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Peter Pan 5th May 2011 02:26 PM

Ground loop isolation to get rid of hum?
Not sure where to put this thread, but the music area seems a good start. In my band, we have a completely self contained system in which all our sound sources get routed through our own mixer. We are very careful about ground loops to prevent excessive hum in performance situations. Ground lifts on virtually everything, a single point to allow grounding when needed, all 120 v equipment has its ground pins cut off, and are plugged into the same "Octopus", etc., etc. All we have to do is plug a pair of line level R/L RCA outputs into a sound system, and we're good to go.

But you know the story! If the main "house" soundboard is in a remote location, and conversion from ground referenced RCA style line to balanced outputs aren't provided, then all it takes is one little bit of leakage and you have a ground loop between our system and the main PA. The result of course is that all you can do is choose between one level of hum or another, based on whether or not your single point ground lift helps.

So what i want is to have our own "emergency" isolators. Something that would allow us to plug our line level RCA outputs into one side, and have completely ground isolated outputs, both RCA and XLR. maybe such a product already exists and is not too pricey, but my experience is that in the world of performance electronics, the simplest item is pricey enough to be able to easilly build 10 for the over the counter price of 1.

Any ideas?

(Moderator: Please feel free to move this post if there is a better area for it).

chrispenycate 5th May 2011 02:44 PM

I'd go through jack, rather than try and stay in RCA. There are any number of good, passive DI boxes out there, giving you isolated, balanced outputs (passive because you don't need power supply, no batteries to go flat, and your mixer has a low impedance drive anyway). Optimum would be a stereo box with independent ground lift switches.

Of course, this being a DIY forum, perhaps you would like to build your own?

Even if an active transformerless output would do the job a little cheaper, I do recommend keeping the iron in there; it gives a much more total isolation. And you can carry your XLR-RCA adaptors with you to unbalance it again, without re-introducing ground problems. Here, however, you're probably going ti find you've lost considerable level (DI boxes are not one to one). Generally, though, an RCA input is minus ten dBs sensitivity, while the mixer can easily drive zero, and this ceases to be relevant.

nightanole 5th May 2011 03:01 PM

Isolation transformers?

EDCOR Electronics Corporation. WSM6400

Just connect them in line, signal and ground on one end, signal and ground on the other end.

Peter Pan 5th May 2011 04:28 PM

@ chrispenycate Not sure what you mean by "go through jack", but I can tell you that mainly due to space considerations, our little Berringer (sp?) mixer has only RCA and 1/4 inch line level outputs (no balanced outputs). As far as the receiving end goes, if the room already has a DI box on stage, of course that solves everything. But sometimes all they have are extra XLR mic inputs), or as happened to us last Friday, actually had RCA jacks on the wall, wired into the sound system. As you can imagine, they were a problem in the making. The hum wasn't bad, but it was disturbing that I was powerless to do anything about it. And yes... I'm much prefer a DIY solution :-)

So... I need to have a box that will cover both cases. And...

@ nightanole yes.. I agree... isolation transformers would probably solve everything.

But here's the bottom line.... these ground loops only seem to rear their ugly heads at performances. So if I am going to DIY this, I think I'd feel better implementing an existing design, in whole or in part. I wouldn't even know (for example) what transformers to buy for such a project, and which kid would leave me replacing ground loop noise with electricl or magnetic pickup noise. Any suggestions? :-)

Peter Pan 5th May 2011 05:09 PM

I guess if I used transformers (like the EDCOR Electronics Corporation. WSM6400) nightanole suggested, I'd almost have to add some op-amps to restore line level again. I can't think of any other way to maintain isolation, low impedance output, AND maintain line level. Of course if I make it a battery powered thing, it won't introduce any additional grounds or hum. And if I use two transformers back to back (10K->600ohm->600ohm->10K), then a a simple (gain=1) op amp circuit would be all thats needed to regain low impedance. Right?

nightanole 5th May 2011 06:02 PM

You are miss reading, if you wire them like i said then they are just isolators and dont need opamps and stuff. What goes in is what goes out. so wired as i put its is 10k/10k no level shifting. You can also wire them for stereo to mono conversion, and 10k/600ohm adapters. The 10k means they will take down to a 10k load and be fine.

