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Title: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on March 29, 2016, 10:12:58 AM
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.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: John Roberts {JR} on March 29, 2016, 10:47:46 AM
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
 
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Nathan Riddle on March 29, 2016, 10:58:36 AM
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.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Stephen Swaffer on March 29, 2016, 01:39:03 PM
   
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?
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Stephen Kirby on March 29, 2016, 01:52:23 PM
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.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: John Roberts {JR} on March 29, 2016, 01:55:12 PM
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).
Quote
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
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on March 29, 2016, 03:52:39 PM
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.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: David Buckley on March 29, 2016, 04:41:14 PM
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".
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Kevin Graf on March 29, 2016, 05:26:20 PM
For the transformer, I would think about the audio output transformer from an old tube amplifier.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Mike Sokol on March 29, 2016, 07:42:41 PM
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.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on March 29, 2016, 09:05:12 PM
Here's an example of the dirty power from the venue I mentioned. The big spike at the left is good, that's our actual 60Hz sine wave. The rest is what we don't want.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on March 29, 2016, 09:08:47 PM
For the transformer, I would think about the audio output transformer from an old tube amplifier.

Great idea.

I'm still going to start with a wall wart transformer because
a) It might work just fine.
b) I've got a box of them salvaged from old broken things.

Ultimately it would be great to be able to say "buy this AC wall wart from Mouser, and the rest is all safe low voltage".  That definitely makes it a friendlier project.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: John Roberts {JR} on March 29, 2016, 09:15:08 PM
I haven't spent much time looking at mains power voltage waveforms but I have never seen a clean sine wave the few times I've looked.

If you properly isolate the mains you could even patch it in and listen too it.  ;D

I repeat, it is unclear how you correct dirty power after you see it (hear it)...?

More data is never bad, but data that you can't take action from is just another source of stress.

JR

PS: You could even connect a modest voltage wall wart directly to a speaker. KISS
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: TJ (Tom) Cornish on March 29, 2016, 09:17:01 PM
Here's an example of the dirty power from the venue I mentioned. The big spike at the left is good, that's our actual 60Hz sine wave. The rest is what we don't want.
It looks like the largest harmonic is 30dB down from the fundamental.  If my math is correct and I'm reading the scale right, that is less than 4 volts amplitude.  That doesn't seem all that dirty to me in the grand scheme of things.  How do you ensure this noise was from the AC mains and not at least partly your testing apparatus?
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on March 29, 2016, 10:57:14 PM
It looks like the largest harmonic is 30dB down from the fundamental.  If my math is correct and I'm reading the scale right, that is less than 4 volts amplitude.  That doesn't seem all that dirty to me in the grand scheme of things.  How do you ensure this noise was from the AC mains and not at least partly your testing apparatus?

The power back at the shop (sorry, no screen shot at this point) did not show most of that junk.

Most important, the "quietest" leg also resulted in the best results when the sensitive gear was plugged into it instead of one of the noisier legs.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on March 29, 2016, 11:01:41 PM
I repeat, it is unclear how you correct dirty power after you see it (hear it)...?

More data is never bad, but data that you can't take action from is just another source of stress.

Sure, there's no easy fix for that. But it would relieve the stress of not knowing where the noise comes from at all.

There might be a choice of power sources in a big venue, and one of the other tie-ins might be cleaner.

It might also be useful for convincing people to live with a little noise instead of disconnecting safety grounds.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Mike Sokol on March 30, 2016, 07:22:31 AM
Sure, there's no easy fix for that. But it would relieve the stress of not knowing where the noise comes from at all.

The problem with this sort of graphical information is that you need a certain amount of expertise to interpret it. Like an X-ray, diagnosis is in the eyes of the technician and doctor looking at it. Many years ago when I worked in a large industrial plant we had a big room full of Ingersoll-Rand air compressors. Each compressor was the size of a VW mini-van, so it was serious stuff. We sent lubrication oll samples of each compressor back to IR every few months to look for signs of abnormal wear. When one of the samples indicated something going wrong, IR sent out a field engineer with a portable frequency analysis monitoring system. Since I was the liaison between engineering and production, I got to assist this guy. He hooked up a contact mic to the side of the compressor, and plugged it into a display that showed all the internal vibrations and their harmonics. He showed me which noises were related to the various gears, cams, rotors, etc... He could tell which component was contributing which fundamental vibration and harmonic because he had a diagram of all the gears, shafts, bearing, valves, etc... While I could easily have hooked this up and got it working, I was lost with interpreting the results because I had no diagram or understanding of the internal workings of this compressor. But he did, and was very good at it, which is why IR could charge $600 per day for him back in 1975 dollars.

