Weird ground loop problem

[Saurav]

Well, weird for me, at least. I got a PE plate amp some time back, and it's hummed from the first day I've hard it. Recently I spent some time trying to narrow it down. Here's what I know so far:

* There's no video/cable in the system, and I added power strips and cables so everything was off the same power strip/outlet, and it did not make any difference.

* I have separate stereo and HT systems with a shared subwoofer, so I have cables from my stereo's XO and my receiver's pre-out going to the L/R inputs on the BFD, then outputs from the BFD to the L/R inputs on the plate amp. At any time, only the L or R side is playing, obviously.

* The sub is quiet if I remove both interconnects from it.

* The sub is quiet if I have the ICs between the BFD and the plate amp, but no inputs to the BFD, and no power cable in the BFD

There are 2 things that make the sub hum:

* The sub hums if I now connect the power cable to the BFD, even if the BFD is turned off, and has no inputs connected. Both power cables (BFD and plate amp) are on the same power strip, and I've tried sending that to the main power strip of the rest of the system, as well as sending that to the wall and unplugging everything else. No difference. Tried different power cords, no difference.

* The other scenario is if I disconnect the power from the BFD (which makes the sub quiet), then hook up the input cables to the BFD. The one from the receiver is fine, but the one from the Behringer XO makes it hum. I haven't diagnosed this too far at the XO end, but I think it's similar - if the power cable is plugged into the XO, then it makes the sub hum. Or it's picking up hum because it's a pretty long cable and maybe isn't well shielded... don't know this one yet.

I had the same setup with the Behringer A500 as the sub amp, and it did not hum.

So... any ideas? I might have a toddler crawling around in that room so I'd prefer not to use a cheater plug as a long-term solution. It almost seems like Behringer products do something with their grounding design that the PE plate amp does not like.

I remember adding an RC circuit to my tube amp to address ground loops, but I don't remember what I did. I think it went between audio ground and earth, and the values were chosen to break the DC connection, but to effectively be a short at higher frequencies... or something like that, but I don't remember. I'll go research this, but if anyone has any ideas for changes i could make to the plate amp's grounding that would be safe and would eliminate this, then I'm interested in hearing them. Of course, if there's a solution that does not require making circuit changes, that would be best.

[chasw98]

* The sub hums if I now connect the power cable to the BFD, even if the BFD is turned off, and has no inputs connected. Both power cables (BFD and plate amp) are on the same power strip, and I've tried sending that to the main power strip of the rest of the system, as well as sending that to the wall and unplugging everything else. No difference. Tried different power cords, no difference.

First step: The fact that you get hum when plugging a power cord into the BFD WHEN the BFD is turned off or powered off definitely indicates a ground loop problem. To remedy this and for troubleshooting purposes, plug a 3 to 2 or cheater plug in the power cord of the BFD and see if the problem goes away. If the problem goes away, you now know where the problem is centered. (You could solve the problem by leaving the cheater plug in)

Next step: Just connect the cables from the BFD to the plate amp and remove the cheater plug. Do you get hum when you plug the power cable in but do not have the BFD powered? If you do not get any hum when just the BFD is connected to the plate amp and the power cord is plugged in, then you probably need to make up some 'special' cables that connect to the BFD.

Last step:
If you move along the chain of interconnects very logically and noting when the hum appears, you can get rid of it without using a cheater plug. What you have most probably is that 'ground' is at a different potential at different points in your system and you will have to work at troubleshooting if you don't want to go the cheater plug route.

* The other scenario is if I disconnect the power from the BFD (which makes the sub quiet), then hook up the input cables to the BFD. The one from the receiver is fine, but the one from the Behringer XO makes it hum. I haven't diagnosed this too far at the XO end, but I think it's similar - if the power cable is plugged into the XO, then it makes the sub hum. Or it's picking up hum because it's a pretty long cable and maybe isn't well shielded... don't know this one yet.

In order to track this problem down without going the cheater plug route, you will need to have numerous connectors (RCA, XLR male & female, 1/4 inch, etc.) on hand and then wire them up according to what grounding scheme will give the most hum free signal. It can be done with a lot of patience and following certain rules. Such as when wiring an RCA plug to an XLR connector only hook the shield to pin 3 and the positive to pin 2. DO NOT jumper pin 3 to pin 1. Or making an XLR cable that only has the shield connected at one end of the cable. There are too many combinations of scenarios to go through here but it can be done.

If you don't want to make cables then you will need a meter to be able to test and find out whether the shields are connected, not connected, or just connected at one end. Then you must mix and match appropriately. Good luck.

Chuck

[Saurav]

Ah, so it's the RCA-XLR cables that might be adding an extra kink in this. Thanks, I hadn't even thought about that. The Behringer is temporary while I fiddle with XO points, eventually I'll roll that into my active XO (which is currently handling the woofer-mid XO, the Behringer is doing the woofer-sub). So I can get back to that once I have just RCA connectors everywhere.

Next step: Just connect the cables from the BFD to the plate amp and remove the cheater plug. Do you get hum when you plug the power cable in but do not have the BFD powered?

This sounds like what I tried, so maybe I'm not understanding what you're saying. Here is the scenario that causes hum:

* BFD - power cable plugged in, powered off, output cables to plate amp
* Plate amp - turned on (power cable plugged in, obviously), input cables from BFD, speaker cable to sub.
* Both on the same power strip, going to a single outlet.

