Ground Noise Filters

[LPdan]

Hello,
I'm helping a bar that has an audio system rack and is trying to connect a previously installed jukebox to it.
The jukebox was wired with L/R cables (RCA over coax), and would be difficult to replace. We are getting bad ground hum/buzz from the jukebox into the audio rack.
We tried powering the jukebox through an extension cord from the rack, and the hum went away.
I've seen RCA ground hum eliminators, wondering if people find they work, or if there is a better way to resolve without rewiring?
Any thoughts would be greatly appreciated!

 

[MNicolai]

Yes, you could try an ground hum eliminator. Only on the low-voltage audio cables. Never lift the ground pin or use a ground pin adapter on power cables.

The principle is that ground hum eliminators have an isolation transformer in them which inhibits the ability of ground loop current to run across the audio cabling.

I would use an RDL AV-HK1. There are certainly cheaper options on Amazon you could try first for a fraction of the cost (and for how cheap they are, probably are worth trying first), but I wouldn't be shocked if some of them are low-quality or not engineered correctly (or not engineered at all). It's a common product for car audio installations so there are a million options online from companies that nobody has ever heard of, some of which are certainly scams.

There are also some RDL AV-HK1's on eBay for $40-60. Since there's almost nothing in them that can break, I wouldn't be concerned about getting a damaged product.

 

[almorton]

Sounds to me as if you need to resolve why you're getting a bad ground hum, rather than trying to mask it. If powering from the same source eliminates it, it suggests an earth problem elsewhere that needs to be investigated.

 

[LPdan]

Yes, you could try an ground hum eliminator. Only on the low-voltage audio cables. Never lift the ground pin or use a ground pin adapter on power cables.

The principle is that ground hum eliminators have an isolation transformer in them which inhibits the ability of ground loop current to run across the audio cabling.

I would use an RDL AV-HK1. There are certainly cheaper options on Amazon you could try first for a fraction of the cost (and for how cheap they are, probably are worth trying first), but I wouldn't be shocked if some of them are low-quality or not engineered correctly (or not engineered at all). It's a common product for car audio installations so there are a million options online from companies that nobody has ever heard of, some of which are certainly scams.

There are also some RDL AV-HK1's on eBay for $40-60. Since there's almost nothing in them that can break, I wouldn't be concerned about getting a damaged product.

Thanks, I'll give it a try!

 

[FMEng]

Sounds to me as if you need to resolve why you're getting a bad ground hum, rather than trying to mask it. If powering from the same source eliminates it, it suggests an earth problem elsewhere that needs to be investigated.

The hum does not indicate a problem with the AC wiring. The noise is caused by a ground loop inducing noise into the long, unbalanced audio lines. This is a very common problem and Mike's solution is the best way to resolve it.

 

[almorton]

Yes, probably exactly so, but I'm us usually keen to eliminate causes than symptoms. I agree, it's a very common problem with a well known workaround by using transformers.

 

[TimMc]

There is a potential difference between the equipment grounding conductor in the booth and the EGC at the jukebox; it IS possible for a couple of mis-wire conditions in the mains AC (at the outlet, usually) that will contribute to this problem, but most likely the difference in potential comes from each use being served from a different breaker box.

 

[FMEng]

Yes, probably exactly so, but I'm us usually keen to eliminate causes than symptoms. I agree, it's a very common problem with a well known workaround by using transformers.

Audio transformers or active balancing amps and receivers should not be considered work arounds. Rather, they are the correct way to make the interface between the two pieces of equipment, when an input or output is unbalanced. If you want to get to the root cause, you have to look at the entirety of the system, from the AC wiring to the internal design of the audio equipment and the interconnections between. Then you'll understand why blaming hum and buzz just on the AC wiring is based on myths. Whole books have been written on the subject of noise in analog audio systems.

I have over 30 years experience of building large and complex audio systems for broadcasting, and a fair amount of it in buildings with old electrical wiring. I'm pretty confident in my knowledge of analog audio interfacing.

 

[MNicolai]

Agree with @TimMc and @FMEng.

It's not outside of the realm of possibility there's a wiring issue on the power side, but the presence of a ground hum is by no means any sign that's the case. Typically it's that you have power circuits fed from different panelboards where they have different ground potentials. There are any number of reasons that could be the case. Sometimes the solution is to use an isolation transformer on the signal cables. Sometimes, for balanced signals (which RCA is not), you can also lift the ground pin of the [signal] cable on one end.

