Mystery a/c hum - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 20 Old 08-30-13, 01:30 PM Thread Starter
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Mystery a/c hum

Good afternoon Y'all'
Im scratching my head on this one and can use your help. I changed my family room around and put my AV set up on the opposite wall. I had the equipment in a vertical tower and TV stand combo and it was working fine. This is where the bizarre ground loop type of hum issue comes in; I recently got an OPPO BDP 103 and decided to eliminate some gear from my rack (and the rack too). I took out a Linn Karik, Marantz Combo SACD/DVD player, and a dvd recorder I seldom used. Mounted my Panasonic Plasma on the wall and used the TV stand to hold my gear. Made for a clean down size, but now I have a ground loop hum, and a pretty good one through all the speakers. I can hear it on the bass & mid amps powering the L&R mains (the treble amp isn't making any noise, due to the Internal Crossover card I think). The center channel amp, and the amp running the surrounds is also transmitting the noise. The noise doesn't increase with volume changes.

Whats odd, is the same a/c outlet is used, same gear minus what was taken out and the added OPPO. I did go to a short S.E. cable set to the main amps from the long set I had b4, but they are all older audioquest cables (forgot the model name). I have it all running through a Cinepro Powerpro 10 balanced line conditioner. The rest of the system is an Active Linn 5140 system with 3) 5105 amps w/ active crossover cards, passive center channel running off an LK100 bi/amped, going through a Marantz 6004 receiver used as a preamp, & a REL Storm sub. Surrounds & outside speakers are running off an Adcom 6000 5 channel amp using 4 channels. Im also using a Directv HD DVR. Video from both the OPPO & the DVR are running through the Marantz and on to the TV by HDMI.

Frustrations:
When I had everything in the tower in the current location, I didnt have anywhere near as loud of a hum as I do now. still had a hum, but it was consistent with the noise level of the previous location.

What I've tried so far;
- I insured the single ended cables from the receiver to each amp were not running near the power cords
- I tried running the amps cords through a power strip, taking out the cinepro. No change.
- The OPPO is the only change to the system. took it out, no change in the hum.
- Unplugged all digital sources - No change
- Unplugged each amp one at a time. This improved it incrementally until all amps were out of the system. I plugged them back in one at a time and got the hum back incrementally louder with each added amp.
- Bought a polarity/ground outlet tester from Home depot - outlets tested correct for ground & polarity.
- I finally ran an extension cord from the old outlet across the room to the Cinepro. This had a big effect on the hum reducing it (but not totally eliminating it) by about 2/3rds
Now Im getting somewhere.

My initial conclusions;
- The noise is picked up after the source components and receiver.
- I may have something in this circuit that is "dirtying up" the A/C feed. other switches, appliances, etc., bad connections, or some old wiring issues. The house was built in the late 70's, so it may be wiring related
- I am considering running a new, dedicated circuit for the audio sysem.

Any ideas I should check before I go to the trouble of running a separate dedicated circuit? Thanks in advance.

Of all the things I've ever lost, I miss my mind the most!
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post #2 of 20 Old 08-30-13, 01:38 PM
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Re: Mystery a/c hum

Most line conditioners have a fault indicater that lights up when there is a ground problem. Does yours have an indicator light? Is it on?

You could put the old cables back in & see if the problem goes away.
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post #3 of 20 Old 08-30-13, 01:55 PM
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Re: Mystery a/c hum

Quote:
Otis857 wrote: View Post
Good afternoon Y'all'
Im scratching my head on this one and can use your help. I changed my family room around and put my AV set up on the opposite wall. I had the equipment in a vertical tower and TV stand combo and it was working fine. This is where the bizarre ground loop type of hum issue comes in; I recently got an OPPO BDP 103 and decided to eliminate some gear from my rack (and the rack too). I took out a Linn Karik, Marantz Combo SACD/DVD player, and a dvd recorder I seldom used. Mounted my Panasonic Plasma on the wall and used the TV stand to hold my gear. Made for a clean down size, but now I have a ground loop hum, and a pretty good one through all the speakers. I can hear it on the bass & mid amps powering the L&R mains (the treble amp isn't making any noise, due to the Internal Crossover card I think). The center channel amp, and the amp running the surrounds is also transmitting the noise. The noise doesn't increase with volume changes.

