Grounding a CRT monitor

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Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 5:11 pm

Okay, I haven't asked weird questions in a while, but it's time. I just discovered a problem tonight and need help from someone with credentials.

I have a large CRT monitor usually working at 129 kHz / 80 Hz, and two relatively cheap microphones (one on a headset, one separately on the desk). I own two desktop computers: my own workstation and a separate one built from my own hand-me-downs for guests. My own computer has a Xonar Essence ST with EM shielding. The headset is for the guest computer, I haven't used it before. The desk mic was purchased recently with the expectation of using it, leading to the discovery below.

The problem:
When the monitor is turned off, both microphones on both computers record a faint low hum, presumably the 50 Hz frequency of the mains. This is expected with cheap microphones and not annoying, and easy to filter out.
However, when I turn on the monitor, I record a much more annoying buzz, higher in tone and much louder than the mains hum, regardless of which microphone, PC, or sound device I use, leading me to believe it propagates through the power cables themselves. The buzz is almost as loud as recorded speech itself, causing problems in online communications. (When "noise canceling" is turned on, the problem goes away, but understandably that also mangles speech, which is less than desirable.)

The question:
How the heck do I ground such a swine of a CRT? Is it even possible? I'd rather not buy an LCD at this time.

Thanks!
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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 5:22 pm

I would guess you are picking up EMF hum from the flyback transformer on the CRT monitor, it's how old now? 10 years+? It could just need tuning or a new flyback (they crack sometimes from overheating).

If could be a ground loop due to bad grounding in the PC and the monitor, so what you could do is take a spare IEC mains cable and snap off the earth prong. This can be used for short TEMPORARY testing only as this stuff is a fire and/or shock hazard (and given that it's a CRT it can totally kill you). Plug the monitor in using this and see if the hum goes away. If it does, tada you found your problem, take it to a local electronics repair shop and have them fix it OR (more likely) buy a 100 Quid LCD monitor and be happy when it pays itself off in the next year vs running that CRT.
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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 5:23 pm

Does the buzz change in volume and/or tone if you move the microphone cables around, or move the monitor relative to the PCs? If so, it isn't coming through the power cord, it is being induced by the magnetic field of the CRT deflection coils and/or radiated EMI from the flyback transformer.
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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 5:24 pm

keltor wrote:buy a 100 Quid LCD monitor and be happy

I would rather drown myself in mare urine.
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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 5:26 pm

just brew it! wrote:Does the buzz change in volume and/or tone if you move the microphone cables around, or move the monitor relative to the PCs? If so, it isn't coming through the power cord, it is being induced by the magnetic field of the CRT deflection coils and/or radiated EMI from the flyback transformer.

The only thing I noticed is that whatever hum or buzz the microphones pick up (whether the monitor is turned on or not), it gets slightly louder if I touch their metal parts.

I'm not good with these things, it's way out of my area of expertise.
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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 5:30 pm

Try looking for something called a "ground loop isolator". Most power conditioners for audio equipment would also take care of the issue, but I don't think you want to spend anywhere near that type of money, considering you're using a CRT.
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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 5:46 pm

The Egg wrote:Try looking for something called a "ground loop isolator". Most power conditioners for audio equipment would also take care of the issue, but I don't think you want to spend anywhere near that type of money, considering you're using a CRT.

That doesn't sound like it would help, based on the responses from the other guys.

Also, I don't still use a CRT because I'm frugal or anything. At the time of purchase, this was the best screen with the highest desktop resolution supporting the highest refresh rates: 2048×1536 @ 80 Hz on the desktop and in most games (if my GPU can handle it), or straight 1280×960 @ 125 Hz for playing Quake Live at 125 fps (for example). It's probably still the best, considering the value I'm getting out of it. If I were to get an LCD, I would either suffer lower refresh rates or a lower resolution, and neither is acceptable.
So, not at this time.
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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 5:57 pm

Does the buzz only occur when the mic is plugged in to the computer?

You could try running an extension cord to another outlet and plug the monitor into that instead.

