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Author Topic: Ground Loop Audio Hum and Cures  (Read 6269 times)
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K1JJ
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« on: February 23, 2020, 01:04:39 AM »

I wonder if anyone has had 60 cycle hum in their audio caused by AC power ground loops or cabling or whatever?  I'd be curious what you did to cure it.

Today I got a report of some 60 Hz hum on my AM audio.  I could hear it slightly in the monitor but always thought it was low enuff to be invisible. Conditions were peaking locally so stations could hear anything that was there.

I spent an hour trying different grounding, changing AC outlet plug positions - but nothing helped. It was a low level hum that went away when I unplugged the 528E processor from the AC line OR I unplugged the balanced 528E output audio cable.

I am using balanced XLR connections thruout.

After no results, I went into the cellar and carried up my old 100 pound Class E power transformer. I configured it for 120VAC to 120VAC, as an isolation transformer.  I put all the audio gear and also the FT-1000D on this isolation transformer circuit.   It did make a difference and the hum is now MUCH fainter.  I must get some actual tests tomorrow and look more closely at the scope..

Are there any other things I should be looking at?

Back when I had my class E rig there was a pot on the PDM board that was a feed-forward circuit that allowed someone to null out the hum. That was slick and worked really well and completely killed any trace.   Is there a circuit that will sample AC ripple either on the carrier or at the end of the audio chain and allow one to use a negative feedback technique to null it out? Maybe the audio would be adversely affected, I dunno.

I'm kinda out of ideas on how to proceed since I already have balanced XLRs and now good isolation from the AC line loop currents.  I wonder if there might be something reversed/ wrong with the audio cabling...   I have all my braided ground leads going to a central point, more or less. All the audio gear cabinets are grounded.

Any ideas to kill the last bit of hum?  How quiet should I expect a good ham audio system to be? IE, assuming our power supplies are clean, if you really turn up the monitor gain or look on a scope with the mic unplugged can you see or hear ANYTHING relating to 60 Hz audio hum when you key up a big dead carrier?

When we think about it, there are so many things in a transmitter system that can cause hum -  from bad tube elements, filament choke imbalance, power supply ripple, AC ground loops, grounding, audio equipment failure, cable connections... yikes!

(Cheeez, I hate hum)  

T
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« Reply #1 on: February 23, 2020, 02:09:07 AM »

Interesting reading:

https://www.prosoundweb.com/the-pin-1-problem-revisited/

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« Reply #2 on: February 23, 2020, 08:19:50 AM »

When using balanced connections, the shield is only a shield — it carries no signal. That’s why balanced unshielded connections work fairly well, in fact there are many commercial radio stations running analog audio through Ethernet cables as a method (there are plenty using digital audio over ip too, as well as many using shielded audio cable too). In recording studio installations, usually a decision gets made to ground only sources and float all the sinks...that is, connect shields at outputs only and float shields at inputs, so loops cannot occur. Make a couple short xlr jumpers with the shield not wired, put them here or there until you find the place that should have the shield lifted.  In pro audio land, we all carry a handful of xlr ground lifts in our briefcase, along with phase reversers, male/male and female/female barrels, and so forth, because ground loops happen.

Ed
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« Reply #3 on: February 23, 2020, 08:25:09 AM »

The shield is not connected at the end being fed for balanced shielded wiring. In the case of XLR cables, the ends with the female connector should have the shield disconnected. The exception to this is microphone cables where you *do* want the shield connected to the microphone case.

Some engineers cut the ground pin off of the power cords or use three prong "cheaters"  with the ground lead disconnected when their equipment is mounted in a rack to avoid the multiple ground paths.
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« Reply #4 on: February 23, 2020, 08:29:04 AM »

Ah, so that was what the "Other messages have been posted!" popup was about.

At least it's proof that great minds think alike.
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« Reply #5 on: February 23, 2020, 08:46:38 AM »

Never mind.
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« Reply #6 on: February 23, 2020, 08:52:41 AM »

By rotating my Shure SM-58 in its holder, I can find an orientation where the hum is mostly cancelled. Moving the microphone around the shack also finds places where the hum is better... or worse. All this, with the transmitter off. The shure SM-7B, which uses a very similar capsule, includes a hum-bucking coil to provide its own hum cancellation. But you have to pay four times the cost of a '58 to have that luxury.

I assume that we're all surrounded by 60Hz magnetic fields and an interference pattern is present which is generated by the many sources producing these fields. This interference pattern has certain sweet spots where cancellation occurs and a dynamic microphone can be placed without inducing much hum.