While your gadget could and should generate a wealth of data, and while many on this forum will eagerly digest it and come up with possible solutions, the average weekend warrior will be lost in the possibilities. For instance, I spend a lot of time on noisy power sites looking not only at meters, but I'm also snooping around to see how the conduits are fitted, if the sub-panel has a secondary neutral bond, where everything is plugged in, if the noise changes when turning non-audio gear on and off, etc... It's similar to a doctor's physical where he or she measures, thumps, feels, and listens to all your bodily functions. The data by itself doesn't help much. But a good diagnostician will see beyond the numbers and symptoms to come up with possible diagnosis which they will confirm with additional tests.

I'm not trying to dissuade you from doing this (as long as you do it safely). Rather I want to point out that AC power noise is pretty complicated because it has distributed and often unknown causes. But keep us posted here since we're all going to learn something from your data.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Stephen Swaffer on March 30, 2016, 08:38:02 AM
I am disputing that the quieter leg got rid of the noise-I'll take the OP at his word.  I do wonder if something else is at play here?

Internally, audio gear runs on DC-which means the mains power is rectified and filtered-in some cases with linear supplies and in other with switching supplies.  Ultimately, the filter is a low pass filter designed to filter out very low frequencies-and noise and harmonics are higher frequencies so they should be quite effectively filtered out.

Ultimately the only wires actually physically connected to the gear are the ground and the neutral-and that should only be through the ground bond at the service.

A true test would be to A-B clean power vs noisy power and look at what we see down stream on the DC buses in the gear when under load.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on March 30, 2016, 08:50:06 AM
"They laughed at me in Budapest... they laughed at me in Prague!" 

Had a great time writing a rant to the naysayers, but I appreciate the "here's how it might work better" advice.
I don't know where we'll end up on this little project, but most of my other crazy measurement projects turned out to be useful.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on March 30, 2016, 09:24:20 AM
If you properly isolate the mains you could even patch it in and listen too it.  ;D

For that matter, I could record it to an audio file while at the test site. Then later in the "lab" I could play it back through a big power amp for 120 volts at the speaker terminals.
Then I could use it as the AC power source to test how it affects various pieces and combinations of audio gear.

Speaking of which, the actual noisy Meyer UM-1P problem I mentioned in an earlier post. Our UM-1P wedges use the Meyer VEAM cabling, which bundles power, audio, and RMS into one multipin cable/connector.  In one venue in particular we often end up using the very short power adapter cables and direct XLR inputs to avoid the noise from the 50-75 foot VEAM cables. But some customers really like the convenience and neat appearance of the one cable. One solution that comes to mind for this problem (bundled AC and audio) would be to find or build a better audio line driver than the ones built into the monitor console de jeur. The current workaround has been to just plug those systems into outlets on the quietest leg on the power distro. This is easy enough to do. But it hadn't occurred to me to try it until I looked at the noise on the various legs.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on March 30, 2016, 10:26:42 AM
Oddly enough I have the morning off. Found a nice 9vac wall wart in my box-o-warts, and slapped together a very simple prototype with a 10k/1k voltage divider and a 1/4" jack. Here's the power from the outlet by the dining room table. My laptop PSU is plugged into the same power strip, but didn't show any significant change when unplugged.  I've not had any complaints about the power quality here.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Mike Sokol on March 30, 2016, 10:30:13 AM
Had a great time writing a rant to the naysayers, but I appreciate the "here's how it might work better" advice.

Be careful not to poke the bear with a stick by doing a rant here... ;D

This is an educational forum where we welcome technical challenges and aren't afraid to offer opinions, many of which may be wrong at first until more data is available. We welcome your challenge and will learn from your data, but few things in audio are as simple as they seem at first. I've been burned a bunch of times from my own knee-jerk quickie diagnostics, so I'm careful not to the be 100% sure about anything until I can make the problem come and go at will.