Whats odd, is the same a/c outlet is used, same gear minus what was taken out and the added OPPO. I did go to a short S.E. cable set to the main amps from the long set I had b4, but they are all older audioquest cables (forgot the model name). I have it all running through a Cinepro Powerpro 10 balanced line conditioner. The rest of the system is an Active Linn 5140 system with 3) 5105 amps w/ active crossover cards, passive center channel running off an LK100 bi/amped, going through a Marantz 6004 receiver used as a preamp, & a REL Storm sub. Surrounds & outside speakers are running off an Adcom 6000 5 channel amp using 4 channels. Im also using a Directv HD DVR. Video from both the OPPO & the DVR are running through the Marantz and on to the TV by HDMI.

Frustrations:
When I had everything in the tower in the current location, I didnt have anywhere near as loud of a hum as I do now. still had a hum, but it was consistent with the noise level of the previous location.

What I've tried so far;
- I insured the single ended cables from the receiver to each amp were not running near the power cords
- I tried running the amps cords through a power strip, taking out the cinepro. No change.
- The OPPO is the only change to the system. took it out, no change in the hum.
- Unplugged all digital sources - No change
- Unplugged each amp one at a time. This improved it incrementally until all amps were out of the system. I plugged them back in one at a time and got the hum back incrementally louder with each added amp.
- Bought a polarity/ground outlet tester from Home depot - outlets tested correct for ground & polarity.
- I finally ran an extension cord from the old outlet across the room to the Cinepro. This had a big effect on the hum reducing it (but not totally eliminating it) by about 2/3rds
Now Im getting somewhere.

My initial conclusions;
- The noise is picked up after the source components and receiver.
- I may have something in this circuit that is "dirtying up" the A/C feed. other switches, appliances, etc., bad connections, or some old wiring issues. The house was built in the late 70's, so it may be wiring related
- I am considering running a new, dedicated circuit for the audio sysem.

Any ideas I should check before I go to the trouble of running a separate dedicated circuit? Thanks in advance.
You mention having active speakers. Where do you have those plugged in? If you have gear plugged into separate branch circuits on opposite 120V legs, that could be responsible for your hum.
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post #4 of 20 Old 08-30-13, 01:58 PM Thread Starter
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Re: Mystery a/c hum

I have not tried the old cables yet, but that is on my short list to check next. But I dont think that its cable related because some of the same cables were use and the hum is coming through all the amps, not just the L&R mains where i used shorter cables.

The Cinepro has a line of LED's across the front that read system voltage, up to 140 volts, Its at 120 now. It also has a system protection led that isnt lit and a status light thats green. I didnt condemn it because when it was out of the system, the hum was the same.

Thanks for the suggestions, Tonto

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post #5 of 20 Old 08-30-13, 02:10 PM Thread Starter
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Re: Mystery a/c hum

Quote:
Glen B wrote: View Post

You mention having active speakers. Where do you have those plugged in? If you have gear plugged into separate branch circuits on opposite 120V legs, that could be responsible for your hum.
They are active, but dont have the amps built into the speaker cabinets. The crossovers are removed from the speaker cabinets and Im using 3 Linn 5105 amps with Linn's Aktiv crossover cards for treble, mid, bass in each amp that match the 5140's, tri-wired for the corresponding drivers in the speaker cabinet. All the amps are plugged into the Cinepro. The Cinepro has 4 outlets dedicated to digital sources, and that's where I have the OPPO, the DVR and the Marantz plugged into.

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post #6 of 20 Old 08-30-13, 05:32 PM
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Re: Mystery a/c hum

Try disconnecting your Cable TV wire, that has a different ground than the AC system in the home.
Alternatively . . . try connecting the shield/ground of the cable tv wire to the ground of your power conditioner.

