Amplifier Hum

I've got the same issue with some very large torodial transformers. It's worse when the oven or microwave are on (even with dedicated lines!). The manufacturer told me it was dc voltage in the lines and there's nothing I could do. Anyone know if some of the line conditioners would help (e.g., the Noise Hound)?

For a transformer to hum, the gapping in a standard (i.e. square) transformer is off by some very small amount, whether in the gap itself or in the lamination spacing. When the gapping is off, the transformer suffers from eddy current losses that emit audible harmonics (i.e. a hum). In other words, the transformer is either loose as mentioned above, or the lamination has separated (both often the same problem). It is possible that there is a frequency problem that can induce such noise but if so, the problem lies in the rectifier and not the transformer. If a toroidal transformer makes noise, the design is most likely bad (that means get a better amp). I would check the transformer to make sure its mount is tight since the hum is actually a vibration that can make the problem worse with time by loosening it. If you put your hand firmly on the transformer, the problem should disappear in this case. Otherwise, take it to a pro for a close look at the rectifier circuit. Good luck! Arthur

I have the 4004 Mk II. There is an audible hum. Mild but apparent.

Yes, the rectifier problem I alluded to is generally a DC offset voltage that can cause a hum as Darryl found out. You might want to take his advice and see.