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Author Topic: Home audio AC buzz  (Read 6301 times)
WV6U
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Posts: 33




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« on: January 24, 2014, 11:47:18 PM »

Sorry, this isn't directly related to amateur radio but since several people here are experts at dealing with EMI/RFI, I thought I'd better ask my HAM community for help rather than the audio community.

I have a home theater setup in my basement with a LED projector, Yamaha AVR and Crown external amplifier. A Blu-Ray player feeds audio+video to the AVR via HDMI. Video goes out to the projector via HDMI from the AVR. And, the AVR's pre-outs for Left/Right front speakers go to the external amplifier via RCA cable. The projector and external amplifier, both, have three prong power plugs. The AVR has a two prong with no chassis ground lug on the back. The power outlets are all properly grounded.

The issue is that I hear a buzz in the speakers with this setup. So I tried various combinations to isolate source of the buzzing noise. Disconnected RCA inputs from the AVR to the amplifier and the buzz went away. The buzz stayed even the AVR is turned off. No buzz if I connect speakers directly to the AVR. I changed the RCA cable and buzz stayed. Finally, I found the culprit - it was when I disconnected the HDMI cable going to the projector that the buzz went away. Next, leaving the HDMI to the projector plugged in to the AVR, I disconnected the projector from the power outlet and the buzz went away. I went further and found that simply touching the ground prong of the projector's power plug to the power outlet induces buzzing noise in the speakers (via the AVR and amplifier).

Now to eliminate the buzz, I introduced a Behringer HD400 buzz eliminator between the AVR and amplifier. RCA pre-outs from the AVR go to the Behringer and Behringer's output go to the amplifier's inputs via RCA. This killed the buzz.

However, I wasn't satisfied using the hum eliminator. I wanted to eliminate it at source, that is, the projector. So I went to the projector and unplugged the power cable with the intent of replacing the power cable or testing placing a surge suppressor between the projector and power outlet.

With the hum eliminator between the AVR and amplifier and AVR connected to projector via HDMI, I disconnected the power cable from the projector. To my surprise, a loud buzz re-appeared. I plug the power cable back in and the buzz goes away. With the hum eliminator in place, I plug out the HDMI between AVR and projector and the hum re-appears. Plug HDMI back in and hum goes away. This has me scratching my head.

Projector <-----AVR------>ExternalAmp == Buzz
Projector <-----AVR------>HumEliminator--->ExternalAmp == No Buzz
(Projector disconnected from AVR or power outlet)  AVR------>HumEliminator--->ExternalAmp == Loud Buzz

The projector, AVR and external amplifier are connected to different wall outlets but all outlets are properly grounded.

Can someone please help me decipher what's going on here? And, a possible solution?

73,

Siddhartha WV6U
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AA4PB
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Posts: 12696




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« Reply #1 on: January 25, 2014, 06:29:08 AM »

Often this type of problem is caused by some other device on one of the circuits having leakage and drawing current through the grounding conductor. This causes the ground pin on one outlet to be at a slightly different potential than the other. When you connect your equipment, AC current flows from one device to the other via the shield on the audio leads and the resulting magnetic field couples voltage into the other conductor(s) just like a transformer.

One solution is to route AC wiring so that your devices are connected to the same outlet or at least outlets on the same circuit. Another is to use a transformer (hum eliminator) to couple the signals without having a ground path between equipment via shields.

I quite often see this type of issue (both audio hum and video hum bars) on commercial installations where there long cable runs (especially between buildings). The transformer is usually the easiest fix.
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WV6U
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Posts: 33




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« Reply #2 on: January 25, 2014, 01:05:41 PM »

Since the buzzing noise appears even when, on the projector end, only the ground pin is connected to the power outlet, does it mean the amplifier is the device leaking and drawing current through the grounding conductor? Or, how do I test which device is leaking?

I am also confused as to why the hum appears (and is louder) when a hum eliminator is introduced between the AVR and the amplifier but the projector is disconnected from the power outlet? And, the hum disappears as soon as the projector is plugged back in (hum eliminator still in place).

In terms of a fix, connecting, both, the projector and amplifier to the same outlet is going to be cumbersome since they are far apart. But right now that HDMI cable between the projector and AVR has no ferrite chokes on it. I could replace it with a HDMI cable that has chokes and see if that prevents the interference from coming via the HDMI route.
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AA4PB
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« Reply #3 on: January 25, 2014, 01:24:40 PM »

It could be something else in the house sharing that same circuit that is causing current to flow in the grounding conductor. I'm not sure why adding the hum eliminator would introduce hum when the projector is not connected to AC power.
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WV6U
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Posts: 33




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« Reply #4 on: January 25, 2014, 01:56:10 PM »

I did some more testing. Instead of the projector, I connected a LCD monitor to the HDMI out of the AVR. And, took out the hum eliminator from the circuit.

