Musicians, Amplifiers and Grounding

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jcook980

jcook980

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Gresham, Oregon
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Electrical Engineer
Musicians sometimes have a great deal of audible hum generated by ground loops caused by the signal connections between their instruments and signal processors like amplifiers and mixing boards. Many (far too many...) musicians carry 2-to-3 prong adapters with them to gigs to lift the mains ground to their instruments. In most cases, this cures the hum but obviously eliminates an important electrical safety feature.

It would seem to me, that a far better solution would be to use a portable GFCI adapter. But, every one I've seen apparently carries the ground through the adapter since they all have 3-pin plugs. A kludgy way to do this would be to use the 2-to-3 adapter to lift the ground and plug the portable GFCI adapter into that. What's really needed is a NRTL listed portable 2-pin to 3-pin GFCI device.

Now I know that many will recoil in horror at this idea. As an electrical PE, I don't like it either. But the bad practice of lifting the mains ground is going to continue and musicians are going to continue to get injured and killed when an internal fault occurs. Surely the GFCI will restore some of the protection offered by the mains ground.

Comments or other ideas?
 
Re: Musicians, Amplifiers and Grounding

Go wireless and use hum eliminators like ART CleanBox II for example. They are cheap and effective.
 
Re: Musicians, Amplifiers and Grounding

I have long argued that the problem should be fixed at the signal side but the price of isolators or direct boxes keeps many from going this route. They'd still rather float the mains ground. A GFCI solution, if practical and safe, might be cheaper and therefore more attractive.
 
Re: Musicians, Amplifiers and Grounding

I have long argued that the problem should be fixed at the signal side
Click to expand...
Nope
The problem should be fixed at the power supply side. If this is not possible then the the signal route should be taken but the grounds should never be lifted ever! When someone get's electrocuted from a lifted ground I would not wan't to be on the bad end of that law suit!

The most common cause of a 60hz hum in a P.A. system is current flowing on the grounding system in the building. Most of these can be traced back to improperly wired circuits and or sub panels. If there is current on a EGC for a branch circuit, it must be found what is causing it and fixed (IE. bootlegged neutrals bad extension cords etc...). If the PA system has a sub panel feeding some of the equipment then look for the neutral bonded to the ground in the sub panel and or only 3 wires feeding it as this is also not allowed per the NEC. Most NEC compliant installations will provide a grounding system that is free of common mode current and will not produce any hums in a PA system. Other hums can be caused by dimmers but you will know them from the different frequency of the hum and if the dimmer is changed so will the hum. Knowing what to look for and how to go about tracking down the source is very important to having a hum free system. Hope this helps
 
Re: Musicians, Amplifiers and Grounding

How is a gigging musician supposed to control the electrical system where he or she is performing? I know that is where the problem is likely rooted but the occurance of this problem is often very random. Unless every performer complains, the venue owner is likely to ignore electrical noise complaints.

The problem typically occurs most often with grounded instruments connected to amplifiers or house mixers. Lifting the power ground or using a DI box to isolate the signal always cures the problem. A 2-pin to 3-pin adapter is far cheaper than a DI box, so it should be no surprise that many musicians always toss a couple into their gig bag and compromise their safety.

In the absence of an inexpensive, listed GFCI product that could safely isolate the ground, I'll recommend keeping the instrument "solidly" grounded and use the ART DTI to isolate the instrument from the amplifier or house mixer.
 
Re: Musicians, Amplifiers and Grounding

Gig musicians are at a disadvantage. On the other hand the venue opperator may incur a great expense to get the repairs done.

I am the sound technician for our church. We have a building that is over 100 years old, and quite frankly, has its share of problems. I have been working for some time now trying to find the source of the noise in m system, and have had a terrible time. It could be because of the 5 sub-panels in the building. I have tried my darndest to isolate the hum to one or a half-dozen circuits, but had no luck so far. Even if I do find it I'm not sure if I will have access or means to fix it.
 
Re: Musicians, Amplifiers and Grounding

I am probably in need of an education on this subject as much as the next guy.

But.

In my experience, I haven't seen the ground prong make any difference.

It's been a while since I've had any of this kind of equipment disassembled but I remember the typical power supply being a transformer followed by typical rectifying, filtering and regulating circuitry.

Up to this point you have a bipolar DC supply that is electricly isolated from the AC supply source by it's transformer. It's also floating.

If I remember right, The 0 volt of the bipolar supply gets a chassis ground. Also causing there to be a building ground on the DC side of the supply.

The reason I've gone so far into this is because this is the point where the noise would be introduced. And I'm interested in any thoughts on what happens there.
 
Re: Musicians, Amplifiers and Grounding

Sam read my post above.

I Will add the reason current on the building and branch circuit grounding causes a hum in audio equipment is because different pieces of the equipment have been fed from different circuits. And the low level audio cable shield that runs between these pieces of equipment will be a parallel path for this current. This current is now running on the shield and will induce the 60hz AC hum into the amp. This is why I said finding the source of the current on the grounding is the proper way to remove the hum. But if this can not be achieved then a 1 to 1 isolation transformer 600 to 600 ohm or 1k to 1k ohm needs to be installed in every path between equipment and the main PA amps. The grounding should never be removed from equipment as this can present a shock hazard.

Yes the hum might be irritating but being electrocuted will be much more irritating to the people who care about this person that has been electrocuted and you could wind up in court.
 
Re: Musicians, Amplifiers and Grounding

I may never have had this problem because of everything being on the same circuit?
 
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