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Fluctuating high-pitch noise in op-amp circuit
Fluctuating high-pitch noise in op-amp circuit
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Old 20th November 2019, 12:57 PM   #11
Douglas Blake is offline Douglas Blake  Canada
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Quote:
Originally Posted by Rolox View Post
All I have at hand is those 0,1uF MLCC caps, could I use those on the input RCA?
No. The value is high enough to cut into the treble range of the audio.

You're going to want to keep those values under 100pf since all you want to do is shunt off any RF that might find it's way into your interconnects. Small ceramics are not expensive, probably about 10 cents each and they're common as rice. Almost any electronic repair shop should be able to sell you a few.
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Old 20th November 2019, 01:01 PM   #12
Rolox is offline Rolox
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Okay so I've changed my decoupling scheme.
Now I've put 0,1uF MLCC from each power pin to ground - instead of between V- and V+.

But now they are paralleled with what was originally in the circuit: some 22uF (those will stay or be replaced with 100uF), but also some 0,1 (apparently) plastic (?) caps that are between each supply and ground but just a few mm farther.

I wonder if it's ok to leave those plastic 0,1uF there or if I should take them out now that I put the 0,1 MLCC closer to the power pins?
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Old 20th November 2019, 05:06 PM   #13
ubergeeknz is offline ubergeeknz  New Zealand
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I doubt they will do any harm.
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Old 20th November 2019, 05:10 PM   #14
Rolox is offline Rolox
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In any case nothing is doing anything; I still get this fluctuating high pitched noise coming and going. Noise floor between those peaks is quite hearable as well - but I could live with that.
Probably this cheap crossover is just badly conceived and there's nothing that will fix it... without redesigning everything wich is above my pay grade.
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Old 20th November 2019, 05:24 PM   #15
ubergeeknz is offline ubergeeknz  New Zealand
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Have you added the RF filtering ceramic caps on the inputs yet? Worth a shot
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Old 20th November 2019, 05:52 PM   #16
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by Rolox View Post
Regarding the "metallic edge", maybe it would have been better to decouple from each supply pin to ground, instead of between supply pins?

Maybe, maybe not - that can inject distortion currents into a local ground trace. HF Decoupling is between supply rails to stop oscillation modes involving unintended internal feedback via the power rails. The internals of the chip are ignorant of ground potential anyway.
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Old 20th November 2019, 08:23 PM   #17
indianajo is offline indianajo  United States
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Put 33 pf disk caps parallel the op amp feedback resistors. Pins 1 & 2 and 7 & 6 usually.
I had to do that to my 33078 when I replaced the 4558 the mixer came with. In addition to a .1 uf between supply rails for every 2 packages.
When you replace a 6 v per microsec slew rate amp with a 20 v per microsec amp, make sure the high freq gain is killed by shorting it out with the feedback cap.
I couldn't hear the oscillation, but my cs800s amp fan was running on high at 1 w/ch. Had a working scope then for 2 weeks, saw ~ 1 mhz.
BTW the 5532 is a pretty quiet part. Just a bit of a hog on power supply current. There weren't any leaded DIP packages in stock the day I was buying "low noise op amp" from newark. Paid all of $.38 ea for ST33078. NJM2068 is another forgotten low noise low cost part. If you need low noise + high drive current for long lines or high capacitance filters, Peavey used NJM4580 in the 90's.
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Last edited by indianajo; 20th November 2019 at 08:28 PM.
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Old 20th November 2019, 10:14 PM   #18
Rolox is offline Rolox
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Quote:
Originally Posted by ubergeeknz View Post
Have you added the RF filtering ceramic caps on the inputs yet? Worth a shot
Well I did try with what I have at hand - the 0,1 mlcc - it did nothing for the noise problem

I’m gonna take them out since value is too big anyways - I wanted to see if it would cure my noise problem

It’s very frustrating and never experienced something similar... maybe something is broken? It’s a really strange noise. It’s just like something frying then it modulates higher then lower then louder then it’s just the fried noise again. I tried disconnecting nearby appliances (WiFi, tv box,...) but it stays the same. I put ferrite beads, I put a filtered IEC cable... noise is always there. Enclosure open or closed won’t change a thing. It must come from within the circuit. But how can a simple filter circuit with 5 op-amps be so difficult to deal with?..

I’m about to give up but it’s very frustrating as, if that noise wouldn’t be there, it would be more musical than my TDM crossover.

Last edited by Rolox; 20th November 2019 at 10:17 PM.
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Old 20th November 2019, 11:07 PM   #19
29285 is offline 29285  Europe
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Could you post a sound record file of your noise here?
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Old 20th November 2019, 11:19 PM   #20
Rolox is offline Rolox
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Quote:
Originally Posted by voltwide View Post
Could you post a sound record file of your noise here?
Yeah Iím gonna try to do that tomorrow. I never did it before but I suppose itís not more difficult than uploading an image?...

I guess hearing it might help understand where it could come from.
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