How to get rid of transformer hum?

Hi Tommy; I'm sorry I can't help you, but you've maybe helped me. I have a pair of McCormack DNA-2DX amps that hum at various levels. I've talked to Steve McCormack about this at length, and he says that most likely there is DC leakage into my AC lines, and that there is nothing they can do about it (is this the sine wave clipping that you mention?).

I have not yet contacted the power Company, but have a hard time believing they would have much interest in it? BTW, my amps are at SMc being upgraded, and I specifically asked them to check out their tendency to hum. Steve has told me that they do not hum in S. Calif.-- probably not enough power to muster up a hum ;>). We'll see. Craig.

The mechanical hum you are talking about could be from a few causes. Magnetostriction causes the transformer laminations to actually change size as the voltage changes. This effect is greatest at high voltages and is independent of the power the amp is delivering. At high currents transformers can have loose laminations that buzz, a loose winding that moves or the transformers stray magnetic fields can cause a steel chassis to buzz.

Do both amps behave identically?

Both amps hums just alike. Before building a filter made up of two caps and a diode I could hear the hum when I walked into the room. Now I hear it when I walk over to the amps. I'm just looking for a fix that doesn't cost 5 grand. Any thoughts?

Well, I think the transformers are either mismanufactured or poorly designed. When you design a transformer you make a deal with the devil. If you use lots of turns the transformer will be quiet, but the large number of turns means smaller longer wire which causes higher resistance so the transformer becomes less powerful. Or, you can make the transformer bigger without increasing the power, that costs more money.

If in fact the transformer is humming because it doesn't have enough turns on the primary, then the only thing one can do that I know of is to lower the voltage to the amp. This could be verifided by using a variac to lower the voltage and see if the hum goes away. More diodes will lower the voltage some. Or a stepdown transformer that dropped the voltage from say 120 to 110 votlts. Of course the step down transformer might hum. I presume you have two diodes in parallel connected anode to cathode. Each time you add one of these it will drop the line voltage about .6 volts. It also increases the lines harmonic distortion but that shouldn't be a problem.

If the hum is being transmitted into the chassis you might reduce hum by placing the amps on something soft. If there is a cover that is vibrating you could try removing it.