The VEAM snake offer another possibility of interference cross-connection between AC power and audio signal paths. Can you provide a diagram or part number of their VEAM snake?   
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Stephen Swaffer on March 30, 2016, 10:35:07 AM
I wasn't laughing at you or trying to be a naysayer-I was throwing out thoughts to learn and understand.  I have spent a lot of my day career troubleshooting and I personally do not like shotgun or let's try this and this and this to get to the bottom of the problem.  I prefer to try and understand the problem.

If the noise is coming from bundled cabling, then higher frequencies/noise would have a tendency to couple through capacitance to the signal lines.  In that case, then I could understand noisy power creating an issue.

Understanding the cause or path of interference helps you to actually solve the problem.  One solution that comes to mind here, would be to use a power isolation transformer to feed power to the bundled cables-perhaps a dedicated one for each cable.  As I mentioned in an earlier post, this is a solution I have used to "filter" unwanted noise in the past-albeit in an industrial/encoder counting application, not audio.  If this solution worked, you would have a solution for even when you can't find a quiet enough leg.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on March 30, 2016, 10:42:59 AM
The VEAM snake offer another possibility of interference cross-connection between AC power and audio signal paths. Can you provide a diagram or part number of their VEAM snake?

http://www.meyersound.com/products/options/veam.htm
http://www.meyersound.com/pdf/products/veam/multi-cable_ds-1.pdf
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Art Welter on March 30, 2016, 03:20:21 PM
Scott,

Have you tried lifting (disconnecting) the audio shield(s) at one end of the Veam connectors?

Art
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Mike Sokol on March 30, 2016, 04:21:56 PM
Scott,

Have you tried lifting (disconnecting) the audio shield(s) at one end of the Veam connectors?

Art

Back to the original question about how to hook up some sort of spectrum analyzer to a 120-volt outlet without getting killed. I talked to an application engineer at Hammond Manufacturing about using a tube audio output transformer for this tester since it would have a high enough frequency response to pass though any real noise of interest. And he thought it would be safe enough and potentially provide real data. I did a little looking for cheap tube output transformers and came up with this one with an 8-watt output and 25:1 turns ratio. https://www.tubesandmore.com/products/P-T31 My calculations suggest it will output around 4.8 volt AC when the primary is connected to a 120-volt source. And it will be well within its design specs. My guy at Hammond gave me the name and number of one of their transformer designers in Canada who can discuss this further.

For now I would council everyone against hooking anything up directly into a wall outlet without an isolation transformer of some kind in between you and the power plant. And if you want actual noise and harmonic content from your measurements you need a transformer designed to pass high frequencies. That's opposite of a standard AC wall transformer where reducing high frequency content is an advantage.   
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on March 30, 2016, 07:38:06 PM
I did a little looking for cheap tube output transformers and came up with this one with an 8-watt output and 25:1 turns ratio. https://www.tubesandmore.com/products/P-T31 My calculations suggest it will output around 4.8 volt AC when the primary is connected to a 120-volt source. And it will be well within its design specs. My guy at Hammond gave me the name and number of one of their transformer designers in Canada who can discuss this further. 

14 bucks, not a bad deal at all.

Tomorrow I'm doing a show at the venue in question, so I'm bringing both the original adapter and the new wall-wart based adapter.

The other measurement I've been mulling over is a neutral to ground measurement. Might see some really interesting stuff there.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Geri O'Neil on March 31, 2016, 01:03:23 AM
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.

Wow, I'd love to know what made you consider the possibility of the problem being actually outside your dwelling and in fact, many feet and in someone else's house!
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Steve M Smith on March 31, 2016, 03:15:45 AM
If I was going to do this, I think I would take a toroidal mains transformer and wind two or three turns around it to give me a signal level output.


Steve.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Mike Sokol on March 31, 2016, 07:09:35 AM
Wow, I'd love to know what made you consider the possibility of the problem being actually outside your dwelling and in fact, many feet and in someone else's house!