So how about condenser mics? Well, they contain sensitive electronics that can be susceptible to hum/noise pickup. I have a Marshall MXL-770 that was driving me nuts with intermittent hum and noise pickup. It turned out that the problem was excess paint on the housing where the metal screen attached to the body. Scrapping the paint off those places to provide a reliable electrical contact cured this, a shielding problem.

As far as RF pickup goes, I use ferrite cores with the audio cables wound around them. I'm not sure how much this helps, but it couldn't hurt, eh?

The sources of hum are myriad. And so are the cures.

Here are a few suggestions:
- Keep interconnect cables as short as possible.
- Use balanced interconnections when possible.
- Keep levels between components high to maximize S/N ratio.


Don
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« Reply #7 on: February 23, 2020, 09:08:42 AM »

Tom,
I'm wondering if you are having an interaction with the FT-1000D unbalance connections with the other balanced connections in some form assuming any unbalanced connection is being used. Your isolation transformer is sort of a clue.

You might be able to isolate the hum with a 3 prong to 2 prong adapter to open or lift the ground and try it on each piece of gear.  And as others have stated, make sure the shields on the balanced lines are grounded at one end of the run only.

Below is a nice write up on ground loops which might provide some clues.

https://www.ranecommercial.com/kb_article.php?article=2107

https://www.ranecommercial.com/legacy/note151.html
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« Reply #8 on: February 23, 2020, 12:29:46 PM »

Excellent info - just what I needed to proceed.

I will make up some XLR jumpers to unground the shield at the sink ends... and experiment with this.  Years ago I remember needing to unground one of the audio cables at one end (unbalanced) and it solved the problem...  but I forgot about trying this.


The hum appears with any rig - yesterday it was with the 4X1 plate modulated rig using the GFZ audio driver.  


I can unplug the Shure mic (balanced XLR with both ends grounded) and it does not affect the 60 Hz hum.  It seems centered on the 528E which I will test first with the "unground" technique.


I'm hoping the 120VAC isolation power transformer just installed last night for the audio gear will give me a new starting point of lower hum to begin again today.  I need to reread this thread and look over some of the articles first.


Thanks much!

T
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« Reply #9 on: February 23, 2020, 01:37:59 PM »

You're having a 'Pin 1 Problem'.

Open up the 528.  Make sure the ground pin on that unit isn't connected with paint under or around it.

Astron power supplies are renown for this. 

--Shane
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« Reply #10 on: February 23, 2020, 05:58:29 PM »


On issues like this, I like to divide and conquer...


On an AM rig:

Turn off Mic switch, or reduce modulator gain to minimum
still hum?
Shut down the modulator
still hum?
Switch VFO over to a crystal
Still hum?
Check power supplies, and possible heater cathode shorts

Also consider RF ingress into the speech amplifier....dummy load time so long as you can monitor Tx without introducing hum from the receiver.

I remember once I added a filter choke into my Central Electronics 20a. Magnetic fields spraying from the choke coupled into one of the low level audio transformers. I had Hum when the modulator tube was pulled! Grounding one side of 6.3v for filament return can really cause a mess as well with low level hum. I replaced the choke with a choke inside steel enclosure, and hum was GONE!

As to using an isolation transformer, some studios use a big Topaz ultra isolation transformer with an electrostatic shield. Then the AC can be made balanced instead of hooking neutral to ground (at transformer secondary) as is the US convention.

Jim
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« Reply #11 on: February 23, 2020, 06:36:40 PM »

Here are some salient questions:

1. Is the hum detected without RF present? (Can you hear it just monitoring the audio chain) because that would indicate an AC to equipment loop..(maybe pin 1)
2. Do you hear it while transmitting into the dummy load?
3. Finally, is it only present while transmitting into the antenna. (Gobs of RF present)..

 Important “Hum-Sleuth” questions!
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K1JJ
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« Reply #12 on: February 23, 2020, 09:04:43 PM »

Yo Guys -

I appreciate all the help.  It's looks like it worked and the hum is non-existent on the carrier (scope)  or in the audio monitor.

A report from most of the same guys who spotted it yesterday said the hum was now completely gone.   I cannot hear any even with the audio monitor cranked up all the way.

The isolation power transformer, 120VAC > 120VAC to isolate the audio gear from ground loops helped the most.  The XLR cables were not changed nor grounds lifted at this point.

