MTB Vince, yes I plugged the subwoofer into the preamp circuit. No change in the hum. One other interesting thing--the hum volume is independent of pre-amp volume, but does increase with sub volume and RCA-to-XLR level compensator volume. Turning off the preamp or muting the preamp, however, does eliminate the hum. One other interesting thing is that this is really a noise-floor issue--the level and frequency of the hum is equivalent to a large truck idling a few hundred yards from my house.Have you acted on the suggestion I previously made in this thread @ACHiPo? If not, I would do so prior to pointing the finger at your preamp...
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