Maybe your ground is just plain bad. Get an outlet tester and check whether the ground lead on the outlet is wired correctly. Or try temporarily moving the entire system to another electrical outlet and see if the problem persists.
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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 6:03 pm

just brew it! wrote:Does the buzz only occur when the mic is plugged in to the computer?


Yes. I forgot to tell this: I checked the "Listen" option on the microphones so I can hear what they record. The speakers do not buzz by themselves, but the microphones appear unusable due to this phenomenon.
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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 6:39 pm

If you don't have it set up already, configure the computer to blank the screen after a period of inactivity. This will leave the monitor power on but turn off the HV and beam sweep. If it goes away, you know the problem isn't a ground loop in and of itself. Also, change the refresh rate. Go to 60Hz, or 50 even, if you can. If you hear the tone of the noise change, then its related to the refresh rate and could very well be magnetic coupling.

Are the mics condenser or dynamic?

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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 6:41 pm

SecretSquirrel wrote:Are the mics condenser or dynamic?

No idea. They were cheap.
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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 6:49 pm

Meadows wrote:
SecretSquirrel wrote:Are the mics condenser or dynamic?

No idea. They were cheap.


Re-reading some of your posts, the headset mic is almost certainly an electret condenser design. I'd bet good money that the desk mic is too.

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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 7:08 pm

SecretSquirrel wrote:If you don't have it set up already, configure the computer to blank the screen after a period of inactivity. This will leave the monitor power on but turn off the HV and beam sweep. If it goes away, you know the problem isn't a ground loop in and of itself.

I tried just that, the sound went much fainter but did not disappear completely and the tone did not change either. The sound only went away completely after I turned off the screen, even in that blank state. (Someone recommended checking the VGA cable but unplugging that hasn't changed a thing.) So what does this mean? Do I own a weapon of mass distraction?
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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 7:16 pm

Meadows wrote:
SecretSquirrel wrote:If you don't have it set up already, configure the computer to blank the screen after a period of inactivity. This will leave the monitor power on but turn off the HV and beam sweep. If it goes away, you know the problem isn't a ground loop in and of itself.

I tried just that, the sound went much fainter but did not disappear completely and the tone did not change either. The sound only went away completely after I turned off the screen, even in that blank state. (Someone recommended checking the VGA cable but unplugging that hasn't changed a thing.) So what does this mean? Do I own a weapon of mass distraction?


Did you try the refresh rate change?

I means that it is something to do with the power supply of the monitor and not the HF circuits or the sweep circuits. The change in volume is due to the monitor drawing much less power when in blanking mode. Are the monitor and computer plugged into the same outlet? Power strip? You mentioned 50Hz, can I assume that mains voltage is 230/240V?

This sounds like a ground loop, but curing it may be hard.
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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 7:18 pm

If you've got a laptop handy, try plugging the mics (one at a time) into it while it's running on battery and approach the monitor from whatever distance seems appropriate. If the buzzing noise is present on the laptop speakers (with the mic input active) and the noise increases as you approach the beast, then the mic cable is acting as an antenna, and improper mains grounding is not your problem.

If the noise is NOT present with the laptop in close proximity to the beast, then you may indeed have noise being injected through the mains. Perhaps there's another circuit in the area you can plug the beast into, isolating it somewhat from the rest of your setup. Or, you can get an isolated power supply for the mic, allowing you to use lower impedance line inputs.

My money's on the antenna effect. Try turning down the gain on the mic input. There's usually a 20dB buffer enabled by default in your sound card's control panel - if you can disable that and still have enough output from the mic, you'll usually get better noise performance.
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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 7:24 pm

SecretSquirrel wrote:Did you try the refresh rate change?

Now I did. The tone momentarily goes higher as the monitor switches modes, but then settles on the same tone after it sets. I tested 80, 125, and 160 Hz.

Also, sadly no laptop here.
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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 7:32 pm

I just gave my mighty Sony 34" XS955 away. Still a very nice TV but my 30" 2560x1600 Korean monitor is so much nicer. I did not expect it to be so good.
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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 7:33 pm

PenGun wrote:I just gave my mighty Sony 34" XS955 away. Still a very nice TV but my 30" 2560x1600 Korean monitor is so much nicer. I did not expect it to be so good.

