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?

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.

Cleaning up the sine wave certainly won't hurt anything, but oddly enough it may not help. Magnetostriction is related to volt/seconds. The transformer stores up energy during each half cycle. If you begin to push the storage capacity to its limit it begins to saturate and hum. Since you are stuck with 60 hz only the volts are available to change. It is the area under the curve and not the shape of the curve that counts for volt/seconds.

It would be interesting to know what your line voltage is. Use an RMS reading voltmeter if you can, rat shack sells them along with some nice speaker cable (opps getting off the subject :-) if you don't have one of those a regular volt meter will work OK. If the line voltage is high then the following option might help.

There are ferroresonant transformers you can buy for probably 3 to 5 hundred bucks that clean up the waveform and stabilize the voltage. They also provide tremendous surge protection. The problems are that they are (at least the ones I have seen) very noisy. This is because they deliberately run them in saturation. So, you would have to locate the transformer somewhere away from your listening area. They also waste alot of power. Something like a third of their rating. So it is best to disconnect them when not using your system.

A step down transformer would be cheaper. It can be an autoformer, since you are not looking for isolation. An autoformer has no secondary. This makes them smaller and therefore cheaper.

Try four or five more of those diode pairs. If they don't help you are not out much. Try the cheapest stuff first.

Geeez, Tommy, it just doesn't make sense that they are humming at only 120 volts. If it weren't for the fact that both are doing the same thing I would think it was something mechanical.

I don't know much about the audiophile line conditioners. I did look up a couple of brands on the internect in response to a post someone had made. It said something like "No capacitors, no inductors, no transformers to interfer with the purity of the sound!". It cost 6 or 7 hundred dollars and they were advertising that there was nothing useful in the box?! The ferroresonant transformer I mentioned won't help because the line voltage is not too high.

The diodes are like little valves. One is pointed in each direction because AC needs to go both ways. It takes .6 volts to open the valve. ten pairs would give you a 6 volt drop. So, you just make some more pairs and put them in series with the one you have. They can be soldered to the one you have. They can go anywhere in either leg and will work the same, just not in parallel with each other. You could just temporalily cobble it all together with alligator leads to see if it helps before taking the time to do it right.

I'm not familiar with the amps. Is the manufacturer available for advice?