA from the Arp Odyssey. It's dead simple. It sounds quite warm IMO and will impart an old school vibe. You can get 3080s from Small Bear for cheap.

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cackland
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Post by cackland » Fri Nov 01, 2019 5:06 pm

Thanks for sharing. Have you built this circuit or is this just a concept at the moment?

Also, devinw1, couldn't find the CA3080 at Small Bear.

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SphericalSound
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Post by SphericalSound » Fri Nov 01, 2019 5:20 pm

Lovely circuit. I would do it dual. The circuit is so simple that you can put 2 in the same hps

Question: is it linear or expo?

ixtern
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Post by ixtern » Fri Nov 01, 2019 5:31 pm

devinw1 wrote:A nice old school VCA is using the CA3080 and the VCA from the Arp Odyssey.
VCAs using differential pair are much older than any 3080 VCAs so they have priority being "old schoolers" :)
MapacheRaper wrote: Question: is it linear or expo?
I think it is linear as it has linearizing resistor in the diff pair emiters.

Skullandcircuits
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Post by Skullandcircuits » Sat Nov 02, 2019 5:29 am

Thanks for sharing. Have you built this circuit or is this just a concept at the moment?
I've build a few of them. They work quite well. No frills VCA.
Question: is it linear or expo?
Linear.

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khakifridge
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Post by khakifridge » Sat Nov 02, 2019 6:27 am

cackland wrote:Also, devinw1, couldn't find the CA3080 at Small Bear.
http://smallbear-electronics.mybigcomme ... -ca3080ae/

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guest
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Post by guest » Sat Nov 02, 2019 3:08 pm

circuit looks good! a few points of feedback, if interested:

1. you could get rid of the 15k resistors that go between the diff pair collectors and the opamp inputs. the gain will go up (double), but otherwise should function the same.
2. the offset trim could get reduced to 500ohm. the imbalance in the circuit shouldnt be more than a few percent.
3. im not exactly sure what the response curve is for this circuit. the output of the CV opamp will go from 0V to -5V as the input CV goes from 0V to +5V. the 10k voltage divider that follows will reduce this to a 2.5V swing with some offset determined by the trimmer. but, the transistor will go on/off over a range of 0.6V, so will a lot of the range be in saturation? what if R10 resistor was attached to the emitter of the transistor, instead of the collector (still going to -12V, and the emitter disconnected from ground). then, if you assume Vbe somewhat constant, the current through the transistor will be varying linearly as the base voltage drops. you could probably eliminate R9. the main problem here is that the response is now inverted, which i suppose could be fixed by changing the input opamp stage to a non-inverting stage with some offset.
openmusiclabs.com

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CLee
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Post by CLee » Sat Nov 02, 2019 5:04 pm

The CGS VCA is a similar circuit

https://sdiy.info/wiki/CGS_VCA

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devinw1
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Post by devinw1 » Sat Nov 02, 2019 6:04 pm

ixtern wrote:
devinw1 wrote:A nice old school VCA is using the CA3080 and the VCA from the Arp Odyssey.
VCAs using differential pair are much older than any 3080 VCAs so they have priority being "old schoolers" :)

Won't argue that!

OTA is much simpler than discrete though. :)

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