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    Building, troubleshooting and testing of these amplifiers should only be
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Power Transformer Hum


2012-11-22 4:39 pm
I have an Edcor power transformer secondary all wired for heater and b+, but nothing plugged in - only the power cord for the amp. When I turned on to test the voltages I can hear a hum, but nothing is plugged in. Is this normal for the PT or is there a way to troubleshoot it?


2008-08-20 9:42 pm
Depending on how hot it gets under load, I've used various items to shut them up. I agree to try to tighten the bolts, but Edcor tightens them pretty well usually.

I use adhesive 1/4" foam on the inside of the bells, and have also used automotive "dynamat" type material.
My Edcor power transformers hum too!

Did you resolve this problem?

I just finished a pair of 50W Williamson monoblocks and have the same problem.

They've both hummed every since I first turned them on with no tubes installed.

Unfortunately, I can just hear the hum from my listening position.

Painful but I'll try removing the transformers and bells and insulating the insides.
Yes ....

Transformer hum comes mainly from the applied primary voltage, not application of a load. The mains voltage 'activates' the ac field in the core, causing it to vibrate wherever there is a loose lamination.

Often the solution is rather cumbersome - taking the transformer apart and mechanically checking for a loose lamination with mains applied - care of course not to short out anything, including self! With luck it can be stopped by wedging something in between core and former etc. With no luck .... one has to live with it or return the component, or pour some sticky stuff down somewhere hoping .... Though irritating, the manufacturer should replace it; mechanical power transformer hum is unacceptable.

Late thought: I hope a ferrous metal chassis has nothing to do with it. I had such an occurence; remedy then obvious.
Perhaps troubleshooting may be practical if the PT could have its mounting bolts removed (easy for bolts that just hold down bell-ends, but not so easy for bolts used by the power transformer itself). Then try and sit the core on some neoprene rubber or similar to remove the chassis from the 'mechanical equation', and test again.

Hopefully its just a loose or unconstrained lamination on the end of the stack, hidden under the bell ends.
Think vibration isolation.

Create a cork and rubber sandwich. First try the isolator between the amplifier and the table top. Also think of the vibration getting into the tubes creating electrical hum coming out of your speakers. I like the power supply separate from the amplifier chassis.

The combination of rubber and cork together attenuates vibration better than the sum of the cork and rubber alone. It is a composite technology thing.

An example from industry http://www.mason-industries.com/masonind/_doc/pdf/VP.pdf

Finally got around to addressing this problem. Simple fix was to cut some large grommets into slices and place one under each transformer foot.

Can still hear the transformers humming close up but that will do.

I feel I'm treating the symptom rather than the cause though because I have 2 similar amps that I acquired rather than built (both electronic design and physical layout, just lower power) and they don't hum at all. Mains transformers on those are probably recycled I'd say - unknown brand.


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