Nice troll, but if it can't do >120 Hz, it is inferior.
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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 7:56 pm

Meadows wrote:
PenGun wrote:I just gave my mighty Sony 34" XS955 away. Still a very nice TV but my 30" 2560x1600 Korean monitor is so much nicer. I did not expect it to be so good.

Nice troll, but if it can't do >120 Hz, it is inferior.

It was $389 shipped. It will do 80Hz I'm told, I'll leave it at 60Hz. It is so much better than the TV it's amazing. It runs ESO real nice too. With a 2560x1600 screen even a GTX 780 does not get much past 70fps or so with the goodness turned on.

I had a nasty humm from the power supply but it has a long cord and I found a place it's OK.
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Re: Grounding a CRT monitor

Postposted on Thu Mar 20, 2014 8:54 pm

Meadows wrote:
The Egg wrote:Try looking for something called a "ground loop isolator". Most power conditioners for audio equipment would also take care of the issue, but I don't think you want to spend anywhere near that type of money, considering you're using a CRT.

That doesn't sound like it would help, based on the responses from the other guys.

Also, I don't still use a CRT because I'm frugal or anything. At the time of purchase, this was the best screen with the highest desktop resolution supporting the highest refresh rates: 2048×1536 @ 80 Hz on the desktop and in most games (if my GPU can handle it), or straight 1280×960 @ 125 Hz for playing Quake Live at 125 fps (for example). It's probably still the best, considering the value I'm getting out of it. If I were to get an LCD, I would either suffer lower refresh rates or a lower resolution, and neither is acceptable.
So, not at this time.

It's possible that I've dealt with this issue before. The interference is being injected into the ground by the monitor, and then picked back up by other devices which are connected to the same ground. Considering you can find highly rated units for about $12, it's unquestionably the least expensive fix. If it doesn't work, you're only out 12 bucks at most.

By the way, I used to be a CRT guy myself. My last unit was a 22" Mitsubishi 2070SB-BK, which I dropped quite a bit of cash on, and used until mid-2008 (I believe it's still around; you want it?). I liked CRTs for their high refresh rates and resolution flexibility (within the usable range). Unfortunately, all CRTs look like absolute horsesh*t above 1600x1200 @ 60hz, so it's not even worth mentioning those resolutions because they're unusable. This is because the ancient DE-15 analog VGA cables are severely bandwidth limited, and once you cross the bandwidth threshold (on ANY quality of CRT), the picture looks as if you rubbed sandpaper on a pair of $4 safety glasses and then wore them in a sauna. Eventually I got tired of being stuck at 1280x960, even if it was at 100hz+. If you're happy with that resolution, then enjoy.......I guess.
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Re: Grounding a CRT monitor

Postposted on Fri Mar 21, 2014 12:43 am

My Sony CRT does 1920 x 1200 @ 85 :D
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Re: Grounding a CRT monitor

Postposted on Fri Mar 21, 2014 3:24 am

Starfalcon wrote:My Sony CRT does 1920 x 1200 @ 85 :D

This is a 22 inch IBM CRT, it does:

2048×1536 (4:3) @ 80 Hz, default
1920×1440 (4:3) @ 86 Hz
1600×1200 (4:3) @ 100 Hz

Weighs 33 kg (73 pounds) too, so I'd rather not take it anywhere if I can help it.
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Re: Grounding a CRT monitor

Postposted on Fri Mar 21, 2014 5:15 am

The Egg wrote:It's possible that I've dealt with this issue before. The interference is being injected into the ground by the monitor, and then picked back up by other devices which are connected to the same ground. Considering you can find highly rated units for about $12, it's unquestionably the least expensive fix. If it doesn't work, you're only out 12 bucks at most.

Thanks, I just might try that. I've just read some help articles on this so-called "ground loop" and while I'm a far cry from an electrician, I think I'm beginning to see the nature of the problem.

