I took some measurements from each amp using the ground tab on a cheater plug with nothing turned on, just plugged in. I'm seeing 16 vac on the ground tab from one tube amp and 14 vac on the ground tab with the other tube amp. The SS amp shows no potential. I didn't bother measuring the ac current.
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Just having one of the other amps plugged in but not turned on or connected in any other way is enough to cause the hum.Wow! That's different!
When the amp that is not connected to anything except power is turned on, does the hum get better, worse, or stay the same, compared to when it is turned off?
A possible explanation that occurs to me is that, referring to Figure 1 of the Jensen paper that Jim and I both linked to (great minds think alike :-)), when you plug in the unused amp, and it is in the turned off state, you are placing CPS4 (as defined in Figure 1) of the unused amp in parallel with CPS4 of the amp or amps that is/are being used. That would worsen Vy (as shown in Figure 2) for the amp(s) that is/are in use, thereby worsening the ground loop situation between the amp(s) being used and the preamp or whatever is driving it or them.
If the amount of hum decreases when the unused amp is turned on, it would add credibility to that theory, because doing so would place CPS3 of the amps in parallel, as well as CPS4, which would to some degree reduce the effects of the increase in CPS4.
I presume, btw, that the preamp to amp interconnections are unbalanced, as I would expect these kinds of effects to be much less significant with balanced interconnections.
Yes, all unbalanced. I put the two tube amps on cheaters. When they are both off I see about 2.5 vac and that drops to about 1.3 vac when both are switched on. That seems to go along with your theory, Al.
I have also been referencing this paper from Equitech's web site.
Lifting the Grounding Enigma
They seem to be saying that the balanced power approach will do the trick for my situation but I can't reason it out.
I read through the Equitech paper, and it strikes me as excellent. However, it also strikes me as not inconsistent with the comments on balanced power in the Jensen paper, which is to say that there would probably be significant hum reduction if you used a balanced power arrangement, but not necessarily a reduction that is fully satisfactory.
From the Jensen paper:
It is also very unlikely that the two capacitances, CPS1 and CPS2 or CPS3 and CPS4, would be exactly matched in any piece of equipment. Mismatch ratios of two to one are common.... Although intuitively attractive, [the balanced power] approach can completely cancel interchassis currents in a system of three or more devices only in the case where each of the devices had such matched capacitances. This would be an extremely rare occurrence.... 10 to 15 dB hum reductions ... would be more routinely achieved.From the Equitech paper:
If any aspect of the circuit is applied OR LOADED [emphasis added] in an unbalanced manner, noise will appear in the ground.... On the average, 16dB improvement in background noise has been noted.In other words, any difference between the stray capacitances CPS3 and CPS4 in your particular amplifiers (which is unpredictable) will limit the amount of hum reduction balanced power would provide.
In considering the 10 to 16 db numbers, keep in mind the rule of thumb that a reduction of 10db is subjectively "half as loud."
Thanks for your interpretation. I believe I have the "extremely rare" occurrence mentioned in the Jensen paper. :-) I realize this probably won't eliminate all of the hum, but cutting it in half might make it liveable. I'd like to put each of these two amps on their own balanced supply. These are around 60 watt PP amps so I don't need too large of transformers. Maybe a pair of 500va on each. Finding them for an experimenting price might take some looking.
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