You could solve the problem by leaving the cheater plug in

I thought this was a safety no-no?

I'll pick up a cheater plug and see what that does.

[Dennis H]

Pics of the custom cables Chuck was talking about.

http://www.rane.com/note110.html

PS - better get two cheater plugs, one for each Behringer box.

[chasw98]

Pics of the custom cables Chuck was talking about.

http://www.rane.com/note110.html

PS - better get two cheater plugs, one for each Behringer box.

Thanks Dennis, I was too lazy to go find the URL.

[Saurav]

Wow, that's a great article. Thanks.

So what are the risks associated with adding a couple of cheater plugs and just leaving it at that?

Edit: After reading that, I realized that my CX2300 - BFD connection is through an XLR-RCA cable, and an RCA-TRS adapter. I could probably improve that by putting a TRS connector on the end of the cable, that way the shield connection goes all the way through without touching signal ground. But like I said, this may become RCA-RCA -> TRS in the near future, so for now I'm more interested in removing the hum that's coming from just plugging in the BFD power cord.

[ThomasW]

Wow, that's a great article. Thanks.

So what are the risks associated with adding a couple of cheater plugs and just leaving it at that?

Much less than the odds of being hit by lightning, or your house destroyed by a tornado, hurricane, or tsunami.....

[Saurav]

Heh, OK.

I also found this interesting little device:

http://www.zzounds.com/item--BEHHD400

Haven't tried to look up how it works. There are some other devices in the $50 range that go between the power cord and socket, and claim to break the ground loop there, while providing a solid ground. Again, no idea on how they work.

[ThomasW]

Need to be very careful with the devices that filter the audio signal, as often as not they remove frequencies we want to hear...

[Saurav]

So maybe it's a sharp notch filter at 60Hz? That would suck. I'll start with trying some cheater plugs

[cjd]

Do none of the Behringer products come with a "ground lift" switch? I know my Face Audio amp has that.

You have ground loop because multiple products are providing a direct path to earth along a common ground (likely that carried on the RCA cable). So, as long as you have all the cables hooked together, cheater plugs should, theoretically, never leave any product un-grounded. Some may just have a longer route to earth.

So, the biggest risk is unplugging the RCA cables from devices with the cheater plugs in place that actually need this earth.

Keeping everything on the same circuit and using a power strip will solve issues caused by being on different halves of the 220V into the house (it's +/- 110 at the box and there IS a central 0...) but not generally ground loops if your equipment has issues. Using pro gear expecting balanced and having either a floating or earthed neg signal path may contribute to this.

C

[Saurav]

So there's a chance that 2 outlets on the same wall in a room could be on different 110V legs? I think the upper outlets might be wired different since a switch controls some of them, but I have the power strip with BFD and plate amp, and the other power strip with everything else, plugged into the lower outlets on one wall.

Regardless, that's 2nd stage problem, first I need to fix the hum with only the BFD and plate amp connected to each other and nothing else. AFAIK, there are no ground lift switches on these boxes.

So, as long as you have all the cables hooked together, cheater plugs should, theoretically, never leave any product un-grounded.

That's a good point, didn't think about that. I also read about running separate wires to hook up 2 chassis, apparently that helps sometimes.

[Saurav]

Picked up a couple of 67c cheater plugs today. Putting one on the BFD took care of that hum. But the cable coming from the stereo turned out to be more of a problem. I'm not really sure where that's picking up the ground loop from, I disconnected various power cables at the stereo end and it didn't help. So in the end I just put one cheater plug on the plate amp, and that's taken care of all the hum for now. I'll try some other tests once I've moved the woofer-sub XO off the Behringer and onto my active XO.

And I found a thread on Audio Asylum where someone was unhappy with putting a $1 cheater plug on his $2700 power cord, and was wishing someone made a high end cheater plug.

[Dennis H]

And I found a thread on Audio Asylum where someone was unhappy with putting a $1 cheater plug on his $2700 power cord, and was wishing someone made a high end cheater plug.

Hey no problem. I have a $2700 cheater plug I'll let you have for only $1000.

[ThomasW]

PS Audio makes expensive power cords that have a removable ground pin.

[wettou]

Hey no problem. I have a $2700 cheater plug I'll let you have for only $1000.

What a :E why buy a power cord at $2700 is it solid Gold!!! Even Silver is not that expensive

[cjd]

What a :E why buy a power cord at $2700 is it solid Gold!!! Even Silver is not that expensive

You pay more for your car than the raw materials are worth also...

You're paying for the engineering.

Now, whether you find that a sound deal, totally different story. There's a reason I drive a $15k car and not one of the 3-series BMW's I was constantly beating back when I raced...

C

[ThomasW]

You're paying for the engineering.

Not much new engineering in a power cord unless it's made with nano-tube technology...

[impala454]

Just a quick hum question, if you use separates, and can go balanced XLR from the processor to the amp, is that pretty much it for hum since all the audio goes to the amp? Or can hum still be introduced by other equipment which is connected to the processor via RCA/HDMI/other cables?

[wettou]

You're paying for the engineering.C

Not even close that is snake oil!!

[cjd]

Oh, BMW's aren't *that* bad... definitely not snake oil.

Oh, the cord? :P

I didn't value the engineering or try to suggest you were getting a deal...

C