It's not like I have much to lose, I already tried separating the devices and computers into different wall plug arrangements but that had almost no effect. On the other hand, I noticed something cool: the loudness of the buzz slightly decreases if I'm touching the desk microphone with one hand and the glass of the CRT with the other. :P
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Re: Grounding a CRT monitor

Postposted on Fri Mar 21, 2014 6:55 am

Someday wired analog connections will be considered "old school" and hum from ground loops in audio equipment will be history. We're nowhere near that point yet, unfortunately.
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Re: Grounding a CRT monitor

Postposted on Sat Mar 22, 2014 9:47 am

UPDATE: I did as you told me to do and bought a ground loop isolator, but the problem didn't go away, instead I got a new problem. When plugged into the isolator, the microphone does not work: instead of isolating the buzz, the apparatus isolates the microphone, so I only hear the buzz. Playback devices (speakers and headphones) continue to work fine using the isolator, so the apparatus itself is probably not at fault.

What could be the cause?
Edit: as l learnt later on, the Xonar Essence has a perk: the microphone input is the only port exempt from the EMI shielding, which is probably why every output sounds clean and dead silent on it except for the microphone port.

(Bonus question: a computer technician told me that these noise and ground interference problems can be alleviated if I feed the monitor (or the computers, I forgot) from a UPS with a "true sine wave output", but I don't know what that means or why it would help. I'm wondering if I'd be better off with a breakout box or a mid-range microphone instead.)
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Re: Grounding a CRT monitor

Postposted on Sat Mar 22, 2014 6:07 pm

UPDATE 2: Talked with an audio nut friend of mine, he smiled and explained that condenser microphones are supposed to not work through isolators, it's by their design.
See, I told you I'm bad with electric stuff. :(

If only the Xonar's EMI shielding would extend all the way to the microphone port... cheap bastards.

I managed to disconnect the ground on the CRT using a wall socket converter and just for testing purposes I disconnected the ground on the PC itself too, and the issue never went away. Thus the only thing left is the microphone cable itself. The monitor's electromagnetic field is probably significant for some reason, so we'll look into shielding the microphone cable so it stops acting as a radio receiver. Friend said I wouldn't have had much better luck with a dynamic mike with double the price tag either -- assuming this disturbance really does go through the cables -- so the el cheapo product I got is probably only partially responsible, at most.

A sad story, but it's soon coming to a close.
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Re: Grounding a CRT monitor

Postposted on Sat Mar 22, 2014 7:09 pm

Cheap shielding: dismantle your tinfoil helmet and use it to cover the entire length of your microphone cable. :P

(more seriously, I'm not sure if that'd really work without the ends being connected to ground)
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Re: Grounding a CRT monitor

Postposted on Sun Mar 23, 2014 8:12 am

Ahh, yes... I believe condenser mics typically draw power from the mic port. A ground loop isolator would prevent that from working.

Hmm... a USB headset might be your best bet, to take the noisy mic input out of the loop. Though if the monitor is radiating badly enough there may not be much you can do, short of using a pro grade mic and preamp or replacing the monitor.
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Re: Grounding a CRT monitor

Postposted on Sun Mar 23, 2014 3:03 pm

Meadows wrote:I managed to disconnect the ground on the CRT using a wall socket converter and just for testing purposes I disconnected the ground on the PC itself too, and the issue never went away. Thus the only thing left is the microphone cable itself. The monitor's electromagnetic field is probably significant for some reason, so we'll look into shielding the microphone cable so it stops acting as a radio receiver.

Well.....definitely not the ground then. You might try a couple cheap ferrites for the mic cable as a last resort.
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Re: Grounding a CRT monitor

Postposted on Sun Mar 23, 2014 4:00 pm

The Egg wrote:Well.....definitely not the ground then. You might try a couple cheap ferrites for the mic cable as a last resort.

The audio enthusiast friend I mentioned (as in for real, not like how audiophiles are) promised to build me a few cheap microphones from parts and endow them with fancy shielded cables and the works, so first of all we'll test that before investing in anything serious. Also, an electrician will look at the place tomorrow.
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