Peter Pan 5th May 2011 07:28 PM

@nightanole: Ok, I'm a little confused how the same centertapped output secondary could be both 600 ohms and 10K. But assuming it is and can be wired as a 1-1 transformer, is it possible I might need a no gain (gain=1) driver amplifier on the output side? Consider that the mixer output probably does have an output amplifier to ensure that as a signal source, its impedance is very low. If I drive a 1/1 transformer that is basically 10K->10K impedance, is it possible I'd be creating a new avenue for noise pick up to get in? It just always seems to me that high impedance sources are greater noise makers than low Z outs. And line level is almost always pretty low Z out, right?

chrispenycate 5th May 2011 08:55 PM

If you use a 1-1 transformer, the impedance is reflected through, even if the current drive isn't. And the quarter inch jacks on the output of your Behringer; are you certain they're not three pole balanced? A lot of Behringer gear is wired like that (I can't remember if it's servo balanced or just shorts out one leg and loses 6dB when you plug in a guitar jack).

As regards ohmic separation for ground currents, just about any transformer will do; concentrate on getting a good frequency response. There should be minimal risk from capacitive coupling between windings, as the grounds should both be at the same voltage apart from minimal inductance currents.

If you want to drive high levels into a low impedance, the trabsformer is going to be big and heavy, Reduce the level, or increase the impedance (that pair of RCAs is probably at something like 50k, hi-fi values. Bleh.) and the transformer shrinks nicely. But it's a good idea, if you can manage, to have transformers capable of taking anything the desk can pump out:- +18dB into 300Ω, perhaps, screened. And that's chunky. Something you want to attach to the box, not a PC.

There should be no need for an active driver; I haven't got the specs for your Behringer on hand, but it should have no trouble driving any reasonable line. And a screened transformer (or a steel box, rather than a diecast) will eliminate all reasonable electromagnetic interference from there (although not from guitar pickups and leads, Fender Rhodeses, Mellotrons…

Check about those quarter inchers; it could be that a simple ground lift cable will solve - no, better to have switches. then doing comparisons is immediate.

What model number is the mixer? I could check up on its output wiring.

Peter Pan 5th May 2011 09:46 PM

Thanks chrispenycate . Well there's a question that sent me hastily rushing to re-read the manual! But no, it plainly says the main outputs are unbalance Mono jacks. Its their Model XYNYX- 802. Oh wel... still an amazing mixer for under $100! VERY quiet Mic pre-amps, but thats another story. But OK.. thats good to know about the impedance being reflected through from one side to the other. Geesh... it has been a LONG time since I learned anything about that in school :-(

Well size and weight is a concern too. And I DO remember from tube amp days how the output transformer on a good amp was significantly bigger (and heavier) than the power transformer, for having to work with lower frequencies. Since I am pumping full range audio through this thing, it does make even more of a case for some kind of active amplifier, if only to allow a small transformer to do the job of a bigger one. Just let the transformer output drive a hi-Z amplifier input.

Andy yes, controling grounds has been very helpful! With virtually every piece of gear now running on a transformer, all my power feed cords except one have their ground pins cut off. The one that still has it is broken out with an alligator clip so I can decide to manually clip power system ground to an audio ground, for the slim number of case where that will actually help.

I'm just trying to have a bag of emergency tricks at this point. Unfortunately, its nearly impossible to predict in advance what you're going to run into when you jack your mains into a house system, and you often don't have a lot of time to remedy things. I'm always buying and building little "magic" solutions, just in case one will work in a pinch.

Hey speaking of which, here's a little ground isolation thingie by Crutchfield. It's only $15 so i ordered one. Wonder whats under the hood?...

PAC SNI-1 RCA Ground Loop Noise Isolator at

metalsculptor 10th May 2011 07:10 AM

If you want a wideband solution with CMRR over 120dB an isolation amplifier will do the trick. Isolation available up to 3Kv or so, a DC to DC converter module is needed to power the amplifier on the isolated side.

For something slightly less demanding buy or build an in line instrumentation amplifier. It still puzzles me why audio equipment uses ground referenced signals given the amount of effort spent on removing ground loops, time much better spent on a few dollars of op amps to get a differential input or output.

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