After I had already tried shutting off EVERYTHING in my own house and still had the sunset buzz, I guessed it was coming in from outside electrical power. I could see the 240-volt transformer on the street feeding 3 houses in total, mine and two others. So I sat outside in my yard looking for any changes that occurred when the sun started going down. When I saw my neighbor's dining room lights come on when the sun went down and ran inside to look at the o-scope, I knew I was onto something. The confirmation was calling her on the phone and watching the Triac pulse on the scope phase-shift as she turned her dimmer up and down.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Stephen Swaffer on March 31, 2016, 08:40:11 AM
14 bucks, not a bad deal at all.

Tomorrow I'm doing a show at the venue in question, so I'm bringing both the original adapter and the new wall-wart based adapter.

The other measurement I've been mulling over is a neutral to ground measurement. Might see some really interesting stuff there.

Sounds like a plan.  I know your focus is on a measurement tool to diagnose the problem.

I would curious to see if the noise is eliminated by using either a separate power cord (to the same "dirty" power) or signal cable.  That would definitively localize the path the noise is taking to the cable.  I would have thought the Meyer engineers would have spec'd the shields connected at only one end-but maybe the manufacturing engineer didn't think it mattered and was easier to manufacture-certainly wouldn't hurt to ohm out the shields just to see.

You mentioned a better line driver.  The problem is, if the noise is in the audio spectrum, how does the circuit discriminate between noise and desired sound?  Some "music" I hear intentionally amplified is "noise" to me :).

While noisy mains may be the source, it is unlikely you will be able to fix that.  Even power factor correction won't entirely eliminate harmonics-and it can be a pricey fix.  It certainly won't happen before the show starts.   I think it more likely you will be able to break the path the noise takes-if you do that, then it works in every venue.

Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: John Roberts {JR} on March 31, 2016, 10:08:09 AM
Sounds like a plan.  I know your focus is on a measurement tool to diagnose the problem.

I would curious to see if the noise is eliminated by using either a separate power cord (to the same "dirty" power) or signal cable.  That would definitively localize the path the noise is taking to the cable.  I would have thought the Meyer engineers would have spec'd the shields connected at only one end-but maybe the manufacturing engineer didn't think it mattered and was easier to manufacture-certainly wouldn't hurt to ohm out the shields just to see.
To reject RF and very HF interference shield grounds should be connected at both ends. Professional gear should transmit the audio as separate + and - stems (pins 2 and 3), ignoring ground noise (pin 1).
Quote
You mentioned a better line driver.  The problem is, if the noise is in the audio spectrum, how does the circuit discriminate between noise and desired sound?  Some "music" I hear intentionally amplified is "noise" to me :).
Source impedance of the signal driver can make a difference in noise pickup. Consumer gear can be 1k ohm or more, pro gear much lower.
Quote
While noisy mains may be the source, it is unlikely you will be able to fix that.  Even power factor correction won't entirely eliminate harmonics-and it can be a pricey fix.  It certainly won't happen before the show starts.   I think it more likely you will be able to break the path the noise takes-if you do that, then it works in every venue.

Proper gear design should ignore all but the nastiest mains noise. I can imagine troubleshooting mains power quality as being more appropriate for fixed installs or perhaps recording studios, not sure what you can do, on the spot at live gigs. While it's never bad to have more information.

JR

 
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Mike Sokol on March 31, 2016, 11:25:31 AM
To reject RF and very HF interference shield grounds should be connected at both ends.

Many, many years ago I was in a band playing in the front window of a bar with my back to the street. Yes, a really low-class establishment. Less than 20 away from this window was the road with an intersection and a traffic light. This was back in the day of CB radios and such, so every time the traffic light was red some yahoo in his car would get on their CB and start yapping. My PA system was normally pretty quiet, but somehow the RF from the AM modulated CB radios would show up in my speakers. Of course, this was during the slow ballads, which drove us crazy every time we played there. I did some studying on the problem (1975 style with actual books) and found that sometimes speaker cables could feed RF back into an audio amplifier, where it could be demodulated inside the amplifier's negative feedback loop, then amplified by the output stage as significant wattage. One of my ham radio buddies suggested putting tiny ceramic capacitors right across the speaker wires to shunt out the RF before it got inside the audio amplifiers, and it worked like a charm. I can't remember the exact value cap, but it was in the pico-farad range. Now, I'm not saying that's the same problem here... just that RF frequencies have a way of getting inside of audio gear and being detected by any stray corrosion on a shield bond or a P-N junction in its path. 
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Jonathan Johnson on March 31, 2016, 11:48:10 AM
My PA system was normally pretty quiet, but somehow the RF from the AM modulated CB radios would show up in my speakers. Of course, this was during the slow ballads, which drove us crazy every time we played there. I did some studying on the problem (1975 style with actual books) and found that sometimes speaker cables could feed RF back into an audio amplifier, where it could be demodulated inside the amplifier's negative feedback loop, then amplified by the output stage as significant wattage.