Had a project a few years ago -- large sports arena with broadcast equipment. Part of our scope was adding an isolated ground power distribution system. That involved a power-side isolation transformer with multiple panelboards, a company switch, and shore power to the OB trucks -- and long distances between them. The way to maintain a low-noise grounding scheme in that configuration is to upsize the conductors bonding everything together. Code-minimum conductor sizes are based on safety. When it comes to audio and what could be audible ground hums, it's a matter of making the grounding and bonding components have the lowest impedance reasonable by going above and beyond what is required for safety so the ground potentials between these distant panelboards is as equalized as possible. This is where electrical engineers go nuts because eliminating audible ground hums in audio cabling is a completely different beast from providing power distribution that won't kill anyone or set anything on fire. That's something you do when you want to avoid having to use low-voltage isolation transformers on tons of your signal cabling, but if you're talking a couple channels of audio cabling, adding an isolation transformer on the signal cabling is a perfectly viable, right-sized solution.

 

[almorton]

That's the sort of investigation I was thinking, Mike. But I'm not pedantic over this; if there was a quick fix (like ensuring ground potential strapping was quick ...) then I'd want to look at that, but I appreciate that there are other situations.

I'd still want to be sure there wasn't a potentially hazardous fault, though.

 

[MNicolai]

I'd still want to be sure there wasn't a potentially hazardous fault, though.

You could use an outlet tester for that. There are other ways with a DMM that would be more comprehensive, but without knowing your electrical knowledge and comfort level messing around in someone else's panelboard, I'm not sure I would recommend them so you don't risk blowing up a DMM.

You could also deenergize the circuit and pull the receptacle off the wall and do a visual inspection for anything hinky like a missing ground wire or if the neutral is split between the neutral terminal and ground terminal. Those would be the most obvious signs of shenanigans. To be clear, the neutral and ground networks should be bonded together, but only at the service entrance. Sometimes in much older buildings that didn't originally have ground conductors to all the receptacles, someone would swap the receptacles to add ground pins and just tie the grounds in with the neutral at every receptacle. I have never seen it personally but I have heard of it being done, and the only fix for that would be to pull ground conductors everywhere. It would be unusual to see that in a commercial building though -- that's something you more likely to be done in residential by homeowners who know enough to be dangerous.

 

[almorton]

That's the sort of thing I was thinking of - soggy neutral, bad bonding, loose screws in receptacles, that sort of thing. My experience with power circuits is UK 240V 50Hz (and associated 3-phase) but the principles of earth bonding at incoming neutral and star point and so forth are largely the same.

 

[TimMc]

You could use an outlet tester for that. There are other ways with a DMM that would be more comprehensive, but without knowing your electrical knowledge and comfort level messing around in someone else's panelboard, I'm not sure I would recommend them so you don't risk blowing up a DMM.

You could also deenergize the circuit and pull the receptacle off the wall and do a visual inspection for anything hinky like a missing ground wire or if the neutral is split between the neutral terminal and ground terminal. Those would be the most obvious signs of shenanigans. To be clear, the neutral and ground networks should be bonded together, but only at the service entrance. Sometimes in much older buildings that didn't originally have ground conductors to all the receptacles, someone would swap the receptacles to add ground pins and just tie the grounds in with the neutral at every receptacle. I have never seen it personally but I have heard of it being done, and the only fix for that would be to pull ground conductors everywhere. It would be unusual to see that in a commercial building though -- that's something you more likely to be done in residential by homeowners who know enough to be dangerous.

"Bootleg ground". Toss in some reversed-polarity wiring at an outlet and they've energized the entire grounding system. Potentially lethal.

I think of it like an old Fender guitar amp with a mains AC "polarity" switch. Flip it one way and things are relatively safe, flip it the other way and the amp chassis is hot, along with the guitar strings. PA microphone is properly wired, mixer and other equipment is properly grounded/earthed. Guitarist's lips touch the mic grille and funeral ensues. And yes, that's happened.

 

[LPdan]

Thanks for all the additional responses!
We did verify correct AC "polarity" and ground at the rack and juke box. I've ordered the transformer isolator, will give it a try and report back.

 

[TimMc]

Referencing @MNicolai and @FMEng suggestions about transformers and telescoped signal grounds on balanced pairs... these come from 100+ years of telephony and broadcast best practices. Back when I was even younger than Ron Hebbard gave me credit for being a lot of what I got "skooled" in were the why's of best practices. I came to appreciate the beauty of MIL-SPEC electronic assemblies; the elegance of lacing cord support, Hellerman sleeves, and wire dressing (white tie, black tie, tails? ). It got to the point that when I encountered a systemic way of doing something in a repair or service situation that I emulated, best I could, the existing termination methods and wire dress. I figured it was done "that way" for a reason and, not knowing what that reason was, decided to maintain consistency.

Clair Global has a standard: any time a signal is imported/exported outside the Clair Ecosphere to another vendor or venue connection, it is transformer isolated. In the bottom of a drawer in a work box, they have a dozen or more iso transformers tagging along with every tour.