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In Reply to: RE: You would still have a ground loop. posted by Al Sekela on July 03, 2007 at 15:13:22
Let me make it clear that I never objected to having two separate power circuits. I haven't been back since my prior mentioned post, but my point in my mentioned post was that IF you had a ground loop, then having two separate circuits would make that loop even larger. You can have ground loop problems in as small a loop as in a single enclosure, so NO you don't need to be large building to have the ground loop created by two circuits matter. The area of the loop is what matters. Even if they are close together, there is some space between them, and it is a lot longer to the circuit breaker box, so the area adds up (length x space between).
My recommendation to plug into the same outlet was merely to aid in diagnosis, in changing the size of the ground loop. However, the proper result could only be obtained by breaking the loop, no matter how large the loop. If there is more than one circuit, the two circuits are "star" grounded at the breaker panel. There should be no other ground connection between the circuits.
Even if you have a ground loop, there must be a coupling mechanism into the signal for it to have an effect. If you have two monoblocks, each with a separate ground back to the breaker box, you get a huge loop if there is a common ground connection between them in the signal lines, which would very likely occur in your preamp, and the coupling would be in the common impedance of the shields. One way to easily diagnosis this is to disconnect ONE of your input to a power amp (after turning everything OFF first!!!) Turn everything on and see if the hum disappears. This should be done after testing the disconnection of any antenna or cable line first, to see if that is the problem.
You don't want to use a cheater plug with the amps!! Although this might break the loop and have the cheated amp grounded through the other amp, it would be through signal lines and pcbs. Any fault might smoke your low level electronics as well as cause a safety hazard.
I would consider using one circuit for one channel, and the other circuit for the other channel, and try to break the ground between the channels in the preamp. Lacking that ability, you're almost resigned to an input transformer on at least one of your amps, which is rather expensive.
Follow Ups:
Here are the facts, as we know them:The wiring,
* One truly dedicated 120V branch circuit. Consisting of one each dedicated, hot conductor, neutral conductor, and equipment grounding conductor.* Two separate 120V circuits. A 3 wire multi wire branch circuit consisting of two hot conductors (each connected to a breaker fed from each Line in the electrical panel L1 and L2). One shared neutral conductor, and one shared equipment grounding conductor.
* The truly dedicated circuit feeds the preamp and what ever source equipment.
* The two separate circuits,
if a power amp is feed from each separate 120V circuit a hum is heard through the speakers.
Note..... There is only one equipment grounding conductor for both circuits feeding the power amps....
Added note, at idle both amps theoretically are drawing the same amount of current so the two power transformers primary windings are in series with one another and being fed by a difference of potential of 240 volts nominal.* If both power amps are plugged into only one of the separate circuits, that circuit being fed from the same Line in the electrical panel as the truly dedicated circuit, the final result, no hum is heard through the speakers.
*Still the same equipment grounding conductor.
* Why no hum??
Difficult to say without more diagnosis. First I would disconnect the preamp cables from the power amps. Then I would try the power amps in both power supply connections to see if one still hums more than the other. If, for example, it still hums in the three-wire two-circuit mode, I would suspect transformer leakage adding into the ground line instead of cancelling. It may be that a reversal of the transformer leads in one of the amps would correct this. (Don't do this with the power line, since the hot should go through the switch.) It may also be that one circuit is providing a ground loop for the other through the common ground.
If both power arrangements work with no noise when not connected to preamp, then there is probably a ground loop situation that the preamp brings when connected.
Difficult to say without more diagnosis. First I would disconnect the preamp cables from the power amps.
> > > > > > > > > > > > > > >
I believe in one of Steve's earlier posts he did disconnect the ics from the preamp and the two amps. No hum with the ics disconnected.
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I would be curious what the effect, result, would be if Steve left the ics connected to the preamp but unplug the preamp from the 120V power outlet. Also disconnect all input ics from the preamp.
This test would still have the two power amps signal ground tied together at the preamp jacks. Would there still be a hum?
Your thoughts, or Al's, or any one else's.......
The preamp circuit and the power amp circuit have separate safety-earth wires that go back to the circuit breaker panel. According to Steve, this setup, with both monoblocks fed from one hot wire, does not hum.
Therefore the use of the second hot wire for one monoblock is the proximate cause of the hum. Whether it is because of the small residual current in the neutral wire, or mismatched power transformer parasitic capacitances, would have to be determined by some simple experiments and measurements.
Therefore the use of the second hot wire for one monoblock is the proximate cause of the hum. Whether it is because of the small residual current in the neutral wire, or mismatched power transformer parasitic capacitances, would have to be determined by some simple experiments and measurements.
> > > > > > > > > > > > >
How about this for a test, have Steve plug one of the power amps into the dedicated circuit with the preamp. Leave the other amp plugged into the separate circuit outlet that is on the same Line, leg, as the preamp recept.
We would have all the audio equipment being fed from the same Line, leg, of the electrical panel. Will there be a hum?
----------------------
I wonder how many guys out there think they had two dedicated circuits installed by an electrician for the audio equipment, and instead have two separate circuits.......
If there is hum with this configuration, then it is time to check the individual amp power transformers.
I should add that reversing the transformer leads to correct this might only be a bandaid approach for this particular power circuit arrangement. It may compromise the noise in a normal arrangement.
Tranformers have a different leakage capacitance from each side of a winding and ground. Therefore, one lead orientation will couple a different amount of noise to ground than the opposite lead arrangement. This is why reversing the plug may sound better one way than the other. To test for this, disconnect the PC ground from the chassis. Power up, and measure the voltage between the disconnected PC ground line and chassis. Shut off, reverse transformer leads, and measure voltage between the disconnected PC ground line and chassis again. The transformer connection having the lowest voltage between the chassis and the ground line is the one to use, because less leakage will flow to ground.
Hard to say if the best arrangement was done when amps were made, but perhaps putting the amps on differently phased circuits caused the leakage currents to add.
I'm not entirely confident of this is the entire reason for the noise. It could also be from one circuit being a loop coupled to the other via the ground line. There's no way of telling without going through the process of elimination.
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