Toroidal transformer in power amp is singing

On occasion, my toroidal transformer sings/buzzes. It varies from almost unnoticeable to where I can hear it 30 feet away, but it is not coming through the system. I’m pretty sure it’s a mechanical issue. Even isolating it from the utility line and using my back up generator only connected to the amp, it still sings. So any interference from harmonics in the power line would not be present in the dedicated power source of my generator. I considered checking the inner area of the windings to see if it’s potted properly, and/or to remove the mounting bolts and put 1/2 “ semi soft foam under the transformer and leave the mounting bolt out. I figure the chassis is acting like a speaker and is probably transmitting 75% of the buzzing I hear. But I can’t easily access the transformer to do these things without a lot of effort. My other thought was to wrap a steel band (gauss bad) wrapped around the outer circumference, which is easier than the first two options. Any thoughts on what else I can do, short of replacing the transformer.

Transformer to what?

Contact PS Audio service 1st. Discuss what’s happening with them. You sound as though you are okay for small diy and they may well
Suggest something. They may also say - send it back, return to point of sale etc.
My BHK Signature 300 monos hummed from new, PS Audio replaced transformers and now one is quiet and one still quietly hums :frowning_face: . I wish both were silent.

Frank- For freaking $15K they should both be Dead silent. I’m sure PSA will make it right. Or should get you new product?

Actually this is a Plinius SA250 Mark III which used one, single massive transformer. The next gen Mark IV used four smaller transformers. Probably because of the issue I have. And yes, the amp was 15k in 1995. Should run silent

No deal. PS Audio say that’s the best they can do.

All transformers vibrate, the large the transformer the greater the potential and louder it is when it happens. The older they get the more likely they are to develop a hum. If it is a potted transformer, and you cannot find a replacement you might be able to find some one to re-pot it. The foam will probably melt or only work a short time, once it dries and then gets loosened by heat or vibration all you have is a mess and potential fire risk.

Have you tried calling the manufacturer? They look to still be in business.
I would say, if you have had the amp for 28 years, you had a good run, might want to look at a replacement.

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I cannot handle transformer hum. Happens more with E-core than with toroids. Krell ties down the toroid with big rubber washers on top and bottom and a big screw to hold it in place. Is your toroidal tied down well? Perhaps the center screw just needs to be tightened?

It is so crowded in there with the two large caps on top of the transformer, but I’ll see if I have access from the chassis to the center bolt.

If there is any way to check those smoothing caps, perhaps worth doing so. 28 years is over their life expectancy and they may not be up to the task anymore. They usually go open and/or ESR increases with age, but who knows, they may be straining the transformer if they start leaking DC. You’d need ESR and Capacitance meter. Blue are for the output stage and smaller black ones appear to be for the preamp stage. (Make sure they are discharged before measuring or dismantling if you decide to go that route)

Thanks for the comment about the caps. I had the amp recapped 6 months ago, but did not replace the two big main capacitors. The Plinius MKIII was the only amp to ever use two monstrous computer grade capacitors. They tested fine and unlike all of the other parts in the amp, I am not sure I can buy better caps today for that position in the circuit. Also, two of them are around $400.

There seems to be a lot of room there that can accomodate taller caps if needed. Pre stage is usually very sensitive sonically to power supply changes, but output stage is less so, so any right cap or multiples of them can go in there. It is not uncommon to have tens of smaller caps than 1 big, it offers lower ESR, lower noise floor, ripple handling, as a result can have positive sonic effect, but it would no longer be original. I’m sure though some who recapped the amp checked original ones, they are most likely fine.