Would twisted-pair vs. parallel zip cord (for the speaker cables) make a difference in a situation like this?
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Kevin Graf on March 31, 2016, 12:49:47 PM
Yes twisting will help.
Jim Brown Power Point and paper on the subject.
Just skip the Ham radio sections.
Jim Brown was past AES committee co-chair on EMT/RFI and is a Ham radio operator.

"RF Interference in Audio Systems"
http://www.audiosystemsgroup.com/AES-RFI-SF08.pdf

"RFI, Ferrites, and Common Mode Chokes For Hams"
http://www.audiosystemsgroup.com/RFI-Ham.pdf

About 50 more Jim Brown papers:
http://www.audiosystemsgroup.com/publish.htm
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Geoff Doane on March 31, 2016, 01:12:13 PM
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.

I chased a problem like this a few years ago, and posted some pictures in an old version of this forum, but it seems to have disappeared.  Here's the 2016 version.

At my day job, we discovered that we couldn't get a clean signal out of some db Technology monitors (model 12-4M).  We have 8 of them, and they all exhibited the same problem, a slight "rizz" when plugged in to the building's AC power.  They hadn't always done this, but the problem showed up shortly after an 80 KVA building UPS was installed in the basement electrical room.  None of the other studio gear seemed to have a problem (the building housed radio studios and a music/drama recording studio) and various portable consoles and other powered speakers all seemed to be unaffected.  The problem was not a ground loop, because it happened as soon as the speaker was powered up, even before an XLR feed was attached.  It was also constant regardless of the gain setting on the speaker's amplifier.

I tried all the usual (and unusual) tricks to eliminate noise: lifting the ground, putting a 2KVA isolation transformer in the AC feed, even bypassing the diodes in the bridge rectifiers with small bypass caps.  Nothing worked.  Since the only thing they were connected to was the building power, it was tempting to blame that, but difficult to eliminate it in an A/B test.  Finally, I came up with the idea of running the speakers off a UPS, and the noise went away.  At the time I only had a fairly ordinary APC UPS (not double conversion), so I had to unplug its input to get it to generate a sine wave, but the speaker was happy when I did.

I didn't have Smaart to look at harmonics, but did look at the outputs with a scope.  I used an ordinary bell transformer to drop the voltage down to about 6V.  I suspect that most 60 Hz transformers will pass frequencies a lot higher than that.  The other interesting point I discovered was that the AC was noisy whether the speaker was plugged in on the line or load side of the UPs, although the character of the noise was a bit different.

Here are the waveforms.  The first is the line side (of the 80KVA UPS), second is load, and the third is the rackmount UPS output, with no load except for the scope and powered speaker.  The building UPS was running at about 60% of capacity.

GTD
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: John Roberts {JR} on March 31, 2016, 01:15:04 PM
Many, many years ago I was in a band playing in the front window of a bar with my back to the street. Yes, a really low-class establishment. Less than 20 away from this window was the road with an intersection and a traffic light. This was back in the day of CB radios and such, so every time the traffic light was red some yahoo in his car would get on their CB and start yapping. My PA system was normally pretty quiet, but somehow the RF from the AM modulated CB radios would show up in my speakers. Of course, this was during the slow ballads, which drove us crazy every time we played there. I did some studying on the problem (1975 style with actual books) and found that sometimes speaker cables could feed RF back into an audio amplifier, where it could be demodulated inside the amplifier's negative feedback loop, then amplified by the output stage as significant wattage. One of my ham radio buddies suggested putting tiny ceramic capacitors right across the speaker wires to shunt out the RF before it got inside the audio amplifiers, and it worked like a charm. I can't remember the exact value cap, but it was in the pico-farad range. Now, I'm not saying that's the same problem here... just that RF frequencies have a way of getting inside of audio gear and being detected by any stray corrosion on a shield bond or a P-N junction in its path.
Not extremely common but yes, speaker wires can act like antennas at RF, and since many solid state power amps have an inductor in series with the output (to decouple from capacitive loads), that output can look like high Z at RF.

