Qu for an electrical engineer re: amp transformer hum

[nickbaba]

Hi - I have a technical question for anyone with expertise in the area of electronics and AV equipment.
I recently decided to upgrade my standard "B&Q" surge-protecter plug-board to a more expensive 'audiophile' unit, as I wanted to see if it would have a positive effect on the sound from my amp - a McIntosh MA7000.
I went for the IsoTek Evo3 Polaris plug-board (& IsoTek mains cable) as an entry level upgrade.
However, the first time I plugged the amp into it and powered it up, I immediately noticed a hum coming from the MA7000's transformer which had not been there before. Not loud, but audible when standing next to the amp.
Disconnecting it and trying different configurations e.g. plugging amp directly to the wall socket, revealed that the hum was now here to stay.
Previously the amp had always been absolutely quiet internally when powered up.
My question is, is there any scenario whereby using a unit like the IsoTek with an amp like the MA7000 (which has a lot of onboard features for handling mains power) could be detrimental to the amp and possibly even end up causing the transformer hum? Or is this simply impossible (or at least a vanishingly small probability)?
It seems unlikely to me, as I thought these types of mains units were designed to protect sensitive electronic equipment, if anything. But that hum did appear to start the first time I plugged it in, so I am suspicious.
Can anyone please give me an expert opinion?

[synsei]

Not an expert opinion as such but an account of a similar experience. When I moved from using a Tacima mains block to a Belkin PF30, initially I powered most of my equipment from the PF30 and a Hafler power amp from the Tacima. The PF30 and the Tacima were plugged into the same power outlet. The hum which resulted was horrendous. Plugging the Hafler directly into the socket solved the issue. The Tacima now does sterling service powering my computer bits and bobs but it is running from a separate socket in the same room without issue. It was suggested to me that the power conditioners in the two products were interacting to cause the hum and that power amps should never be plugged into power conditioners anyway as they can limit the amps headroom ;)

[Ali Tait]

Do you mean the hum is still there even when the Isotek is completely out of the system?

[nickbaba]

Yes, I do mean that.
When I took the IsoTek out of the system completely, and plugged the amp directly into the wall socket (no plugboard) I could now hear a faint hum from the transformer area of the amp that I'm sure wasn't there before.
Thats why I want to know if there's any possibility that an interaction between the 2 could have damaged the transformer somehow?
I'm not an electrical expert, so I don't know if that is even a possibility, or if I'm just being paranoid... but as I say, I'm sure my amp didn't hum before, and it sure does now.
Why can't I just learn to leave things alone? ..:doh:

[Barry]

Yes, I do mean that.
When I took the IsoTek out of the system completely, and plugged the amp directly into the wall socket (no plugboard) I could now hear a faint hum from the transformer area of the amp that I'm sure wasn't there before.
Thats why I want to know if there's any possibility that an interaction between the 2 could have damaged the transformer somehow?
I'm not an electrical expert, so I don't know if that is even a possibility, or if I'm just being paranoid... but as I say, I'm sure my amp didn't hum before, and it sure does now.
Why can't I just learn to leave things alone? ..:doh:

There are a couple of ways by which a transformer can 'hum'.

One way is due to the transformer laminations becoming 'loose'; the cure being to tighten all the clamping nuts and bolts. The second way a transformer might hum, is if there is a DC offset on your mains supply. A 100mV DC offset will almost certainly cause transformer hum.

Either of these faults will cause mechanical hum, which won't be heard through the speakers. If you do hear hum through the speakers, then the fault lies with the power supply and inadequate smoothing.

[nickbaba]

Thanks Barry.
Yes, the hum I can hear is actual mechanical hum coming from inside the amp's transformer section (clearly indicated on McIntoshs), when I stand next to the amp or put my ear against the amp casing.
There is no 'ground-type' hum coming from the speakers. Sound from the amp appears normal.
I'm sorry but my technical knowledge is very limited. Assuming that the mechanical hum is indeed caused by a DC offset from the mains, could you please recommend a solution to the problem (if there is one!).
Also, am I correct in thinking that your suggestion implies that the issue could not have been caused by the interaction of my power-conditiuoning plugboard with the transformer of the amp?

[Barry]

Hello Nick,

No, I don't believe any damage was caused by your power-conditioning plugboard.

Before we discuss the use of a DC block, we need to know if there is substantial DC on your mains supply. You will need to have an electrician check out your supply.

[Sonority]

A few more things that 'can' make tx's hum :-
Anything with a 'wall wart' from a laptop to a mobile charger.
Wireless telephones
Most plasma tv's
Low voltage lighting
Compact Fluorescent Lights
Fridge compressors
Freezer compressors
Central heating pumps
Hair driers
Next door neighbours lawnmower (used to send my amp into apoplexy, right up to when the mower literally blew up)
-> ad naeuseum, ie, pretty much anthing in a modern home!
Most are not worrisome to *most* transformers, some transformers just don't like any of them.
I doubt as said the mains block has caused any prolems, but would have been surprised if it had not sat on the dynamics of the amp.
Good luck with finding the cause..

[nickbaba]

Thanks guys - yes, happy to hear that I haven't done anything bad to the amp by plugging it into a power-conditioning plugboard!
Have retired the new board to the computer room - don't want to limit the amp's dynamics!