How to get rid of transformer hum?

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.

I realize that you have measured a 5% sine dip. But are you certian that this is the cause of the hum? I have tracked down a lot of hum problems and most are generated from within the home. I will relate my own personal experience. I always had a slight hum in my tv and Tice isolation transformer for my gear. I lived with it until I bought a Classe CA400. The amp hummed very loud! I finally isolated the source to my heat pumps outdoor unit. Something in the prehaet circuit of the compressor. Right now I just turn it off when I listen. The background is dead silent! I'm going to look into it deeper when I get the unit pm done. I would try turning things off in the house first to see if the hum is caused internally. (off means off at the breaker) Good luck!

A while back some funny Agon poster suggested that the way to stop an amplifier from humming was to teach it the words.