Any significant RF on the output line can come back into the amp via the feedback resistor. If the amp isn't fast enough (and few are fast enough to handle RF) that amp can slew rate limit and rectify the radio signal. Good amp design can minimize this with several design strategies that are TMI for this forum.

I've heard of this happening more typically in churches that are in some unusually strong radio station broadcast pattern.   

Be careful about arbitrarily throwing capacitors across power amp outputs, it can literally make some amps break into oscillation and release their magic smoke. I recall Jack Sondermeyer (Mr. CS800) would hang tens of uF on the output of every new amp he designed to protect them from Peavey Customers (don't try that at home). 

JR

PS: Zip cord should make a lousy antenna but enough RF can cause grief, twisting won't hurt but generally this should not be problem. Note: It isn't just power amps that can have RF snake back into their outputs, but again prudent design anticipates and manages this.

PPS: I really discourage people from lifting grounds, shields or otherwise but expect many here to ignore my advice. :P If you must lift, a "hybrid" grounding scheme, well known in broadcast circles, uses small caps to provide a good RF bond to ground at multiple points, while being modest to high Z at hum frequency. Using a cap in series with a safety ground is only acceptable to me in combination with a GFCI (0.15uF so it can still trip the GFCI), and so far never acceptable to UL or electrical code. 
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: John Roberts {JR} on March 31, 2016, 01:30:52 PM
I chased a problem like this a few years ago, and posted some pictures in an old version of this forum, but it seems to have disappeared.  Here's the 2016 version.

At my day job, we discovered that we couldn't get a clean signal out of some db Technology monitors (model 12-4M).  We have 8 of them, and they all exhibited the same problem, a slight "rizz" when plugged in to the building's AC power.  They hadn't always done this, but the problem showed up shortly after an 80 KVA building UPS was installed in the basement electrical room.  None of the other studio gear seemed to have a problem (the building housed radio studios and a music/drama recording studio) and various portable consoles and other powered speakers all seemed to be unaffected.  The problem was not a ground loop, because it happened as soon as the speaker was powered up, even before an XLR feed was attached.  It was also constant regardless of the gain setting on the speaker's amplifier.

I tried all the usual (and unusual) tricks to eliminate noise: lifting the ground, putting a 2KVA isolation transformer in the AC feed, even bypassing the diodes in the bridge rectifiers with small bypass caps.  Nothing worked.  Since the only thing they were connected to was the building power, it was tempting to blame that, but difficult to eliminate it in an A/B test.  Finally, I came up with the idea of running the speakers off a UPS, and the noise went away.  At the time I only had a fairly ordinary APC UPS (not double conversion), so I had to unplug its input to get it to generate a sine wave, but the speaker was happy when I did.

I didn't have Smaart to look at harmonics, but did look at the outputs with a scope.  I used an ordinary bell transformer to drop the voltage down to about 6V.  I suspect that most 60 Hz transformers will pass frequencies a lot higher than that.  The other interesting point I discovered was that the AC was noisy whether the speaker was plugged in on the line or load side of the UPs, although the character of the noise was a bit different.

Here are the waveforms.  The first is the line side (of the 80KVA UPS), second is load, and the third is the rackmount UPS output, with no load except for the scope and powered speaker.  The building UPS was running at about 60% of capacity.

GTD
I am inclined to blame the gear design, power is routinely dirty and probably getting worse with home automation and utilities trying to piggyback control data on the mains.

Years ago (way last century) I investigated transmitting audio over mains power wiring. Just like those cheap hifi speakers that did that. This would have been a huge cost saving for the fixed install business if they didn't have to pull wire for every install they did. Most buildings had mains wiring already in place.

