Single ended to XLR Question

Thank you for the feedback. It really isn't an issue as long as the preamp is turned on first and turned off last. Just was puzzled as to why I would have 2 different results from 2 different preamps.
Scott

Have you tried plugging the speaker power cords into the same receptacle as the rest of the equipment ?

It's probably just a matter of what the output impedances of the two preamps happen to do when they are turned off.

Probably the output impedance of the Mc in the off state is high, allowing pin 2 (the non-inverted input signal line) of the ATC's xlr input connector to "float" in an uncontrolled manner, while xlr pin 3 (the inverted signal line) may be exposed to inter-chassis hum voltages caused by ground-loop effects. Since balanced input circuits respond to the difference in voltage between those pins, hum will result.

The Aesthetix probably has a lower output impedance when it is powered down, resulting in pin 2 being "pulled" to a voltage level closer to the Aesthetix's ground, which in turn would reduce the difference between the voltages on pin 2 and pin 3 of the ATC's xlr connector (pin 3 presumably being connected to ground in the cable).

One thing I would do, though, is to use a multimeter to verify that pin 3 of the cable's xlr connector is connected to pin 1 (the ground pin). That is what should be done by a cable or adapter that is used to drive an xlr INPUT with a single-ended signal. (Pin 3 should ideally not be grounded, btw, when an xlr OUTPUT is adapted to drive a single-ended input, although in practice it is frequently done, for no good reason IMO).

Also, I would make a point of comparing sonics using the balanced and the unbalanced outputs of the Mc. If the balanced driver circuit in the Mc is of good quality, I would expect it to be preferable. I'm not sure why the dealer would have steered you to the single-ended cable.

Finally, as you may realize it is good practice in a system that has passive speakers to turn the power amp on last and off first, relative to the preamp and other components, to prevent turn-on or turn-off transients that may be generated by the preamp or upstream components from being applied to the speakers. Similarly, it would be good practice with your active speakers to turn them on last and off first, which would eliminate the hum issue.

Regards,
-- Al