W o o f e r h o u n d
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post #7 of 20 Old 08-30-13, 06:07 PM
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Re: Mystery a/c hum

Measure the resistance between the ac neutral and ground, as well as the ac voltage between them. Do the same between the cable or other grounds on any signal lines into the system with those lines disconnected. Listen for a change in the hum when you disconnect.




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post #8 of 20 Old 08-31-13, 12:52 AM
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Re: Mystery a/c hum

I am new but I hope this helps.

Try what Wooferhound said, ground loop is a parallel path to earth ground. The fact that it got louder when you ran an extension cord to the other outlet may have deceased the current through the loop.

I believe you got it right when you said you had a hum from a ground loop. You are bi-amping these units I am guessing running the mid and bass off of one amp and the treble off the other.
First check all the bi-amped speakers and insure 1. insure that the shorting plates (metal piece connection the terminals is only shorting the lower two terminals on both +/- sides. This leaves one hole empty on each shorting plate at the bottom. 2. Then make sure you have one set of cables in the top two post and the other set connected to a pair of the bottom 4 post. I am sure you probably found all this in order but you need to check.

Amplifier power supplies are very good at removing 60 Hz or anything near it from the AC lines. Amplifiers are not really built to remove any ac from there ground plane. Somewhere tough you are getting a parallel path to your earth ground. Start by shrinking your problem to a manageable number of boxes. Isolate 1 amp and its speakers. RCA cables between amps and components are usually shielded to chassis ground so disconnect them all. Power up the amp. If the speakers do not hum any more you are going to find either the hum returns when you plug in one other unit or the hum returns when you plug in anything else to the amp.

If the speakers hum while the amp isolated and you tried what Wooferhound said. (Wow) I would have to think a little more.
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post #9 of 20 Old 08-31-13, 04:14 PM
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Re: Mystery a/c hum

Here is how I understand it . . .
If you have two different pieces of equipment that are grounded in two different locations
you may get a voltage difference between the two grounds that will appear on the unbalanced signal
DC voltage is filtered out
But AC radiated from the grid power wiring that is around us can cause an AC voltage difference on two different grounds which will appear on the unbalanced signal
that is the 60 hz buzz you hear, and sometimes you can hear a radio station
it's a Ground Loop

-- Ground Everything --
I've done this with great success
I got alligator clips that were large enough to clip onto a Phono/RCA jack
connect the alligator clips at 1.5 foot intervals on a long wire
one end of the wire should connect to the Power Ground
all the alligator clips connect to each piece of equipment: on an unused phono jack, a chassis screw, the chassis it's self, the cable wire, the TV ...Everything...
it's a common ground tied to the power ground
this greatly decreases the chance of ground loops and the hum it causes

W o o f e r h o u n d

Last edited by WooferHound; 08-31-13 at 04:22 PM.
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post #10 of 20 Old 08-31-13, 05:19 PM
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Re: Mystery a/c hum

Quote:
Here is how I understand it . . .
If you have two different pieces of equipment that are grounded in two different locations
you may get a voltage difference between the two grounds that will appear on the unbalanced signal
DC voltage is filtered out
But AC radiated from the grid power wiring that is around us can cause an AC voltage difference on two different grounds which will appear on the unbalanced signal
that is the 60 hz buzz you hear, and sometimes you can hear a radio station
it's a Ground Loop

-- Ground Everything --
I've done this with great success
I got alligator clips that were large enough to clip onto a Phono/RCA jack
connect the alligator clips at 1.5 foot intervals on a long wire
one end of the wire should connect to the Power Ground
all the alligator clips connect to each piece of equipment: on an unused phono jack, a chassis screw, the chassis it's self, the cable wire, the TV ...Everything...
it's a common ground tied to the power ground
this greatly decreases the chance of ground loops and the hum it causes
Agreed,
Good commercial electronic installations always begin with a grounding strategy and most will feature a copper ground buss bar with all equipment starred from that bar. A ground buss bar makes a lot of sense for home installations also but, I will admit that I do not use one or know anyone who does. It makes a lot more sense than some pieces of equipment for noise prevention.
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