isolated ground for noise rejection

[dopplermhz80]

I have read a few different things about isolated ground systems and was looking for someone with real knowledge to set me straight. I recently installed a subwoofer and amplifier for my home theater. I have a terrible hum from the speakers when there is nothing on except for the amplifier. So I have read about the ground isolation. I have plastic boxes in my house. So I cannot ground the recept. box to the breaker ground bar. I could only run a 14/2 NM. The copper went straight back to the breaker ground. I still had a hum. I guess my question is... Do I have to have a metal receptacle box and 14/3 wire in order to lose the hum? Is an earth grounding rod a good idea? Any suggestions would be greatly appreciated.

 

[iwire]

With plastic boxes supplied by NM and assuming this circuit runs back to your service panel it already is an isolated ground at the receptacle.

Be aware we can not give advice to do it yourslfers here at this forum.

 

[haskindm]

As iwire said, you already have an isolated ground receptacle. You may try an isolation transformer for this unit. They are fairly inexpensive and available from Radio Shack. You may also try connecting the amplifier in other circuits (perhaps using an extension cord) to see if another circuit causes the hum to dissapate. There may be something else on this circuit that is inducing noise into the system (assuming that this is not a dedicated circuit). Of course it is also possible that you have a bad amplifier. Has it ever provided a clean signal in another location?

 

[petersonra]

isolated grounds are not especially effective at reducing 60 cycle hum, which seems to be your problem.

additional ground rods will not resolve it either and might cause a hazardous condition to be introduced if done incorrectly.

you might want to go to your favorite electronics store and see if they have a plug in RFI filter. you would plug this into the outlet supplying power to the amplfier and plug the amplifier power cord into the filter.

or try powering the amplifier from another outlet.

 

[dopplermhz80]

Thank you all for your advice. The amplifier is brand new and has never produced clean sound from any outlet. Perhaps isolated ground is not the correct terminology. I ran a dedicated circuit with no resolve. I will try the RFI filter. I had what I thought was a great idea, to buy a surge protecter with RFI/EMI filter built in. This did nothing for me. I fear that if nothing helps, then I must resort to spending $500 on a speacialty device to filter and regulate power (maintain constant 120V). But again thank you all for such a speedy response and keep up the good work.

 

[zbang]

Does the amp hum without the incoming signal leads connected? If so, then the amp is probably faulty, take it back. Otherwise, your problem probably has nothing to do with power. Look for a grounding problem in the signal leads; it's likely that you have a ground loop there. (Try a search for "ground loop".)

One potential solution is to lift the -signal- ground at one end of the connection (not the equipment ground). Another is to install a signal isolation transformer in that line, which is what the sound pro's do.)

(try asking on a home theater forum or USENET newsgroup)

z!

 

[haskindm]

The isolation transformer that I referred to above is supposed to provide a "clean ground", RFI filter, etc. They are about $150 for a 500 watt unit. It is really starting to sound like you have a bad amp.
DO NOT connect this amp to a seperate ground rod. This is a violation of the NEC and VERY DANGEROUS. An isolated ground receptacle allows you to take an insulated grounding conductor back through any sub-panels in the system to the ground-neutral bar at the main service. If you have a dedicated circuit installed in NM cable and a plastic box, fed from your main residential panel, then you have an isolated ground. There is nothing that you can do to "clean" this ground.

 

[LarryFine]

Doppler, if I may, try looking at this problem from the low-voltage side of things. Do you have cable TV? If so, try disconnecting the incoming (from the wall to the tuner) coax cable. I bet the hum stops the moment the coax shield loses contact. If so, let us know, and I'll suggest more.

 

[shockme77]

I realize it's a little late to be suggesting this now, but, wouldn't AC or MC cable cancel out any stray magnetic fields and therefore cancel out any humming noise in the speaker?

 

[dopplermhz80]

Thanks Larry, removing the coax cut the hum in half, or so. Not completely gone. You see, I have a home theater hook up and the wires are all in wall. The subwoofer cable and the coax (as well as other speaker wires) all run in the same cluster. Do you think this may cause some problems as well? I really dont want to go into the attic again. I don't know how you guys do it. O do you think that if I ran a dedicated 15A circuit for the sub would help? I think I have a lot more troubleshooting to do. I sincerely appreciate your input, as well as everyone else. Electricity and wiring is not my specialty. I do ultrasounds for a living. The physics may be somewhat similar, but that's about it.

 

[dopplermhz80]

shockme, what is the MC cable? Oh you guys probably know this one too, do I have to run AC wires through 2x4's in the attic, or can they just lay on the insulation?

 

[Jacob S]

I agree with others. The problem is most likely not a result of your AC wiring and circuit configuration. Make sure you hook the amp to the speakers with the input to the amp unplugged to make sure the amp is ok. If the buzz goes away, you know the problem is somewhere else. Then, I would hook the receiver to the amp (with all other connections to the receiver disconnected). If you have a buzz, you know the problem is between the receiver and the amp and an isolation transformer on the signal lines to the amp will probably fix the problem (like others have suggested). If you hook the receiver to the amp with no buzz, your problem is most likely somewhere else. By connecting equipment one by one to the receiver, you should be able to figure out what the source(s) of the buzz is. Let us know what you find.
Hope that made some sense.
Jacob

 

[dereckbc]

You need to isolate the problem to either power problems or components, and quit guessing.

To do this here is the how too:

Get yourself a term strip capable of pluging all your A/V components into it. It doesn't make a bit of difference if the term strip is on a dedicated circuit or not. What you are after is a single point ground, and common neutral.

Turn everything on and listen. If you still have noise, it is one or more of your components. If it is clean, you got a power problem.

 

[zbang]

Also, while you're at it, do a web search for "ground loop"- http://www.epanorama.net/documents/groundloop/ is a nice one, so is http://www.jensen-transformers.com/an/an004.pdf. Larger home theater systems and most professional installations can have these problems. The pros go to great lenghts to avoid them.