BillFitzmaurice.info

Well, I'll tell you the whole story.

I have an Onkyo TX-SR606 receiver (with a freshly re-capicatored HDMI board; that was fun) plugged into a surge protector with a 2 pronged, non-reversible plug. The inputs for the receiver are both HDMI - cable box and home theater computer. Output is a single HDMI cable to the TV (also plugged into the surge protector; also two pronged, IIRC). The 5 speakers are run with standard wire around the room. The LFE remained unplugged for this troubleshooting.

All is fine with very minimal hum as-is. However, once I plug in the compressor/limiter which is three-pronged to the same surge protector, the speakers begin to hum, even with no other cables attached to the compressor/limiter other than the power cable, of course.

I tried a different outlet, but being on the same circuit, the hum was still there. The cheater plug which removed the ground connection makes the hum disappear. At this point I can plug in my RCA(LFE) to the (XLR)compressor/limiter using an RCA->XLR cable and still there is no hum. I can then plug in my homemade compressor/limiter output "cable" - mono 1/4" TRS to stereo 1/8" TRS adapter to 1/8" TRS 'unbalanced' stereo cable to RCA to the subwoofer amplifier. Again with very little added hum. Admittedly, that cable is laughable to say the least, but the real unbalanced XLR to RCA cable will be here shortly. I just needed to make sure it would work.

So in short, just the compressor/limiter being turned on and plugged into and grounded the same circuit is causing the hum. Why that is, I do not know.

The first thing I'd try is bypassing the surge protector.

That was my first thought as well. Directly into the same outlet as well as into one across the room - same problem.

chjade84 wrote:That was my first thought as well. Directly into the same outlet as well as into one across the room - same problem.

That means the problem is related to the ground wire, in some way that's not obvious to me. Defeating the ground isn't recommended, but if that's the only way to make it work you have to do what you have to do.

chjade84 wrote:Well, I'll tell you the whole story.

I have an Onkyo TX-SR606 receiver (with a freshly re-capicatored HDMI board; that was fun) plugged into a surge protector with a 2 pronged, non-reversible plug. The inputs for the receiver are both HDMI - cable box and home theater computer. Output is a single HDMI cable to the TV (also plugged into the surge protector; also two pronged, IIRC). The 5 speakers are run with standard wire around the room. The LFE remained unplugged for this troubleshooting.

All is fine with very minimal hum as-is.

Reading this, I have to ask, "Shouldn't it be dead quiet?"

chjade84 wrote: However, once I plug in the compressor/limiter which is three-pronged to the same surge protector, the speakers begin to hum, even with no other cables attached to the compressor/limiter other than the power cable, of course.

I tried a different outlet, but being on the same circuit, the hum was still there. The cheater plug which removed the ground connection makes the hum disappear.

I wonder if it is the receiver since it was just recapped. The polarized prongs could be hooked up backwards. The ground prong and the 'neutral' prong of a three-prong plug both end up on the 'ground' buss in the breaker panel. That, in my mind, would eliminate the compressor/limiter.

Tom Smit wrote: The polarized prongs could be hooked up backwards.

It's a 2 wire cord, so in theory it's impossible to have them wrong. But as with all theories...
The entire notion of polarized 2 prong North American plugs is intrinsically flawed, as it assumes the person wiring the outlet won't screw up the job.
To account for the fact that you can't assume that the ground prong is actually connected to ground two wire cords never actually connect the alleged ground wire to the device chassis. That way a building wiring error won't result in the chassis being hot.
OTOH it is possible with some devices that use a capacitive shunt from the AC line to the chassis that reversing the line cord position will have a positive result, so grinding off the fat ended blade as reversing the plug is worth a shot.

Tom Smit wrote:Reading this, I have to ask, "Shouldn't it be dead quiet?"

I wonder if it is the receiver since it was just recapped. The polarized prongs could be hooked up backwards. The ground prong and the 'neutral' prong of a three-prong plug both end up on the 'ground' buss in the breaker panel. That, in my mind, would eliminate the compressor/limiter.

I've never had a truly quiet home theater system. You don't notice the hum unless you put your ear to the speakers, but it's still there.

As far as the capacitors go, nothing should have changed. I just replaced some surface mount capacitors as well as adding some of my own.

Bill Fitzmaurice wrote:

Tom Smit wrote: The polarized prongs could be hooked up backwards.

It's a 2 wire cord, so in theory it's impossible to have them wrong. But as with all theories...
The entire notion of polarized 2 prong North American plugs is intrinsically flawed, as it assumes the person wiring the outlet won't screw up the job.
To account for the fact that you can't assume that the ground prong is actually connected to ground two wire cords never actually connect the alleged ground wire to the device chassis. That way a building wiring error won't result in the chassis being hot.
OTOH it is possible with some devices that use a capacitive shunt from the AC line to the chassis that reversing the line cord position will have a positive result, so grinding off the fat ended blade as reversing the plug is worth a shot.

I could give that a shot I guess. Although it's a three-pronged cord, so maybe not. Rewire inside perhaps.

chjade84 wrote:
I could give that a shot I guess. Although it's a three-pronged cord, so maybe not. Rewire inside perhaps.

I was referring to the reciever, which you said was 2 prong.

I would try to physically relocate the compressor. One possibility is that the compressor has a crappy transformer in its power supply and is throwing out a lot of noise. The other, and more likely, problem is If it has a metal chassis touching a metal chassis of one of your ungrounded components. One solution for either problem is simply some method of physical and electrical separation (spare rubber feet, some wood blocks, using two shelves, etc.) If it's a metal-to-metal ground loop, just add a grounding wire to the ungrounded devices—preferably replacing the original power cord with a grounded one.

For more on this and grounding for audio systems in general, I highly recommend reading through Bill Whitlock's "generic seminar" pdf (warning: it's 1.4 MB).