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[Mr.Michaelz]

Hello,

I noticed that my monitors emit a fluctuating hum when turned on - even with no audio cable connected to them, and isolated on their own wall socket.
It sounds like it is coming from the power supply (not from the drivers).
Does anyone have pointers to troubleshoot this?

Thanks!
Matthieu

[James Perrett]

The make and model number might help. Do you know if they have a switch mode or linear power supply? If it is a linear power supply it may well be down to loose laminations in the transformer. Switch mode power supplies also use transformers and coils which can be noisy but the noise is usually more of a buzz/hiss than a hum.

However, I recently had a similar issue with a Digitech delay line which went away completely when the cover was removed. This implies that the stray field from the transformer was causing the cover to vibrate. Digitech must have known about the problem because there was a foam pad between the transformer and cover but the foam pad wasn't working effectively. I still haven't worked out the best way to fix it short of replacing the transformer with a toroid.

[Mr.Michaelz]

Thank you very much for your answer James.
The monitors are ATC SCM25A's. I don't know which type of power supplied is used in them.
I will open them to check if there is anything loose around the transformer and if it is tightly attached.

My understanding is that if it indeed is a loose laminations problem, there is no way to deal with it except replacing the transformer. Do I understand correctly?

[Hugh Robjohns]

Pretty sure it's a linear supply in those -- ATC are quite traditional in their electronics. I think James is probably right that it's a transformer laminations rattle problem.

Have you contacted ATC directly? Ben Lilly is the guy to talk to and is very good with his technical support -- and would be the first to appreciate that an acoustic hum degrades the performance of ATC's monitor speakers.

It might not require a transformer replacement -- it is sometimes possible to tighten the transformer frame, or jiggle the assembly slightly to remove or damp the sounding-board effect.

H

[Mr.Michaelz]

Thank you for your answer Hugh.

Yes I have been in contact with Peter at ATC. He recommended that I try to tighten the transformer. I will open the speakers anyway to replace the faulty power switches, so it will be a good time to check if everything is tight.

[Mr.Michaelz]

Just to close this topic: I replace the power switches on the SCM 25a's.
It's very easy to do: only the ground wire needs to be soldered. Just a touch of glue gun and they were back in business.

The transformer is open style toroidal, so there was no frame to re-tighten. I tried to re-tighten it to the back panel, but it did not really change anything. It's not that bothering though, as I do not really notice it when working.

I would like to stress that the people at ATC have been really charming throughout, and it has been a real pleasure to deal with them. It really feels like they take pride in their products and in providing good customer support, which is becoming quite a rarity these days in my experience...