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Amplifier started humming

paulraphael

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Not through the speakers ... the box itself started making a ~60 cycle, kind of ragged sounding hum. Loud enough to be audible over quiet music. I unplugged it and started it back up again, and the hum immediately came back ... and then vanished.

Background: it's a 20-year-old Unison Research Unico integrated amp, with a tube preamp (pair of 12au7s) and mosfet power section. It's been in storage for 9 years, and a month ago I set it up again after we moved into a house that has room for this stereo system. I've never had any trouble with it. My plan was to sell it, and replace with something more modern and sensible (and solid state), but I remembered that I really like the thing. It's just so pretty, and feels so good, and has this cool remote that's carved out of wood and that works through walls and has only 2 buttons: louder, quieter. So I'd be bummed if the thing died.

What could cause a whole amp to spontaneously hum, and then spontaneously stop? Could it be electrical interference in the mains? A capacitor starting to give up the ghost? A tube problem?
 

sam_adams

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DC on the AC power lines or—most likely—delaminating power supply transformer. Long Island is not the driest place on the planet, so a device in storage might not fare well after nine years.
 
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paulraphael

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DC on the AC power lines or—most likely—delaminating power supply transformer. Long Island is not the driest place on the planet, so a device in storage might not fare well after nine years.
The thing has a giant toroidal transformer, made by the amp manufacturer. Would this be the power supply transformer? Seems unlikely I'd be able to replace this.

What are likely sources of DC on the line?
 

Doodski

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If you take it to another house and no hum exists then it's possibly DC on your mains power. Otherwise if you own a meter or a oscilloscope you can check out your AC mains power yourself. If it is not the AC mains power then the toroidal is probably a bit loose and you need to tighten the mounting bolt(s).

Is this it?>
 
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paulraphael

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If you take it to another house and no hum exists then it's possibly DC on your mains power. Otherwise if you own a meter or a oscilloscope you can check out your AC mains power yourself. If it is not the AC mains power then the toroidal is probably a bit loose and you need to tighten the mounting bolt(s).

Is this it?>

Yes, that's the one. The review shows the guts. I'll definitely reach in and see if the transformer needs tightening.
 

Zek

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... and look at the condition of the power electrolytic capacitors in the power supply, after so many years of non-use they may have dried out or they need some time to form properly again.
 

DVDdoug

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My amplifier used to hum & vibrate when the TV was on. From what I remember it was more vibration than audible hum.

I knew it was some kind of "interference" on the AC line but at that time I didn't know DC could cause this. Nothing "bad" ever happened and now that I have a different TV the problem is gone.
 

Bob from Florida

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Not through the speakers ... the box itself started making a ~60 cycle, kind of ragged sounding hum. Loud enough to be audible over quiet music. I unplugged it and started it back up again, and the hum immediately came back ... and then vanished.

Background: it's a 20-year-old Unison Research Unico integrated amp, with a tube preamp (pair of 12au7s) and mosfet power section. It's been in storage for 9 years, and a month ago I set it up again after we moved into a house that has room for this stereo system. I've never had any trouble with it. My plan was to sell it, and replace with something more modern and sensible (and solid state), but I remembered that I really like the thing. It's just so pretty, and feels so good, and has this cool remote that's carved out of wood and that works through walls and has only 2 buttons: louder, quieter. So I'd be bummed if the thing died.

What could cause a whole amp to spontaneously hum, and then spontaneously stop? Could it be electrical interference in the mains? A capacitor starting to give up the ghost? A tube problem?
The tube preamp section has a high voltage power supply and your MOSFET section has it's own power supply. It was in storage for 9 years. Normally, you would have used a variac to bring the power supply up gradually. Given that it hummed mechanically then quit that behavior the second power up I'd say you came close to blowing a cap. You may have lucked out and be okay. Still it would not hurt to limit the voltage with a variac for awhile just to be sure.
 

TrevC

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The tube preamp section has a high voltage power supply and your MOSFET section has it's own power supply. It was in storage for 9 years. Normally, you would have used a variac to bring the power supply up gradually. Given that it hummed mechanically then quit that behavior the second power up I'd say you came close to blowing a cap. You may have lucked out and be okay. Still it would not hurt to limit the voltage with a variac for awhile just to be sure.
Drying out caps would cause hum through the speakers. They wouldn't cause the transformer to hum.
 
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paulraphael

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The tube preamp section has a high voltage power supply and your MOSFET section has it's own power supply. It was in storage for 9 years. Normally, you would have used a variac to bring the power supply up gradually. Given that it hummed mechanically then quit that behavior the second power up I'd say you came close to blowing a cap. You may have lucked out and be okay. Still it would not hurt to limit the voltage with a variac for awhile just to be sure.
I brought it out of storage 2 months ago and have been using it since then. The humming just happened for the first time (that I noticed) today.
 

sq225917

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Dc, delamination or loose bolt will cause traffo hum. Similarly being over voltage would do it.
 

Count Arthur

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The problem:

The cure:
 

Bob from Florida

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Drying out caps would cause hum through the speakers. They wouldn't cause the transformer to hum.
He did not say anything about hum through the speakers, just the box. Transformer laminations can vibrate under heavy current loads. This often happens with large electrolytic's directly connected to the rectifier. On initial power up the transformer vibrates a lot at first and tapers off as the cap charges and the ripple goes down. It is now clarified that it was in use for a couple of months then this suddenly occurred. At least 2 possibilities - cap and or rectifier failure brewing and the already mentioned DC on incoming power. Output section may be going as well. If it were me I would pull the cover - after unplugging and waiting 20 minutes - and inspect the inside for bulging caps, burned components, etc. You could wait for the problem to manifest itself in a more spectacular manner - think "let the smoke out".
 

TrevC

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He did not say anything about hum through the speakers, just the box. Transformer laminations can vibrate under heavy current loads. This often happens with large electrolytic's directly connected to the rectifier. On initial power up the transformer vibrates a lot at first and tapers off as the cap charges and the ripple goes down. It is now clarified that it was in use for a couple of months then this suddenly occurred. At least 2 possibilities - cap and or rectifier failure brewing and the already mentioned DC on incoming power. Output section may be going as well. If it were me I would pull the cover - after unplugging and waiting 20 minutes - and inspect the inside for bulging caps, burned components, etc. You could wait for the problem to manifest itself in a more spectacular manner - think "let the smoke out".
Stop digging. LOL.
 

solderdude

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Could be the transformer itself (replacement or potting could help)
Could be the chassis it is mounted in (tightening could help)
Could be DC on mains (a DC blocker could help or an AC 'restorer' could help)
If the latter the hum may come and go. Take it to someone else's home (not neighbours, a few miles away at least) and plug it in the socket there and listen if it audibly hums.
There will always be some hum.
 

Bob from Florida

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Could be the transformer itself (replacement or potting could help)
Could be the chassis it is mounted in (tightening could help)
Could be DC on mains (a DC blocker could help or an AC 'restorer' could help)
If the latter the hum may come and go. Take it to someone else's home (not neighbours, a few miles away at least) and plug it in the socket there and listen if it audibly hums.
There will always be some hum.
On the other hand if the amp knew the words it would not be humming. :cool:
 
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