Hi everyone, first post since the forum is back, woo-hoo!

On a more serious note, I have a technical question that I was hoping someone could answer. At work we have an installation with 4 buttkickers fixed to a floor for a shaking effect. We're currently doing a refurbish of the spaces and there's new audio scheduled to go through them so me and some colleagues decided to test them as they have been there for a long time (and they're too expensive to just pull out for this).

There's two identical BKA10000-N poweramps that drive two buttkickers each, wired in series. One of them works perfectly, but the other is making a weird high pitched noise in the rack. When I turn it on it won't work until I raise the volume past a certain point (I think this might be because it used to be in the same position all the time for years, not sure if related to the noise). Then it starts to make a weird high-pitched noise that changes according to the signal I input. So far I've sent some LF sine pulses and pink noise, and I can hear like a lower volume & high pitched version coming from the amp (the buttkickers are on another room two floors above).

My thinking after looking at it is that maybe the electrolytic capacitors on the power supply are causing this, but I don't know that much about electronics so I was wondering if anyone had experienced this before?

Usually it's wound wire elements that actually emit high frequency audible noise, like inductors, transformers, or maybe wirewound resistors, but it's bad filtering (i.e. bad caps) that can cause this increased high-frequency ripple to appear on the power supply rails.

The fact that it changes relative to the input signal is because as the output level changes, the loading on the power supply changes, which causes the ripple on the supply to change, which causes the audible noise to change.

TL;DR: bad caps

Thanks for the reply! It makes sense what you say, there's a toroidal transformer as well so according to what you say that will be the piece that rings and but it's likely cause will be the caps. That makes sense to me.

Thanks again!

The mains toroid might ring, but it's more likely the smaller inductors in the switching power supply, as they probably have a 300+ kHz switching pulse going through them. That gets aliased down into the audible range depending on loading.

Ah, OK. Thanks for the clarification, much appreciated.