Humming amplifier

pdf27 · Post by **pdf27** » Tue Apr 06, 2004 2:07 pm

My amplifier (Cambridge Audio A1) has a fairly noticeable 50Hz hum. It's pretty faint, but is currently significantly louder than my computer. Any ideas what might be causing it and suggestions for how to fix it?

sthayashi · Post by **sthayashi** » Tue Apr 06, 2004 5:56 pm

A ground loop could cause this. Try doing what I did with my amplifier. Run a wire (speaker wire worked for me) between your case and the amplifier chassis.

pdf27 · Post by **pdf27** » Wed Apr 07, 2004 1:03 am

Now you've lost me

al bundy · Post by **al bundy** » Wed Apr 07, 2004 1:35 am

I hate ground loops.

Does the hum disappear when you unplug the wire(s) connecting the amp to the soundcard?

Curtis · Post by **Curtis** » Wed Apr 07, 2004 1:55 am

Ground loops should be fairly easy to fix - simply make sure every device is connected to one and only one power point. If this means chaining 3 powerboards together, do it.

pdf27 · Post by **pdf27** » Wed Apr 07, 2004 2:48 am

al bundy wrote:Does the hum disappear when you unplug the wire(s) connecting the amp to the soundcard?

No. I get a far louder hum of the same frequency when the cable isn't plugged in properly, but the hum I'm trying to fix is the same volume whether or not the computer is turned on.

al bundy · Post by **al bundy** » Wed Apr 07, 2004 3:03 am

pdf27,

Regarding your comment "when the cable isn't plugged in properly":

Are you saying the hum is still there, even when all computer-to-speakers cable(s) are completely disconnected?

pdf27 · Post by **pdf27** » Wed Apr 07, 2004 3:35 am

al bundy wrote:pdf27,
Are you saying the hum is still there, even when all computer-to-speakers cable(s) are completely disconnected?

Yep. To make things completely clear, the hum is coming from the amplifier itself (not the speakers) and is there even if there is no input connection at all to it.

sthayashi · Post by **sthayashi** » Wed Apr 07, 2004 5:11 am

pdf27 wrote:Yep. To make things completely clear, the hum is coming from the amplifier itself (not the speakers) and is there even if there is no input connection at all to it.

Don't buy Cambridge Audio?

A low 50Hz buzz that comes from an amplifier that is ONLY plugged into the wall, means that there is some signal interference coming from the transformer within the amplifier.

I don't think there's any way to fix your problem unless you're willing to a) RMA the amp or b) take it apart and try your hand at fixing something.

pdf27 · Post by **pdf27** » Wed Apr 07, 2004 5:27 am

RMAing it probably isn't on - it's a few years old now, and the hum is pretty faint. I'm perfectly willing to have a bash at fixing it myself, once I know what the problem is and how to deal with it. Best guess is that it might be transformer related (it's that kind of a noise) but as right now I wouldn't know what to do with it even if it was then that "knowledge" isn't a lot of use to me

silvervarg · Post by **silvervarg** » Wed Apr 07, 2004 7:50 am

Since this isn't even computer related SPCR is probably not a good place for this discussion, but I will give some input anyway.

It is probably a problem at the power transformer, so first thing is to find out if it is noise that originates from the transformer itself (usually the coils) or if it from interfearence with other components.

If it seems to be from interfearence you might be able to reduce or eliminate the interfearence with metal isolation or ferrite cores around wires (if it is buildt up from parts connected with wires internally).

If noise seems to be from coils it is hard to fix, and similar to what many people experience in CRT monitors. Perhaps replacing a coil will do the trick. This is often not that easy to do.

Be very carefull when trying to figure out anything of this since you need to have power on to be able to listen to the noise.

