Loud humming coming from one BHK 300 mono at start-up

Check out this video and let me know if anyone has ever heard this humming sound before when powering up their BHK amps. The noise slowly fades until totally gone after about 2 minutes, and everything sounds fine afterwards. Only happening on right side amp, and just started after about 6 months of ownership.

Unplugging the XLR in back of the amp does nothing to stop the sound when happening, and it is coming directly from the amp, not the speaker.

Thoughts or experiences with this?


Does this occur after the “blinking” delay happens or after the blue logo is steady? Thinking that some thing is wrong with the relay switch

Happens right after the blinking has stopped, and the amp clicks on to full power (solid blue). It slowly gets quieter and quieter, until the sound totally dissipates after about 90 seconds.

Did you try switching out the tubes between the units and see if the noise would show up in the other amp?

My first thought is mechanical transformer buzz. If that’s it, it would explain why simply disconnecting your XLR changes nothing. Because it is mechanical, not electrical. I also note it goes away in a couple of minutes, which is likely to be the time constant of the power supply capacitance (lots of filter capacitance in a big amp like the 300). During that brief period the amp is likely drawing surge current and that’s when a transformer’s lamination buzz will be loudest. Don’t go into a panic, though. It could simply be the transformer mount bolt (or bolts) has become loose, letting it rattle against the chassis. The 300 has two (I think) and the mount bolts might even be accessible on the bottom of your amp chassis. My two cents worth and could be completely wrong. But if I’m right it’s trivial to fix the problem. Do NOT get inside unit or check this without unplugging the amp and giving it plenty of time for internal power supply discharge!


Step one is typically, switch tubes left to right amp and see if the noise goes with the switch.

Agree, sounds like a tube that is noisy on initial warm-up. May give up the ghost soon or last for a long time. Swap them from one side to the other.

Just as a reminder to all tube swappers:

As far as I’m told, tubes are not sensible at all in cold condition, but extremely sensible to vibration and shock when warm. That’s why I never pull out tubes or move tube equipment in warm condition, not even the NOS military grade ones. I guess for similar reason and effect to sq, my pre even has suspended input tube boards.

So take care you don’t damage them while trying to fix things.

That would certainly be my guess. Mechanical noises from transformers are common under load and one might be a little noisier than the other. It’s near impossible to get them mechanically identical.

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Swapped tubes between the two amps…

New video posted. Noise now comes on briefly when blue light blinking, then goes quiet within a few seconds. Once the amp kicks on (blue light solid), noisy as hell for about 3 minutes, then quiets down to silent, and performs perfectly (it did finally stop buzzing about 60 seconds after that video stopped).

BHK v2

What are my next steps? Not looking forward to shipping an 80 lb amp back to PS Audio.

Swapping tubes didn’t solve the problem. Which is what would be expected if it were transformer rattle. Note Paul agrees with my best guess as well. I’d recommend calling PSA customer service, but again if it is simply a loose transformer mount you may not need to ship it at all. You might be able to solve it yourself rather easily, in consultation with the good folks at PSA. Everything you’ve posted, btw, suggests the amp is electrically sound.

Will contact support. Thanks all for the input!


It sure sounds like a mechanical buzzing to my ear. Probably best to consult with PSA Technical support.

PS Audio should know the cause. A couple of months ago I saw similar issues having to do with mounting plates, transformer and grounding in both BHK 300 and BHK 250 on this forum.

I recommend for your safety sake, do not turn it on anymore and disconnect it from everything including power, it certainly does not sound safe.

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Mine as well.

I have had this happen on a few pieces of equipment. The cause each time was a transformer mounting bolt which had come a little loose over years of use. Gently tightening the bolt resolved the issue.

Transformers naturally vibrate a bit. It is in their nature to sing along.

P.S. The issue to which @Rudolf_Appel refers impacted only a few amps of very early production and was resolved long ago. Subsequently manufactured amps do not have the issue. I suggest contacting P.S. Audio describing the problem and providing the serial number. If you would like, you can email Paul directly at Paul@psaudio.com.

P.P.S. Only the first two months of production were potentially impacted. Any amps built in August 2015 forward are not affected.

If your amp is six months old you are fine. But to read your serial number, here is an example - serial number 5G0032. The first character is the year, 2015. The G corresponds to the month, in alphabetical order. G is the 7th month (counting from A), and then the last two are the unit ID. So, this serial is number 32, built in July of 2015.

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This makes sense. The amp in question has been making expansion/contraction pops every time it warms up from standby. Usually just one pop about 10-15 minutes after on, and also when cooling down back to standby temp. I am wondering now if this expansion/contraction has indeed worked something loose, and is responsible for the buzzing vibration sound.

Now the sound wont even go away…this seems to be getting worse and worse. Time to unplug and reach out for help.

Please do contact our service department. It’s likely something simple.

Will do, thanks Paul.

Well…BHK 300 is in the possession of PS Audio. Very interested to see what the issue is.

Hopefully I will have repaired or replaced monoblock by the end of next week.