I still plan to tidy a few things up based on several of the suggestions above.

Frank / WA1GFZ, an engineer at Collins sent me an email which I posted below. You will see he puts a lot of stress on solid bonding to cabinets, a rack to hold the equipment and generally thinks in terms of the paths of least resistance. I added it for the future archives.

Thanks again, guys.

T



Reprinted with his permission:

Tom,
I spent a few days last week in a training course run by the Collins EMI Fellow. He is kick ass. Let me share a little of what he told us.
Your audio hum problem is due to 60Hz current riding on the shields of your audio cables. The source of the 60Hz is AC coupling from the line to the chassis of each audio device (bypass caps, EMI power line filters and coupling in the power supply). The third prong in your power leads has a higher resistance than the audio shields than the audio shields so guess where the current flows.

Chopping grounds is totally JS and not safe. Floating grounds or shields at one end is a mickey mouse way to attack the problem and is bad news in an RF environment.
First you need to provide a better path for chassis currents than the audio shields. Mount all your audio devices in a small rack and make sure you have a good chassis ground between each device chassis and connected to the rack chassis as well. Mount a metal quad utility box inside the chassis. Use two of them if you need more than 4 outlets.  All audio devices will get their power from this quad box. Roll up the line cords of each device and make them as short as possible.  Connect a single line cord as a source to the box. You can add an isolation transformer ahead of the quad box in the cabinet but make sure it is also grounded to the cabinet. Isolation transformers couple a little AC from windings to core. Now all the audio devices have a local ground system with a low impedance ground system between audio devices. Now attach a heavy ground lead to the audio rack chassis. I suspect the best place to terminate it is to the power supply ground of the audio driver or the station single point ground you will have to experiment with the termination because you feed multiple rigs. There is one more potential loop. The line cord to the audio rack and the SS driver power supply. They should be plugged into the same outlet.  You may want a low impedance ground directly between them.

Isolation transformer connect the line cord green wire directly to an isolation transformer mounting bolt. This returns primary leakage currents back to the breaker panel. Then you need to reestablish a new chassis ground on the secondary side which is the audio rack chassis ground.

You might be tempted to NOT terminate the audio rack line cord safety ground which would break any primary ground loop there but you could have a shock hazard if the rack ground ever got disconnected. There should be no chassis currents because of the new low impedance between audio device chassis elimination voltage offsets.
I have found a single point ground is good for lightning but for RFI you also want a low impedance ground between devices to avoid ground loops so I actually like both.

  gfz.

 



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« Reply #13 on: February 23, 2020, 10:19:09 PM »

I realize what I am proposing is inconvienient and somewhat js but it just might be a better way for a balanced xlr cable ....

      3  <------------------------------------------------>

      2  <------------------------------------------------>

      1  <------------------------------------------|    -->
                                                                  |
                                                                 ---
                                                                 ---
                                                                  |
                                                                gnd

              this small capacitor is sized to be a good rf bypass but not a good pass for 60 Hz .... its gnd end just might be the pin 1 on the adjacent end and not outside the xlr connector .... the goal is to impede a 60 Hz ground loop but help drain off any rf energy present
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« Reply #14 on: February 23, 2020, 10:26:33 PM »

I don't cut ground pins off, I add them.

When I get an old piece of equipment that has a 2 conductor line cord, one of the first modifications I make is to remove the old cord and install a 3 conductor cord. If I have the time, I'll install an IEC C13 socket so I can have a nice removable cord. These sockets are also available with fuses, switches, and EMI filters. If you have a piece of equipment that is particularly susceptible to RFI, a C13 socket with an EMI filter might be the cure. I installed a fused C13 socket on my DX-100 (pictured).


Don


* IEC C13 socket on DX-100.jpg (108.94 KB, 408x300 - viewed 192 times.)
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« Reply #15 on: February 24, 2020, 10:04:35 AM »

I wholeheartedly agree that lifting AC power grounds is not a safety conscious solution, but you can use this method to troubleshoot.  In a former position I found myself frequently troubleshooting some rather elaborate AV editing / video production systems where ground loops manifested themselves as both audio hums and video “hum bars”. 

I developed a technique to find the offending component that worked 99.9% of the time. Such situations most frequently occurred when the customer had a mixture of devices, using both 2 wire and 3 wire AC power cord.

