Hum help

[Dan Turner]

Hi - I'm hoping for some suggestions as to what might be causing the hum that my system seems to have developed (we're talking sound not smell here), and how I can get rid of it!

I recently purchased an Arcam P38 power amp to replace my A32 which was being used in power amp mode along-side a P35. Due to space constraints the P38 and P35 are stacked on one shelf of my rack. Initially whilst it was running it in I had the P38 serving the bass, and the P35 the treble - at this time the P35 was physically beneath and the P38 on top. Now that it's had 60 or so hours on it I have swapped them round so that the P38 is serving the treble and the P35 the bass, just to see whether there was a difference. Due to various interconnect length factors and wanting to get the signal going first into the amp serving the treble I also changed them round so that now the P38 is on the bottom and P35 on the top.

When I switched them back on immediately the P35 started emmitting a low hum which it definitely wasn't before, worst of all it is loud enough to be audible during quiet passages of music. I'm sure all my connections are correct, nothing is touching anything it shouldn't, everything seems to be in securely etc. Does any one have any idea what might be causing it or what can be done. I don't really want to change things back because I actually think it does sound a little better with the P38 on treble duties.

Any advice appreciated!

 

[matengawhat]

these things will drive you mad - i would say strip it all out and start again - could be caused by the two amps sitting on top of each other and now swapped maybe one more sensative although i ran to p35s for while that were stacked without issue. could be mains cable to close or something or like dust/dirt on an interconnect - unplugging and starting again usually helps

 

[John Duncan]

The stacking sounds like the problem. Can you take it all out of the rack and have them sitting apart to see if that cures it? If so, you're going to need to rearrange them, even if it means new interconnects...

 

[Dan Turner]

OK - bizzarre and infuriating development, i switched them off and back on again (i had already done this before I wrote the post and it didn't stop the hum) and this time that has stopped it. Great. Listened to some tunes, sounded lovely, happy days.

Then I switched the TV on (TV, HC amp and sky HD), the Arcam power amps were still on but obviously not in use now and this awful buzzing started through the speakers (i.e. the ones connected to the Arcams), which changed in pitch seemingly according to the changing picture....Suffice to say I switched of the amps pretty damn quick!

I think I will take the advice to first of all disconnect and reconnect everything and see if that does it....

I can change them back so that the P38 is on top again, but moving them apart is really impractical from an unplanned cost of new interconnects and aesthetic point of view.

 

[Andrew Everard]

Can just be resonances set up in transformers, which disappear when powered off and powered on again.

Is it a through the speakers hum or a mechanical hum? Might be worth trying tightening up the transformer mounts - there's often a big bolt head on the chassis where the tranny is bolted down.

 

[John Duncan]

Andrew Everard:there's often a big bolt head on the chassis where the tranny is bolted down.

I think I saw that musical once.

 

[laserman16]

JohnDuncan:Andrew Everard:there's often a big bolt head on the chassis where the tranny is bolted down. I think I saw that musical once.

That don't sound like something you could take the family to.

 

[a91gti]

On the subject of humming trannies (!), I had a similar problem with a Quad 303.
I found that giving it a firm slap would stop it humming.

This is starting to read like a "Carry on" script.

 

[Dan Turner]

Andrew Everard:

Can just be resonances set up in transformers, which disappear when powered off and powered on again.

Is it a through the speakers hum or a mechanical hum? Might be worth trying tightening up the transformer mounts - there's often a big bolt head on the chassis where the tranny is bolted down.

Thanks Andrew. The first hum seemed mechanical as it was coming from one of the amps itself, and as I mentioned above that did go away after being powered down and back on a couple of times. The new one is through the speakers - it seems to be linked to the P38 as it comes and goes when that is powered on and off. It seems to be caused by the TV (or something used in conjunction with it) as it started when I turned it on and seems to change in pitch as the picture changes. I'll see if the bolt for the transformer can be tightened on the P38.

 

[laserman16]

andrewknight:On the subject of humming trannies (!), I had a similar problem with a Quad 303. I found that giving it a firm slap would stop it humming. This is starting to read like a "Carry on" script.

Well perhaps humming trannies like a firm slap.

I think its time for bed.

 

[a91gti]

laserman16:
andrewknight:On the subject of humming trannies (!), I had a similar problem with a Quad 303. I found that giving it a firm slap would stop it humming. This is starting to read like a "Carry on" script.

Well perhaps humming trannies like a firm slap.

I think its time for bed.

Oooh er!

 

[Dan Turner]

OK - can't access the bolt that secures the transformer to the chassis it goes through the top of the transformer and terminates through the bottom of the chassis, and I ain't dismantling the whole thing.

Just tried powering everything down and it doesn't make a difference. Both P35 and P38 cause a feint hum through the speakers when powered up - nothing to cause too much concern, you can really only hear it when you are close to the speakers - but when I put the TV on now it still goes crazy.

I've got a tacima mains block coming off a single socket (the only one accessible for my system) I've got my Hi-fi going into this, and then a second one coming off that which the HC is connected too. Could that be something to do with it? If so how come it was not a problem before I changed things around?

 

[Dan Turner]

OK, can't understand why, but reasonably happy - I tried unplugging the TV from extension block and plugging it back in, and then made sure the TV mains cable wasn't near the mains cable for either amp, and that seems to have done it. Hardly any any buzz through the speakers when the TV's on even with my ear right next to the tweeter.

Phew. Bedtime.

 

[rod1429]

If the hum is present through the speakers when the power amp is switched on it could be a ground loop.

I had a similar problem with my set up - get an extension lead & plug the power amp into another socket - see if it goes away, if so thats your problem.

I got a non earthed iec lead from my local hi fi shop & the problem disappeared.

 

[Dan Turner]

rod1429:

If the hum is present through the speakers when the power amp is switched on it could be a ground loop.

I had a similar problem with my set up - get an extension lead & plug the power amp into another socket - see if it goes away, if so thats your problem.

I got a non earthed iec lead from my local hi fi shop & the problem disappeared.

Thanks, I think that's it. It was resolved (and seems to have stayed resolved, fingers crossed, when I unplugged and re-plugged the TV (albeit in the same socket), and made sure the mains cables were apart. I mentioned above...

"I've got a tacima mains block coming off a single socket (the only one accessible for my system) I've got my Hi-fi going into this, and then a second one coming off that which the HC is connected too."

...does anyone think that sounds problematic in and of itself?

 

[rod1429]

Funnily enough, I'm using a Tacima mains block.

I wonder whether this is the problem.

I'll try with a normal iec lead & extension block when I get chance & post findings !