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First, determine if the hum is from the power supply or the input signal. Swap in a battery as the supply or try a battery powered input to make this determination. Make sure your signal input is shielded and that the shield is connected to the same ground point as the amplifier.
Hum caused by battery eliminator output voltage ripple should be 120Hz. If it is 60Hz then your battery eliminator is defective (one of diodes in rectifier is broken). Most often cause for 60Hz hum are ground problems with your audio equipement. Ground potentials (chassis potentials) for audio equipement are different (sound source, amplifier), they are not connected, grounded in one point and thus between grounds flows 60Hz current causing the hum in audio. I presume you are using PC as sound source.
When I disconnect the turntable from the preamp, the hum goes away. I have the ground wire from the turntable connected to the ground terminal on the battery eliminator that is powering the amp. I tried putting a ground loop isolator between the amp and the cd player. It didn't help but after doing the test you suggested it looks like the problem is in the ground between the turntable and the amp. The ground loop isolator won't work in there because I read that a phonograph cartridge needs at least a 47Kohm load. The impedance of the ground loop isolator is 10Kohms according to the applications engineer at Radio Shack.
I tried putting the isolation transformer on the turntable output. It did drag the signal down a little but it didn't help with the hum.When I tried removing the turntable from the amp input again I noticed that this itime the hum didn't go away. The amp seems to be generating it by itself. It only happens when both volume lots are at about 75% from full volume. I checked for shorts. It looked OK. My meter is also not with me. I might be able to get it tommorrow. My scanner isn't working either right now, but when I can I'll post a schematic. It's just an emittor follower followed by a volume pot (coupled with caps on both sides) and then a common emittor, for both channels.
---------- Post added at 16:55 ---------- Previous post was at 16:46 ----------
It's hard to tell. Now it seems to be only when the turntable output is connected to the amp in. The ground wire doesn't seem to affect it.
---------- Post added at 17:07 ---------- Previous post was at 16:55 ----------
Upon closer investigation, it seems as though there is a small amount of hum when the input is disconnected but a lot more when it is. Also I tried the isolation transformer again with the ground lead from the turntable disconnted from the power supply. That didn't help either. It might not be the amp, it could be the power supply (battery eliminator) but I wonder why it's only when the pots are in certain positions.
---------- Post added at 18:34 ---------- Previous post was at 17:07 ----------
When I use an MP3 player as the input there is no hum and no noise at all. Very clean. I guess the turntable is the problem, or something between the grounds on the power supply and the turntable.
When I feed the turntable directly into the cd player aux in I get a little hum but it's the same when going through the amp unless the pots are set just right (I perhaps I should say "just wrong"). That's when it gets a lot worse and a deeper pitch. But when I feed the MP3 player in directly or through the amp, there is no hum no matter what position the pots are in.
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