Stupid Q. Gain is affecting the gains of another circuit.



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Stupid Q. Gain is affecting the gains of another circuit.

Postby DRodriguez » Wed Jul 19, 2017 2:53 pm

Fair warning, I have no useful knowledge in electronics besides practical applications learned in practice.

I have 3 circuits in one enclosure. Drive, filter, and clean boost. All on different boards. When I turn on the clean boost, it's inversely affected by the drive circuits gain. Aka, the more gain I add, the less gain the clean boost puts out.

I assume this has to do with ground issues. It's there a way to isolate the ground between the different circuits? Or some other super dumb thing I could be doing. All circuits test fine on their own.
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Re: Stupid Q. Gain is affecting the gains of another circuit

Postby crochambeau » Wed Jul 19, 2017 3:12 pm

How is power distributed between the boards? Is there a decent amount of decoupling?

What sort of topology are you talking about here? Operational amplifiers have a somewhat different rule set regarding feedback paths/etc than do discretes.
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Re: Stupid Q. Gain is affecting the gains of another circuit

Postby goroth » Thu Jul 20, 2017 3:38 am

My first guess would be an impedance problem. Depending on how the first gain stage of the drive is, it could be changing the input impedance that the boost circuit sees. To test this theory just put a unity buffer between the gain and boost. Make it a klon buffer for increased lettuce.
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Re: Stupid Q. Gain is affecting the gains of another circuit

Postby controlFreak » Mon Jul 24, 2017 1:50 pm

Check your wiring on the gain pots.

Are the Gain controls in feedback loops of opamps? Or to ground like a volume control?
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Re: Stupid Q. Gain is affecting the gains of another circuit

Postby DRodriguez » Mon Jul 24, 2017 2:06 pm

Thanks for the suggestions, I now know what paths to chase down and investigate. Will update when I figure it out.

controlFreak wrote:Check your wiring on the gain pots.

Are the Gain controls in feedback loops of opamps? Or to ground like a volume control?

The gain pot does both, haha. Dual gang pot one controlling a feedback loop at the last transitor, One at the input doing a volume control.
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Re: Stupid Q. Gain is affecting the gains of another circuit

Postby crochambeau » Mon Jul 24, 2017 2:24 pm

How are these separate effects joined together in the enclosure?

I'm talking signal path here.

AND

DRodriguez wrote:The gain pot does both, haha. Dual gang pot one controlling a feedback loop at the last transitor, One at the input doing a volume control.


You said transistor, not opamp. Can you point to a drawing of this implementation?
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Re: Stupid Q. Gain is affecting the gains of another circuit

Postby DRodriguez » Mon Jul 24, 2017 2:58 pm

If I remember correctly, it was a EHX LPB into the dirty drive (modified interstellar overdrive) (no real decoupling in that circuit, I'm guessing that's the issue.) into a muff tone control.
It's a transistor based OD (5089's) no OP-Amps

I'll double check what I did when I get a chance to go to my studio and give the proper details. But no op amps in the dirty drive, just the three 5089s.

All circuits tested fine on their own. Signal path was wired like it was 3 separate pedals with one master bypass switch and one to engage the LPB pre OD. All connected to the same 9v jack and ground.

Input > LPB > OD> Filter > Output
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Re: Stupid Q. Gain is affecting the gains of another circuit

Postby crochambeau » Mon Jul 24, 2017 3:05 pm

Yeah, I'd like to see your gain control drawn out. With gain I prefer to control AC (cap in line) bypass of the emitter resistor, doing a feedback implementation may be fraught with challenges.
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