Step 1 was to take a quick check of your AC power, to verify that you didn’t have AC voltage between the “common” line and ground. More than 50% of the time “power strips” caused the problem, so the next step was to get rid of every power strip. Next, for every component that used a 3-wire power cord, (one component at a time) I would temporarily use a 3 to 2 prong “cheater plug”, which will temporarily disconnect the third pin. It won’t take too long to locate the system component that is causing the problem.

Regarding the balanced audio cables, I usually make my own. Get a scrap piece of cable, usually RG-58 or RG-59, and a slightly longer length of network cable. Extract one twisted pair of wire from the network cable. Use the center conductor of the coax to pull the twisted pair through the coax shield. Voila! One balanced audio cable!
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« Reply #16 on: February 24, 2020, 12:35:55 PM »

I wholeheartedly agree that lifting AC power grounds is not a safety conscious solution, but you can use this method to troubleshoot.  In a former position I found myself frequently troubleshooting some rather elaborate AV editing / video production systems where ground loops manifested themselves as both audio hums and video “hum bars”.  

I developed a technique to find the offending component that worked 99.9% of the time. Such situations most frequently occurred when the customer had a mixture of devices, using both 2 wire and 3 wire AC power cord.

Step 1 was to take a quick check of your AC power, to verify that you didn’t have AC voltage between the “common” line and ground. More than 50% of the time “power strips” caused the problem, so the next step was to get rid of every power strip. Next, for every component that used a 3-wire power cord, (one component at a time) I would temporarily use a 3 to 2 prong “cheater plug”, which will temporarily disconnect the third pin. It won’t take too long to locate the system component that is causing the problem.


I've always liked "special trick" troubleshooting techniques to quickly locate a problem. Using the 3 to 2 AC cheater plug and the XLR ground disconnect is like a doctor using a stethoscope to look around and zero in on the problem.   I will be sure to use these two in the future.

Yep, I have at least 8 power strips behind the main table which occasionally crap out or get noisy.  What started out as a good, orderly idea has turned into a big maintenance problem.

T
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« Reply #17 on: February 24, 2020, 02:58:59 PM »

Or do what I did - I built my own. Got several square electrical boxes, some outlets and heavy duty power cord. Wired up correctly and eliminated the cheap Chineseium shortcuts
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« Reply #18 on: February 24, 2020, 07:45:54 PM »

Frank GFZ's comments are as usual, dead on.  Hum plagues everyone at some point with our nice AC power system.

TNX for sharing that Tom!

Peter
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« Reply #19 on: February 26, 2020, 05:40:29 PM »

Yep, I have at least 8 power strips behind the main table which occasionally crap out or get noisy.  What started out as a good, orderly idea has turned into a big maintenance problem.

T


It's not only power strips but basic duplex outlets.  I don't know how many times I've used $2.00 duplex outlets for stuff only to replace them because they can't hold a light weight cord over time.  They loosen up. So can't imagine they're providing a good contact as they age.  
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« Reply #20 on: February 26, 2020, 06:05:30 PM »

Yep, I have at least 8 power strips behind the main table which occasionally crap out or get noisy.  What started out as a good, orderly idea has turned into a big maintenance problem.

T


It's not only power strips but basic duplex outlets.  I don't know how many times I've used $2.00 duplex outlets for stuff only to replace them because they can't hold a light weight cord over time.  They loosen up. So can't imagine they're providing a good contact as they age.  
There are Hospital Grade duplex outlets available. They cost more, but are held to higher standards. They can be identified by a green dot on the face.
https://www.homedepot.com/p/Leviton-15-Amp-Hospital-Grade-Extra-Heavy-Duty-Self-Grounding-Duplex-Outlet-White-8200-W/301361162

If you want to get fancy with grounding to deal with hum problems, there are Isolated Ground duplex outlets that are identified by an orange triangle.
https://www.homedepot.com/p/Leviton-15-Amp-Hospital-Grade-Extra-Heavy-Duty-Isolated-Ground-Duplex-Outlet-White-8200-IGW/301361243
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« Reply #21 on: February 26, 2020, 07:32:02 PM »

Yep, I have at least 8 power strips behind the main table which occasionally crap out or get noisy.  What started out as a good, orderly idea has turned into a big maintenance problem.

T


It's not only power strips but basic duplex outlets.  I don't know how many times I've used $2.00 duplex outlets for stuff only to replace them because they can't hold a light weight cord over time.  They loosen up. So can't imagine they're providing a good contact as they age.  

Use the high dollar 20A capable plugs.

Those are real plugs, and you do get what you pay for with electric equipment.

--Shane
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