AC hum / ground loop

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Rich4349
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Location: Kankakee, IL

AC hum / ground loop

#1 Post by Rich4349 » Thu May 20, 2021 4:06 pm

I have been dealing with a couple of diff hums coming from my PA gear that is now in service as my home theater.

1. Constant very soft hum, though not low. I tried finding the freq via Youtube sine waves but was unsuccessful.
2. When I turn on the light in the bedroom that shares an adjoining wall, there would be a STRONG and obvious hum, diff tone. I know about LED interference and was ready to explore that.
3 I was installing a new light fixture today in the foyer, adjoining in the other direction, and heard a third sound yet. (Prev owners had "Uncle Louie" do a lot of the home improvement. "Interesting" electrical design commented the electrician who updated my main panel.)

What I did:
Lifted the signal cables up off some extension cords in the corner, which were powering the Crown amp and Ahsley 3102 EQ. Did nothing.

Unplugged the amp and EQ (that share power via a passthrough plug), and plugged them in across the room into the same outlet my PC uses, in an effort to eliminate ground loop hum, either actually or troubleshooting wise. This reduced the #1 hum to very very low.

Unplugged the line in from PC to EQ, did not remove any hum.

Cut to the chase, already.

Removed the Ashley EQ from the signal path and ALL the hums of all stripes are gone. What's up with the EQ? I got it used off Ebay in 2012. It's SEEMED to do a good job at gigs, though the general sound levels may have covered it. Is there any hope for it? As an older model I'm sure it lacks design to resist LED interference.
2 DR250s, 2 27" Lab15 T-60s, 2 30" Neo Titan 39s, 1 Autotuba...and looking for more!

Bruce Weldy
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Location: New Braunfels, TX

Re: AC hum / ground loop

#2 Post by Bruce Weldy » Thu May 20, 2021 8:07 pm

Don't know if this will help, but I've found it to be a problem in the past. If you are running with XLR cables, check every end on every cable and make sure that PIN 1 (drain/shield) is NOT tied to the XLR housing. Most of the cables from China come like this. Any time I've had a ground issue and I trace it to a cable like this, I can clip that jumper between PIN 1 and the housing and it clears it up.

I never tie the drain to the housing.

Worth looking at.

You can also run a star ground, meaning that every piece of gear in the system is attached to a single ground spot.

6 - T39 4-25" 2-22" 3012LF
4 - OT12 2512 Melded/NSD2005
1 - T24
2 - XF210


"A system with a few knobs set up by someone who knows what they are doing is always better than one with a lot of knobs set up by someone who doesn't."

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Bill Fitzmaurice
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Re: AC hum / ground loop

#3 Post by Bill Fitzmaurice » Thu May 20, 2021 9:08 pm

95% of ground loops are caused by unbalanced connections. This can be very problematic with consumer gear that seldom has balanced connections. It's why I recommend putting sub plate amps in a box that can be placed close to the AVR, allowing a very short interconnect and the ability to plug the amp and AVR into the same outlet or power strip.

Rich4349
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Joined: Wed Jul 30, 2008 10:18 am
Location: Kankakee, IL

Re: AC hum / ground loop

#4 Post by Rich4349 » Fri May 21, 2021 9:30 am

I do run XLRs from amp to EQ.

For line in, I run a long, reasonable quality, RCA cable from PC to amp, with a 1/8" to RCA adapter at the PC and RCA to XLR adapter at the amp. I've read all of this is bad juju, but it sounds fine this way with the EQ out of the mix.

I took apart my XLRs and found the insulation on the #2 pin wires, on both ends of both cables, to be cracked and open, exposing the wire on all 4 ends, with 2-3 mm of insulation still attached to the pin. But the open insulation is right at the main body of the cable, where it looks like there's some kind of metal jacket, maybe?
2 DR250s, 2 27" Lab15 T-60s, 2 30" Neo Titan 39s, 1 Autotuba...and looking for more!

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Bill Fitzmaurice
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Re: AC hum / ground loop

#5 Post by Bill Fitzmaurice » Fri May 21, 2021 9:57 am

Tracking it down can be difficult, but the cause is simple. The power ground and signal ground are shared. Balanced connections shouldn't cause a ground loop because there is a + and - signal, with no signal ground, so power and signal ground can't be shared on the same wire. If you've got a loop it means that somewhere the negative signal and power ground are sharing the same wire. That can happen even with balanced connections if they're wired wrong. Read about it here, on page 216: https://www.yumpu.com/en/document/read/ ... collection

Bruce Weldy
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Joined: Fri Nov 27, 2009 11:37 am
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Re: AC hum / ground loop

#6 Post by Bruce Weldy » Fri May 21, 2021 11:02 am

Rich4349 wrote:
Fri May 21, 2021 9:30 am
I do run XLRs from amp to EQ.

For line in, I run a long, reasonable quality, RCA cable from PC to amp, with a 1/8" to RCA adapter at the PC and RCA to XLR adapter at the amp. I've read all of this is bad juju, but it sounds fine this way with the EQ out of the mix.

I took apart my XLRs and found the insulation on the #2 pin wires, on both ends of both cables, to be cracked and open, exposing the wire on all 4 ends, with 2-3 mm of insulation still attached to the pin. But the open insulation is right at the main body of the cable, where it looks like there's some kind of metal jacket, maybe?
Not quite sure what you are describing, but a little wire without insulation is fine as long as it isn't touching anything else. I'm guessing that all of the consumer stuff doesn't have a ground pin on the AC cord. The Ashley and the amp probably do. You might try lifting the ground on the Ashley or running a ground wire from the Ashley to the amp. If there isn't a ground lug on the back of the Ashley, just run a wire from one of the screws in the chassis over the the amp and do the same thing.

Really hard to tell what you need to do without seeing it. But, it's obviously a grounding problem.

Just looked up the Ashley unit....probably a newer one though.

https://www.manualslib.com/manual/21053 ... e=2#manual

Scroll down to the picture of the back of the amp and read the disclaimer right above the inputs.......if unbalanced, you need to ground the unused pin - which is most likely Pin 3. Solder a short lead from that Pin to Pin 1 since you are running unbalanced to the Ashley.

6 - T39 4-25" 2-22" 3012LF
4 - OT12 2512 Melded/NSD2005
1 - T24
2 - XF210


"A system with a few knobs set up by someone who knows what they are doing is always better than one with a lot of knobs set up by someone who doesn't."

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