Strange noise from amp

[Hakka]

I have a Rotel RB1070 poweramp for my B&W603 mains. When I power down the system there is a strage whistling noise that decreases in pitch, it starts about 5 second after the amp is switched off and lasts for 3 or 4 seconds. I'm not sure if the noise comes from inside the amp case or if it comes out of the right channel speaker, as the two are side by side, I think it comes from inside the case though.

I've had the amp for about 12 months, I think the noise was always there, I just never got around to working out where it came from.

Sound quality of the amp is fine.

Any ideas on what this could be?

I'll see if I can record the noise and attatch it, its a bit hard to describe what I'm hearing.

Harry.

[F1 fan]

I suspect that what you are hearing is the discharging of the filter caps in the power supply.Amplifiers have large capacitors that can hold their charge for several seconds after the power has been turned off.This can produce some strange sounds as they slowly discharge but it should be no cause for concern.

[Hakka]

Last week I added some fans to my rack to cool my Rotel amps, since I added the fans I do not get the noise after power down so it seems it was heat related. The amps aren't ventilated very well at the top and they used to run quite hot, since adding the fans they run very cool.

Harry.

[Otto]

Wow. When I started reading the OP, I was thinking cap discharge, and that the noise was coming from the speaker itself (actually, probably both speakers). I'm a little surprised it's related to temperature. Well, not that surprised, as temperature can do a lot of weird things. I'd just expect that noise to be there either way.

Sounds like the work you put into the fan solution is paying off.

[Hakka]

Otto wrote:

Wow. When I started reading the OP, I was thinking cap discharge, and that the noise was coming from the speaker itself (actually, probably both speakers).

Yeah, everyone I spoke to suggested cap discharge.

The noise was coming from the right main speaker only, I finally remembered to put my head in front of the speaker before I turned the system off

Thre has been a significant change in operating temp since adding the fans, when I put my hand on the top of the amp it was very hot, not to hot too touch but not that far off. Now they run very close to room temp, they don't feel warm at all.

I've swithched on/off at least 10 times now with no noise.

So can caps be affected by heat?

Harry.

[sur5er]

If the amp is on a shelf, try adding some 1/2" vent holes directly below the amp cool air intakes. You should be able to move a lot more heat. And add life to the components.

[drf]

yes, heat can and will effect all electronic components. It will affect some before others but generally speaking nothing is immune from heat.

[F1 fan]

Hakka wrote:

So can caps be affected by heat?

Yes, the service life of electrolyic type caps like the ones used in power supplys is affected by heat.
Better caps have ratings of several thousand hours at 105 degrees C whereas cheaper ones are only rated for a thousand hours at 85 degrees C. They will last longer with reduced operating temps.

[basementjack]

I just added some fans to my setup - cost was minimal - about $50.

I bought a 2 amp DC power supply with voltage settings of 1.5/3/6/9/12 volts.
I bought a few 'computer fans'
and bought some connectors so the fans could plug into the power supply.

I started with 2 fans clipped to the back shelving to vent air out of the cabinet, but that didn't seem to help much as the amps were still hot.

So it seems that the way to go is a small fan over the amp.

By the way, if anyone is considering doing this - 12v DC fans are the way to go - I originally bought a 120V fan but returned it becuase my system had a 60hz hum when the fan was nearby.

another comment - 12v fans generally not be powered from the 12v trigger - each fan draws about a quarter amp of current, and the 12v trigger doesn't usually have that kind of current.

in my setup, I've got one of those monster power center things - it has a 12v trigger input, and a handfull of switched outlets - so my 12v supply is plugged into that for automatic operation.