Chances are that you end up spending lots of time and resources to fix this, so buying a used amplifier and use as a replacement might turn out to be a better option.

afrost · Post by **afrost** » Wed Apr 07, 2004 8:43 am

that is transformer hum, the only way to improve it is to dampen the transformer....which is a pain. the best way unfortunately is to get a better amp/speakers.

my logitech z-640 amp does the same thing and i solve this by using a seperate power strip with only the logitech sub/amp plugged into this. i turn on the power strip when i am playing a game or music and then the humming doesn't make a difference.

pdf27 · Post by **pdf27** » Wed Apr 07, 2004 9:13 am

silvervarg wrote:Since this isn't even computer related SPCR is probably not a good place for this discussion, but I will give some input anyway.

True, but it's still probably the best place out there to find out. For instance there might be people out there who've had the same problem with PSUs.

I have a suspicion the ultimate fix will be to move the amp further from me, but this isn't possible in the room I'm currently in. However, now my PC actually reaches SPCR standards of "quiet" it is just about noticeable. It's just that it is about the last extraneous noise source left to fix (I can't do a lot about the traffic in the next road, which is slightly louder) so it came to my attention.

Post by **MikeC** » Wed Apr 07, 2004 10:10 am

I'd agree: Most likely hum from the main power transformer. You could try softmounting, but keep in mind that heat is an issue. Soft materials usually are susceptable to heat breakdown over time. You could try several things:

1) set the amp on soft feet
2) place something heavy with a soft base on top of the amp. (Small stone statue? )

pdf27 · Post by **pdf27** » Wed Apr 07, 2004 10:59 am

Looking inside, the transformer is mounted next to (touching) a thin steel plate, and vibrates in operation. I can't figure out exactly what the steel plate is for, as it basically isolates the transformer air circulation from the Amp circuitry. As the unit barely gets warm in operation, I suspect this is surplus to requirements at the volumes I run it at (the unit is 30W RMS anyway).
I'm running it with the top off at the moment to see how hot the transformer gets, but I suspect I will be able to remove the steel steet and soft mount the transformer without too many heat issues.
Come to think of it, I should probably have looked inside first of all. Oh well

Any suggestions for soft mounting material that would likely be able to cope with a fairly heavy transformer attached to it in a stereo that is frequently moved around?

Post by **MikeC** » Wed Apr 07, 2004 11:01 am

sorbothane

sthayashi · Post by **sthayashi** » Wed Apr 07, 2004 11:04 am

pdf27 wrote:I can't figure out exactly what the steel plate is for, as it basically isolates the transformer air circulation from the Amp circuitry.

Though I can't say for sure without actually seeing it, I would think that the steel plate is there to isolate the transformer from the Amp circuitry from an EM/RF noise standpoint.

pdf27 · Post by **pdf27** » Wed Apr 07, 2004 11:55 am

OK, that makes sense. I'll shift the transformer 5mm to the left to isolate it from the plate (or vice-versa).
I can probably steal some sorbothane (or similar) from my lab without too many problems.

Post by **Ralf Hutter** » Wed Apr 07, 2004 11:55 am

FWIW, I've seen a lot of mid-fi power amps with toroidal power transformers that hum. Things like B&K, Adcom etc. This seems to be real typical for this range of product. I've tried everything from mounting the amps on Audioquest Sorbothane feet to placing heavy weights on top of the case (anyone remember "VPI Bricks"?) with little or no success.

pdf27 · Post by **pdf27** » Wed Apr 07, 2004 12:21 pm

Yeah, that's the type of power transformer I've got. Shifting the metal plate that's currently touching the transformer should cut down the volume significantly - it's a thin bit of springy steel that;s vibrating.
I'll try (partially) soft mounting the transformer and let you know how I get on.

fmah · Post by **fmah** » Wed Apr 07, 2004 7:13 pm

One of the other things that could cause the transformer to hum is dirty AC on the AC power line. This might be fixable with a surge suppressor type device that does line filtering, but most likely you'd need something that could smooth out the AC signal, like capacitors. I saw this occur in some lab testing.