v2164 linear VCA problem

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hox3d
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Re: v2164 linear VCA problem

Post by hox3d » Mon Mar 01, 2021 12:53 pm

Ah, that's clever.
I'm quite happy to discover many ways to multiply signals with the same chip: the previous one used selective mixing, but this one use phase-cancellation.
That makes the circuit much simpler, of course, but I can't see any drawbacks compared to the "selective-mixing" one. Maybe there are some, that could justify using one circuit over the other?

In my case though, I need it to be DC-coupled (will get any type of modulation signal, and I need DC it to control cutoff for instance).
What I wanted is basically a simple VC-polarizer; this topology seems to be okay for this need.

Funny you mention the Frequency Shifter (I've read your thread, of course) because we're working on a fully digital version of a FS, and issues are more or less the same in digital (carrier bleed, getting rid of offsets etc), but the topology is the same (even though we'll be talking about Hilbert Transform instead of Dome filters ahah).

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Triglav
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Re: v2164 linear VCA problem

Post by Triglav » Thu Apr 01, 2021 9:07 am

Hi,
I'm trying to build a crossfader with a linearized 2164, and it kind-of-sort-of works, but I'm having terrible oscillation/stability problems.
Could someone please tell me what the problem is with this schematic?

Thanks!
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crossfader.jpg

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neil.johnson
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Re: v2164 linear VCA problem

Post by neil.johnson » Thu Apr 01, 2021 10:05 am

I hope there's more to it than that schematic. It's missing decoupling caps for all the ICs, there are missing stability caps on most of the opamps, and the OPA2134 is quite a spicy opamp (meaning you really need to pay attention to the supply routing and decoupling otherwise it will happily oscillate at hundreds of kHz).

Also your choice of resistor values is about 10x way too high for anything that is passing audio signal.

Neil
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Triglav
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Re: v2164 linear VCA problem

Post by Triglav » Mon Apr 05, 2021 1:41 pm

Thanks for the advice!
Yes, there's more to the circuit, including decoupling caps for every IC and I've managed to find and fix the oscillation issue elsewhere. :goo:

This might be a bit off-topic now, but I have a follow-up question which might be basic, but I'm having a hard time finding a definitive answer:
Is there a way to tell in advance where to use stability caps? Would you just put them on every opamp by default or check for problems and put them in accordingly? The OPA134 datasheet says that they should be unity-gain stable. Does that not hold true here?

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oldenjon
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Re: v2164 linear VCA problem

Post by oldenjon » Mon Apr 05, 2021 3:01 pm

Oops
"We all dance by ourselves, that's why we only play electronic music"

http://synthscalledbeasts.tumblr.com/

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neil.johnson
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Re: v2164 linear VCA problem

Post by neil.johnson » Mon Apr 05, 2021 4:13 pm

Triglav wrote:
Mon Apr 05, 2021 1:41 pm
Thanks for the advice!
Yes, there's more to the circuit, including decoupling caps for every IC and I've managed to find and fix the oscillation issue elsewhere. :goo:

This might be a bit off-topic now, but I have a follow-up question which might be basic, but I'm having a hard time finding a definitive answer:
Is there a way to tell in advance where to use stability caps? Would you just put them on every opamp by default or check for problems and put them in accordingly? The OPA134 datasheet says that they should be unity-gain stable. Does that not hold true here?
Find a good book on opamp theory and it should explain far more precisely what is going in and why you need stabilisation capacitors. Anything from Art of Electronics, or Sergio Franco for example.

There are many things going on both inside and outside an opamp, and so a good understanding of all that would be a good thing to read up about.

Cheers
Neil
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SSI parts and more :: https://www.cesyg.co.uk

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