Originally Posted by jlefors
I had a similar problem when I put my sub system together and got some help here (Thanks again AVS forum!) fixing it. I had two new grounded 20A circuits for my home theater system, all tied into a new grounded service panel, so I was totally flummoxed. The advice I got was to check the grounding for my cable (TV / Internet) and phone lines. Wouldn't you know it, but I had disconnected the ground strap from the cable line while doing some work in the garage. Reconnecting the ground caused my hum to go away (& stay away. Things have stayed dialed for a year + now)!
Hope it's that easy for you...
Well, I spent a few hours trying to solve this. It was mostly academic, since just leaving the low pass filter in place on the amp seems to solve the problem by itself.
The interesting parts of the system are a Denon x4000 AVR, a minidsp Dirac 88a, a minidsp 2x4 balanced, and the iNuke6000.
The interesting wires involved are:
1) a single RCA from the Denon to the 88a
2) a single RCA from the 88a to the minidsp 2x4 (custom pigtail into the phoenix connectors)
3) two XLR to RCA cables from the minidsp 2x4 into channel A and B of the iNuke (custom pigtails to add RCA females to the phoenix connectors)
I disconnected all other equipment from the wall outlets. I made sure all cables were firmly attached. I left the amp in STEREO/No Filter mode, and I tried both all three available circuits.
The hum was there.
First things first. Bypass as much as possible. Remove the minidsp 2x4 and the Dirac 88a. Straight from Denon into iNuke channel A (no channel B). The hum is gone. This is progress.
Since my minidsp 2x4 is currently set to complete bypass, with no filters (the 88a is doing a remarkable job on it's own) it really is just a Y cable. So let's remove it from the system.
Denon into 88a, 88a into iNuke, channel A only. No hum.
Ah hah! The miniDSP 2x4 was causing the hum! Physically remove it from the rack/closet, and wire in a Y jack in instead.
Denon into 88a, 88a into Y jack, Y jack into channel A & B of the iNuke. The hum is back.
WTF? The miniDSP 2x4 wasn't the problem. It's just that having only one channel connected to the iNuke seems to be a solution?
Again remove the minidsp 88a, and go straight from the Denon to the INuke, but this time through the Y cable and connect both channels of the iNuke. Hum.
Okay, bad Y cable? Try another one. Hum. Bad RCA interconnects? Try about 4 more. Hum. How bizarre. Any time I connect BOTH channels of the iNuke, BOTH subs hum. If I connect only channel A, the hum is gone.
Somewhere around this time, I started playing with the filter switch on the amp again, just to prove that I could still make the hum go away with a flip of the switch. I was also back in the mode where only channel A was connected. I either accidentally, or for no good reason, switched the STEREO switch to MONO, and noticed that both subs were still working. I knew that the iNuke had a mode where it would take a mono input and send it to both output channels, but I never looked carefully enough, and thought that it required a 4 pole speakon connection to work. I didn't realize that it would do exactly what I needed it to do...just send the channel A input to the channel A&B output.
Great. So *ALL* of the issues seem to stem from having two channels connected on the input side, but there's now no reason to do that.
I wired Denon to 88a, 88a to iNuke (no more 2x4 or Y cable required) and left the amp in mono/no filter mode. No hum, and all is well.
I still can't explain the issue, but I'm also happy to just sit back and enjoy the room again. *sigh*