Testing an amp with dummy load, can hear music coming from inside the amp.

[ElectroAcoustic]

I was testing a Crown XTi 4000 with a 4 ohm dummy load at near full power with music. I could clearly hear music coming from the amp internally.
This amp has a class AB+B output stage, with a SMPS power supply.
I'm assuming I'm hearing the transformer in the SMPS, but I'm curious if that's the case.
Is there any harm in long term testing this way, up to an hour, maybe?
Do the amps exhibit this behavior also when connected to speakers versus load resistors?

 

[jwilhelm]

Transistors actually make noise. So do inductors. I'm around a 5100 Fincor motor drive all the time. You can hear the transistors start to sing with the softstart and the frequency pick up to 60Hz. The sound is definitely coming from the TO-3s.

 

[Fast Eddie D]

Tube circuits used to really display this phenomenon. I have noticed it many times from many different types of amplifiers. Audio transformers can make a lot of noise when driven hard.

It occurred to me that mitigating whatever causes this phenomenon might reduce distortion. McIntosh used to (still do?) make transformers in house, and they were potted in a big case with lots of potting goop. They were very quiet compared to most other transformers, I recall.

 

[Nigel Goodwin]

McIntosh used to (still do?) make transformers in house, and they were potted in a big case with lots of potting goop.

I seem to remember that many of the better makes of valve amplifiers (Leak, Quad etc.) tended to use potted transformers - I can't say I'd ever thought about why it might be a good idea

 

[AndrewT]

capacitors vibrate/move in response to voltage changes across their plates.

That vibration comes out as "singing", especially at higher frequencies.

 

[djduck]

I've heard this with a TDA2822 chip. Quite odd.

 

[akis]

I used to remember TO-3s singing like this, 2N3055 and 2N3772 from memory, it's been like 35 years ago... If you mount the heatsink onto a proper wooden base you have just made a soundboard.

 

[Mr Evil]

Magnetostriction causes transformers and other magnetic components to emit sound. In the case of capacitors it's Electrostriction. What would make a transistor emit sound?

It can be pretty hard to pin down the source of problems like this. A simple way to narrow it down is to use a tube such as a toilet roll - hold one end to your ear, and move the other end around over the amp.

 

[dmbox]

I was at Altec-Lansing here in OKC before Mark IV industries sold it off. The burn in rack was a wonderful place to visit during burn-in. (laughing)

No, you will not hurt it, as long as you keep it under its clipping limit and it doesn't go into thermal run-away, and yes they will sing back to you...

 

[tomchr]

I build Douglas Self's Blameless amp in the 1990ies. When testing it, I recall it singing as well.

~Tom

 

[ElectroAcoustic]

I've heard transformers make noise before, but nothing quite like this.
My shop was extremely quiet and I had music going through the amp, and at a meter away I could clearly hear the vocals and make out the instruments. It sounded like a speaker with a high pass at about 3kHz was inside the amp!
I figured it probably wasn't too bad, but I've been working on amps with linear power supplies for so long that the nuances of smps' are still a bit new to me.

 

[SY]

Electrostriction. What would make a transistor emit sound?

Electrostriction- transistors are tightly coupled to a very efficient sounding-board, aka "heatsink."