First, I used the same power circuit as the amplifier for the LCD monitor (also has a three prong plug). No buzz. I tried both - a HDMI cable that had ferrite cores and another that did not. Made no difference, the buzz wasn't there.

Then, I ran power for the LCD monitor from the outlet where the projector is connected - again tried both type of HDMI cables. The buzz persisted with both type of HDMI cable. So clearly, using a HDMI cable with ferrite cores isn't going to help.

I guess my options are:
- Use the hum eliminator between the AVR and amplifier but suffer audio signal attenuation caused by the eliminator.
- Get the projector's power outlet re-routed to use the amplifier's power outlet power circuit.

The first option isn't very desirable while the second one could be expensive with having to open up the walls and what not. Did I miss any other option?

I could disconnect the amplifier's or projector's ground pin but that wouldn't be safe especially since the amplifier has a metal chassis. The projector has a plastic casing and is mounted on the ceiling away from anyone's reach so I could let it have a false ground?
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AA4PB
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« Reply #5 on: January 25, 2014, 03:37:17 PM »

No, I definitely would NOT remove the grounding pin on the projector plug. I'd check to see what other outlets are on the same circuit breaker as the projector outlet. What is plugged into them? Maybe you can unplug whatever is drawing the current through the grounding conductor. Disconnect everything plugged into the circuit and see if the hum goes away. If so, start pugging in one item at a time until it returns, flagging the defective device. Don't forget that it's possible to have something (door bell transformer, etc.) permanently wired to that circuit.

You could also have an electrician check for tight connections on the grounding buss bar in the panel and where the ground loops through each outlet box to make sure there are no high resistance connections. High resistance connections could permit any device that has a little leakage current to develop a much larger voltage differential between circuit grounds. The grounding conductors for all circuits originate at the same buss bar in the panel box so something has to be drawing current in one of the conductors in order to have different voltages at the outlet ground pins.

Another thought: Do you have only one breaker panel in the home? Sometimes when electricians run out of breaker spots they'll add a "piggyback" panel. If the grounding is not done correctly between the two panels you could have a voltage difference between outlet grounds fed from one panel and the outlet grounds fed from the other.
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WV6U
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Posts: 33




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« Reply #6 on: January 28, 2014, 11:51:25 AM »

Thanks for the responses. This is a house I purchased last year and ever since we moved in, I haven't had a chance to peek at the electrical panels for various reasons.

So, last weekend, I did peek along with my general contractor. Turns out the house has no central ground. Since the house was build in the 1950s and code was non-existent/sparse back then, most of the power cables coming into the central panel are two-core. Some newer wiring is three-core and for those, the pane's chassis is the ground that eventually goes to this aluminum pipe that runs down side of the house. Along the crawl space, at a couple of places, there are bare copper wires running from the floor above connected to the cold water copper pipe. My guess is, all the older power outlets have the ground coming to the water pipe in the crawl space. The newer ones have ground go to the central electrical panel where they are connected to panel chassis. No wonder, grounds on different circuits don't agree with each other Smiley

Obviously, a permanent fix is going to be cumbersome and expensive (doing all the grounding correctly).

As and when I start installing my radio gear, I guess I will use batteries for electrical and drive a copper pipe in the backyard for all sorts of grounding so I don't connect at any point with the house's electrical wiring.
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N9KTW
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Posts: 22




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« Reply #7 on: January 28, 2014, 05:02:53 PM »

What you most likely have is a "ground loop". This very commonly occurs in audio/ video installations where the equipment is grounded together by the interconnect cabling, but the AC powered components  have grounds which are not all tied together.

As a licensed electrical contractor, I specifically ran into this situation last year. First " test" was to isolate the ground at the projector. This eliminated the hum, but left the projector ungrounded.Next test was to use an extension cord and plug the projector into the same RECEPTACLE (power strip) as the rest of the equipment. NO HUM! (of course this was not a permanent repair. The final "job" required the installation of a new receptacle (isolated ground type) with local code approved cabling to the receptacle box and circuit feeding the rest of the equipment.  (There already was a new +dedicated circuit for all of this)

Sorry if this is a bit of a long post, but I hope this info was of help. Please feel free to E-Mail me off list if you need any more help.


de Howard, N9KTW
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AA4PB
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« Reply #8 on: January 28, 2014, 05:34:52 PM »

You could solve your immediate problem by installing additional 3-wire outlets at the two equipment locations and feeding them from a single circuit, properly grounded, at the panel.
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K1DA
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Posts: 474




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« Reply #9 on: January 31, 2014, 09:09:14 AM »

  I helped a friend resolve a grounding with his  AC blower motors blowing during thunder storms, the contractor who built the house needed to install a ground rod for code purposes which struck ledge about a foot down, rather than fix the problem the proper way the contractor just cut the pipe  at about a foot in length and stuck it back in the hole.  A yank on the nice, thick copper wire running to the "rod" and it came right out of the ground.  NEVER expect things to be done properly.  Obviously the town electrical inspector just LOOKED at the top of the rod and thought it eight feet long.
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