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Author Topic: Cost effective (even for weekend warriors) ways to measure power quality  (Read 19728 times)

Scott Helmke

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After seeing somebody mention on fb this morning how they had to disconnect the power distro ground to get clean sound... it's time to talk about cost-effective (ie dirt cheap) ways to actually measure power quality beyond the usual multimeter.

A couple years ago I finally started doing something about how a particular piece of gear always had some buzz in a particular venue. The first question of course is "why does it buzz?". In this case it was a Meyer UM-1P system with the VEAM cables, in a university fine-arts venue where even a tiny bit of rizz was serious. And of course the first thing to look at is the power, which by the usual multimeter tests is totally fine.

I finally realized that AC power is just sine waves at a high voltage, dangerous but still basically something you could treat as audio. So I built a special test cable - Edison to 1/4" plug, with a built-in resistor network to drop the voltage from 120 volts to about 0.6 volts. (two resistors in a voltage divider, 200k and 1k). This could then be plugged into a passive DI for transformer isolation, and from there straight into SMAART software's RTA screen. And guess what? There are a lot of nasty harmonics on the power in that venue for some reason. But my test also showed that one of the legs had less harmonics than the others, so I could "fix" the UM-1P noise problem by just plugging into different outlets on the distro to find the quietest leg. That's been sufficient, but longer term the results could be used to get the venue to fix their power or to justify rental of a transformer or other piece of gear to reduce the effects of that bad power.

So, this thread is where I'm going to document (with plenty of caveats) my new "fun" project of coming up with somewhat better ways to build an adapter to easily measure power line quality on show site. Pretty much all of us are now carrying an audio-range spectrum analyzer in our pocket everywhere (smartphone with RTA app), so really the hard work has already been done.

Comments are, of course, welcome. I realize that there is some small amount of liability inherent in such public discussions, but I'm also fed up with people still resorting to dangerous workarounds in this day and age.
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John Roberts {JR}

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I would be more comfortable if you used an isolation transformer... I am not sure about linearity, but a small wall wart would drop the voltage down to something that won't kill you and is double insulated. Unloaded it should give a fair representation of the sine wave, while I am not sure what you are supposed to do about "harmonics" (what they call dirty power).   

Be very very very careful about advising the unwashed to play with mains power. Mainly for their safety.

JR
 
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Nathan Riddle

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After seeing somebody mention on fb this morning how they had to disconnect the power distro ground to get clean sound...

My heart paused for a bit when I read this. Wow.

I'm very interested in this! Let me know how I can help good sir.

I too would advise something a bit more sturdy/safe than a voltage divider circuit on the hot leg as a RPBG could bypass that too easily.
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Stephen Swaffer

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Be very very very careful about advising the unwashed to play with mains power. Mainly for their safety.


Especially when they think disconnecting ground is a good solution?  They might as well play with live ordinance tha they have never handled before.

I have used isolation transformers before to attenuate noise-so I am not sure that would give you a good representation.

Given that your options are limited if you find a problem, (to something like switching legs) it might be best for the average Joe to try different legs-after all there are only a few permutations to try.

It might be reasonably safe to supply a voltage divider through a GFCI receptacle or deadfront?
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Stephen Kirby

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Old slow scopes are cheap if not necessarily tiny.  There are interfaces for virtual scopes on tablets/phones.  Haven't looked to see if they can adequately isolate mains voltages but there may be some.

Agreed that disconnecting a distro ground is a bit scary.  You can actually see some of this noise by running a meter on AC between the ground points of equipment.  Then it's a matter of working out if you need to clean a ground pin somewhere, use an iso transformer or lift a signal ground.
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John Roberts {JR}

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Especially when they think disconnecting ground is a good solution?  They might as well play with live ordinance tha they have never handled before.

I have used isolation transformers before to attenuate noise-so I am not sure that would give you a good representation.
yup a small lump-wall wart will look like a bandpass filter but not so severe that it will conceal serious power waveform distortion issues (IMO).
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Given that your options are limited if you find a problem, (to something like switching legs) it might be best for the average Joe to try different legs-after all there are only a few permutations to try.

It might be reasonably safe to supply a voltage divider through a GFCI receptacle or deadfront?
Yup a GFCI could provide some extra protection, it will protect even with no safety ground, but avoid bootlegging ground to neutral in combination with GFCI since that could defeat the GFCI function for fault current flowing into that bootleg ground that actually goes into the neutral return. A bootleg ground to the input side neutral will preserve the GFCI function, but don't do that anyhow for other safety reasons (never bootleg).... just float the safety ground instead.   

JR
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Scott Helmke

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Thanks, a wall wart is worth trying. I'm really not planning a product here, more something that I could DIY for myself and that others could learn from and (if qualified) build themselves.  I'm imagining another tool in the box, not a fix-all wonder unit. IE you start with the usual measurements, you use a NCVT to look for hazards and a voltmeter to look for correct voltages everywhere. If there's a problem, then break out the RTA adapter and look for weirdness there. I do a fair number of service calls where the customer has given up troubleshooting, so something like this would be very welcome in my toolkit.

I'm not really sure at all about solutions to dirty AC, but at least learning to measure it would be a start on the problem.
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David Buckley

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I'd suggest that for most weekend warriors, any sources of buzzes, hums and crackles that are of sufficient magnitude to be noticeable in the context of a weekend warier rig are down to inadequacies in the setup of the rig.  In a contained, well constructed audio system, when everything is done right, its quite hard to get interference into it.

In the context where power quality is important, then one can buy test equipment off-the-shelf to measure "power quality".
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Kevin Graf

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For the transformer, I would think about the audio output transformer from an old tube amplifier.
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Mike Sokol

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For the transformer, I would think about the audio output transformer from an old tube amplifier.

I agree. These are designed to block at least several hundred volts DC, and enough high frequency response to pass enough harmonics to get a look at. It can be pretty small since you really don't need any power, just a voltage change. I remember doing something similar 25+ years ago while troubleshooting a buzz in my home studio that only happened when it was getting close to sunset. Turns out my neighbor had a dimmer on her dining room lights that she turned up halfway while she was eating. Triac dimmers at halfway make gobs of harmonics. I had a tube output transformer connected to my power outlet and feeding my trusty B&K scope. I could see the spike on the 60 Hz waveform and watch it move around while I had her on the phone changing the dimmer switch position. As note above, a perfectly installed sound system should be impervious to most of that AC line noise. But if you have incorrect shields, long runs of unbalanced audio, and dirty shield chassis bonding, that's when the trouble begins. In fact corrosion in a shield connection can act like a diode and demodulate AM radio stations in the area.
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