Tweakers' Asylum

RE: You would still have a ground loop.

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Pooge

In Reply to: RE: You would still have a ground loop. posted by jea48 on July 04, 2007 at 16:50:27

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.

Follow Ups:

RE: You would still have a ground loop. - jea48 07:14:55 07/05/07 (3)
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.
---------------

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.......
- We also know that Steve's setup with two AC circuits does not hum. - Al Sekela 15:59:03 07/05/07 (2)
  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.
  - Agree... - jea48 06:05:22 07/06/07 (1)
    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?
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    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.......
    - Good experiment. - Al Sekela 13:31:32 07/06/07 (0)
      If there is hum with this configuration, then it is time to check the individual amp power transformers.
RE: You would still have a ground loop. - Pooge 17:42:55 07/04/07 (0)
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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