AS3360 VCA oscillating

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CLee
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AS3360 VCA oscillating

Post by CLee » Mon Jan 11, 2021 12:41 pm

I'm breadboarding a build with the AS3360 dual VCA chip and I'm getting high frequency oscillations on both gain cells. Removing the Vref capacitor on pin 8 stops the oscillation but then the chip doesn't work correctly. Nothing else seems to change things. I've bypassed all the power pins with 100n ceramics and sprinkled 47uf electros about with no change. Anyone have a similar problem.

I noticed that the build docs show the gain cells feeding 47K resistors to ground instead of directly feeding the inverting input of an op amp. Does this chip not like to feed a current-to-voltage op amp?

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Re: AS3360 VCA oscillating

Post by jorg » Mon Jan 11, 2021 2:22 pm

Schematic, please.

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Re: AS3360 VCA oscillating

Post by CLee » Mon Jan 11, 2021 2:40 pm

Just the data sheet circuit for linear operation except that the outputs (pin 2 & 13) feed inverting inputs of op amps instead of 47K resistors to ground.

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Re: AS3360 VCA oscillating

Post by jorg » Mon Jan 11, 2021 4:03 pm

Did you include compensation capacitors on the op amps? What op amps did you use?

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Re: AS3360 VCA oscillating

Post by guest » Mon Jan 11, 2021 5:50 pm

does it not oscillate if you just use the 47k to ground? does changing the value of the Vref cap change anything?
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Re: AS3360 VCA oscillating

Post by CLee » Mon Jan 11, 2021 6:44 pm

Yes, had comp caps on the op amps.

I'll have to pull the op amps (072's) out and set it up with a 47K to ground as output and see if it's stable like that. Hopefully I'll have a chance tonight.

Changing the value of the Vref cap had no effect. Going up to ufd values caused it to act weird. It's a bit of a black box, don't know what the cap is really doing so...

(BTW, just noticed that the chip can't have 30V across the supply, so I may be bagging this design. Lot's of folks build 4U with +/-15v supplies.)

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Re: AS3360 VCA oscillating

Post by guest » Mon Jan 11, 2021 6:52 pm

if building it up same as the datasheet doesnt work, id reccomend contacting alfa. they may have some ideas, and minimally they should be informed that there are problems with their chips.
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Re: AS3360 VCA oscillating

Post by CLee » Mon Jan 11, 2021 11:14 pm

Well a 47ufd electrolytic directly on the VCC pin (pin 14) and the circuit acts as expected. Otherwise the circuit oscillates.

I do get an oscillation signal on my ground line, so maybe it's a breadboard issue, though it's not an issue I've had before with the same breadboard setup.

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Re: AS3360 VCA oscillating

Post by guest » Mon Jan 11, 2021 11:20 pm

sounds like its pretty sensitive to that power rail, which is inconvenient, but not a show stopper i suppose.
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Re: AS3360 VCA oscillating

Post by CLee » Mon Jan 11, 2021 11:30 pm

Yeah. I could add an electro to the layout, but now I don't understand the absolute maximum Vcc to Vee of 26V. It's a carryover from the CEM design I see but what are people doing for +/-15v designs. I was looking for a low-parts dual linear VCA to add to a larger design.

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Re: AS3360 VCA oscillating

Post by guest » Mon Jan 11, 2021 11:46 pm

well, you could linearise a 2164, but it will have more passives. an LM13700 might land somewhere between the two in terms of components, but will have more distortion and noise. otherwise its one more part to add a negative voltage regulator.
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Re: AS3360 VCA oscillating

Post by CLee » Mon Jan 11, 2021 11:55 pm

I’ve done linear 2164s before, probably what I’ll end up with.

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