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Noise I can't explain
Noise I can't explain
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Old 21st October 2016, 03:32 PM   #1
borges is offline borges  Norway
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Default Noise I can't explain

Hi guys, could you please help me find the source of the noise my amp is giving off?

The amp is an Adyton Opera, 60W Class-A MOS from the early 90's. I have replaced all electrolytic caps and the volume pot. The amp is full of corroded spade connectors. I have fixed most but probably not managed to do all.

The noise is in one channel. It is independant of channel selection or volume setting. It is only there when the amp is cold. When it is warm things sound good.

The noise is a low-frequency hiss, not a continuous wide-band sound. It comes and goes. Kind of like standing by the road and hear cars go by, only lower in frequency and more random.

Do you have any idea what can make this happen in a MOS amp? It is probably not the (shared) rectifier, although one of my mods was adding snubbers to it. It could be the caps. I have changed all lyts. The amp has a lot of potmeters. I have not touched them.

Could the heat dependant nature of the noise have anything to do with a bad solder joint or broken PCB track?

With a few more ideas I can start chasing the source of the noise. Unfortunately, I don't have schematics for this amp.


Thanks for any feedback,
BÝrge
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Old 21st October 2016, 04:35 PM   #2
Enzo is offline Enzo  United States
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Get a can of freeze spray (circuit chiller), to chill parts of the circuit. See if one area starts the noise back up. If freeze spray is not available to you, those cans of compressed air they sell to blow dirt off your computer keyboard can work.
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Old 21st October 2016, 07:27 PM   #3
indianajo is offline indianajo  United States
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Any resistor or transistor can do this due to internal connection problems. It can also be a solder joint, or rarely, any capacitor or other component.
You can trace through the amp with a sound probe, or a scope, or an analog (not digital) VOm with like a 20 vac or 2 vac scale. You need to protect an analog VOM from DC with a .047 uf cap on the negative lead.
Short the input. Look through the amp where the sound starts, and where it isn't. If you don't have a schematic, the beginning is little parts, and the end is the big parts. More likely this sound is near the beginning, follower parts don't have that much gain on them.
Circuit cooler is cool but I don't keep it around. More often I have sound stuck somewhere because of a bad solder joint or a bad pot wiper or oxidized connector. This technique can find all three problems.
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Old 21st October 2016, 07:31 PM   #4
morinix is offline morinix  United States
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An amp that old, I would be highly suspect an electrolytic cap is dying someplace.
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Old 21st October 2016, 09:08 PM   #5
Chris Daly is offline Chris Daly  Australia
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You could try requesting a schematic or more generally contacting Adyton info@adytonaudio.no
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Old 22nd October 2016, 01:13 PM   #6
borges is offline borges  Norway
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Thanks a lot for your advice guys!

I was hoping it wasn't the caps since they are all new. My personal assumption is oxidized connector or bad wiper. That's what I'm hoping for.

The Adyton Opera is an intricate 3D build, and scoping through it will not be easy! But I'll give it my best.

Cheers,
BÝrge
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