Sorta OT but audio related: GFCI-in-a-box as safe ground lift for AC equipment

[Processaurus]

I've been doing electrician type work at my house recently, and learned a bit about how and why the third prong is important for safety, but most audio people have experienced lots of trouble with buzzes and humms from ground loops, connecting together their AC powered equipment.  A lot of times the quick and dirty solution is to use a cheater plug to turn the 3 prong cord into a 2 prong cord, which will work, but defeats the safety afforded by the 3rd prong and puts you back in the 30's when electrocution was more common.  Arguably the audio cable's shield acts to ground the equipment back through the other equipment's third prong, but that trusts that the audio cable and internal wiring of chained equipment can carry a full 15 or 20 amps of current from a faulty piece of AC powered gear, enough to trip the breaker, and that the audio cable won't be unplugged while the power is on, two serious maybes...

In wiring my bathroom, I learned the basics of how Ground Fault Circuit Interrupters work, in that they monitor the AC current going in the hot wire, and out the neutral, and makes sure it is the same.  If current goes through the hot wire that doesn't come back in the neutral, it assumes current is being  channeled to ground, by a different path, like a person standing in a puddle, etc. and the internal circuit breaker trips, turning off power to the outlet.

GFCI's can be retrofitted as a safety measure into old homes with two prong outlets and no safety ground wire coming out of the wall, as long as they are labeled "No Equipment Ground".  I wanted to run the idea past the knowledgeable people here, of building a safer cheater plug, a 3 prong to 2 prong power adapter that is a UL approved box, like a raco handy box (maybe a 4"x4" box so it sits flat on the floor) with a GFCI receptacle in it, and a heavy 12 gauge power cord coming into it with a polarized (one blade bigger than the other) two prong plug.  The ground would be left floating, breaking the ground loop, but, if there is a fault in the equipment, the gfci trips and cuts off power to the outlet.

The usual caveats of these kind of projects apply:  don't do any AC wiring unless you are 100% confident you can do it safely, remember you are responsible for the things you build to be safe forever, like when your grand kids find it in the attic in 30 years.  Also, with GFCI's, they need to be tested throughout the year with the little button on them.  You can solve ground loop problems with transformers in the signal path, Jenson has many excellent app notes of how to do this.

[GibsonGM]

By all means, use a GFCI as your power source for PROPERLY WIRED equipment.  But please don't do this as a means of providing a ground lift.

Yeah, it's been done forever (and lots of folks have died because of it), but you know - properly designed equipment fed by good power doesn't make a ton of noise that would require a lift.   We're not setting up a concert stadium in here - OUR stuff should be pretty quiet as it's designed.

I think of stuff like this as trying to make a net on the ground under a roof, where someone is working without a harness....once you go over the edge, kinda late, isn't it?  That's what a harness was for!   Well, our earth grounds are our harness, and are there for a good reason.     Neat thinking, but I'd use the GFCI in addition to proper earth grounds...

[Processaurus]

Yeah, it's been done forever (and lots of folks have died because of it), but you know - properly designed equipment fed by good power doesn't make a ton of noise that would require a lift.   We're not setting up a concert stadium in here - OUR stuff should be pretty quiet as it's designed.

Ground loops happen with properly designed equipment, including line powered pedals with a 3 prong cord, as the problem is outside of the equipment - it is differences in ground potential causing current to flow from one outlet to another, through the audio cables, causing little voltage drops and polluting the earth ground as a solid signal reference to zero volts.  It's not the fault of the equipment, beyond that the inputs are unbalanced, and the chassis ground and the signal ground are connected together.  I would bet anyone that uses an older pedal that is line voltage powered has had some kind of noise problem from the safety ground, using pedals like the old EHX memory man with a 3 prong plug, or the other day for me it was a mu-tron phasor II that has a 3 prong plug that was causing lots of noise on my pedalboard until I used the ill advised cheater plug.  Also anyone that uses 2 amps for their guitar rig will likely have trouble, if not in their practice space, then out gigging.

  Neat thinking, but I'd use the GFCI in addition to proper earth grounds...

The idea here is not to just lift the earth ground indiscriminately on all of the equipment, but on one of them, like for example, one guitar amp in a 2 amp rig, or a line powered pedal that is plugged into an amp.  There still is a proper earth ground, through the audio cables to the grounded piece of gear (for us here, probably the amp), it's just to get rid of multiple earth ground connections to multiple outlets that are causing noise problems.  The grounds of everything (guitar strings>pedal chassis's>amp>receptacle 3rd prong> earth ground) are all connected safely the earth ground, it's just that here with the proposed GFCI thing, the extra piece of line powered gear with its 3rd prong ground connection lifted will be turned off immediately if it starts to operate unsafely.

Just wanted to check in with the people here if anyone could think of a specific scenario where this would be dangerous.  It is a bandaid, ideally things like this wouldn't be necessary, but I was shooting the idea around because I know most people in audio who are desperate to get noise out of a system would turn to the cheater plug despite it removing a layer of safety.

[ashcat_lt]

There are a lot better and safer ways to deal with a ground loop.

The first step is to plug everything into as close to the same outlet as possible.  The same power strip is usually the best you can get.  

If that doesn't help, the next step is to lift the ground connection on the audio side of things.  Cut the shield on the cable at the input end of the connection.

A transformer can also be used to isolate the grounds.  Some folks I think prefer this to lifting the shield.  I'm of the opinion that straight wire is preferable to relatively large, expensive, possibly noisy and/or lossy components.

If that doesn't help, then it's likely that there is a problem with one of the pieces of equipment, and you should track it down and fix it.

The GFCI thing is great in conjunction with the safety ground as are often faster than the fuse in the amp or the circuit breaker at the box, and in serious fault situations microseconds can matter.  It is also not a bad idea as a cheap temporary fix for an old piece of gear without a proper safety ground, but it would always be better to ground the thing properly.

[GibsonGM]

The GFCI thing is great in conjunction with the safety ground as are often faster than the fuse in the amp or the circuit breaker at the box, and in serious fault situations microseconds can matter.  It is also not a bad idea as a cheap temporary fix for an old piece of gear without a proper safety ground, but it would always be better to ground the thing properly.


I know exactly what constitutes a ground loop, and why older components were designed to be 'lifted', Processaurus.   I'd be a lot more convinced of the safety of the approach doing it the old fashioned way, literally being sure the chassis' are bonded to each other with one lifted.  For many, being certain this was done correctly may be above their level of experience, so you noticed my balkiness at the idea of relying solely on a GFCI to protect you.  Sorted sounded like a cure-all, which it's not, and altho you, me and dozens of others may know what's going on there, there could be the new guy who doesn't, so it's worth talking about.

The 'different outlets' part is especially problematic for me.  I've played in too many miswired venues to have any faith at all in setting up more than a one-outlet rig, fast, and having faith in its safety.   I've done it dozens of times with no bad results, but then that ONE time I didn't test the outlet and almost got knocked out from a miswire between 2 outlets...those hazards can be anywhere.  Lil' different than differing ground potentials, but you see what I'm saying, I'm sure.

Ashcat said it, use a GFCI for extra assurance by all means, but make sure what you're doing is correct. There would be no NEED for a GFCI if you're bonding two properly wired chassis' with no internal issues.  If a line-powered pedal is not playing well with other ones, find out why and correct the issue.  As long as you understand how to do it, for real.    Just my 2 cents, I'm not knocking your idea but pointing out that there could be more to it than just popping a GFCI in line...