The ugly secret is they don't work all the time. It would be a disaster for a sound contractor to bid on a job without the cost of pulling wire, only to find out after the customer turns on their copy machine, or whatever, that the system is noisy and they have to go back and hard-wire the job.

I shut down that product development after about a year with a junior engineer working on it... It worked acceptably well under ideal (clean mains voltage) conditions, but the real world is not ideal, and loves to bite design engineers in their ASSumptions. 

JR
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on March 31, 2016, 02:19:01 PM
Good news - I did head-to-head comparisons between the original adapter (voltage divider down from 120v) and an AC 120v to 15v wall wart into a voltage divider. The transformer in the wall wart showed all the same harmonics all the way up - not an exact match, so frequency response isn't that great, but definitely enough to show the problem. 

I might try to figure out how to take a video (maybe just with my phone camera), because the noise in the right half of the display tend to move around in interesting ways. Almost like there's a big elevator nearby, though I don't think this building has one.

Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Mike Sokol on March 31, 2016, 02:34:34 PM
Good news - I did head-to-head comparisons between the original adapter (voltage divider down from 120v) and an AC 120v to 15v wall wart into a voltage divider. The transformer in the wall wart showed all the same harmonics all the way up - not an exact match, so frequency response isn't that great, but definitely enough to show the problem.

Aren't these all odd-order harmonics at 180, 300, and 420 Hz? Since this is 3-phase power, methinks you might be looking at Triplen currents generating voltage spikes....
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on March 31, 2016, 02:48:46 PM
Aren't these all odd-order harmonics at 180, 300, and 420 Hz? Since this is 3-phase power, methinks you might be looking at Triplen currents generating voltage spikes....

Yeah, I was curious about that too.  These are all from our power distribution unit.  When I get a chance I'll process and upload a couple readings I got from regular wall outlets. They have even more junk at the higher frequencies.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Mike Sokol on March 31, 2016, 03:27:17 PM
Yeah, I was curious about that too.  These are all from our power distribution unit.  When I get a chance I'll process and upload a couple readings I got from regular wall outlets. They have even more junk at the higher frequencies.

See this from Hershey Energy Systems: http://www.hersheyenergy.com/harmonics.html

If there's any large switch-mode power supplies or variable speed motors in the building, they can excite all sorts of Triplen harmonics.

Harmonics:

Harmonics by definition are a steady state distortion of the fundamental frequency (60 Hz). Harmonic distortion of current occurs when sinusoidal voltage is applied to a non-linear load (ex. electronic ballast, PLC, adjustable-speed drive, arc furnace, any ac/dc converter). The result is a distortion of the fundamental current waveform. This distortion occurs in integer multiples of the fundamental frequency (60 Hz).  Hence, the 2nd Harmonic has a frequency = 2 x 60 = 120 Hz, the 3rd Harmonic = 180 Hz and so on.  Voltage distortion, on the other hand, is generated indirectly as result of harmonic currents flowing through a distribution system.

It is important to note that the vast majority of harmonic currents found in a distribution system are odd-order harmonics (3rd, 5th, 7th, etc.). Secondly, more often than not, the sources of the harmonic currents in a distribution system are the loads in operation within that facility. Interestingly, these are frequently the types of loads that are the most sensitive to distortion in the current and/or voltage.

Triplen Harmonics The triplen harmonics are defined as the odd multiples of the 3rd harmonic (ex. 3rd, 9th, 15th, 21st etc.). Triplen harmonics are of particular concern because they are zero sequence harmonics, unlike the fundamental, which is positive sequence. The consequence of this fact is that the magnitude of these currents on the 3 phases are additive in the neutral. This can lead to very large currents circulating in the neutral, and unless the neutral is sufficiently oversized this can present a fire hazard. These currents can also circulate in the transformer causing significant overheating there too. Single-phase power supplies for equipment such as electronic ballasts and PCs are the most significant source of Triplen harmonics.

5th and 11th Harmonics The 5th and 11th harmonics are also of particular concern to industry today.  Although the 5th harmonic is much more prevalent, both have a negative sequence.  This means that when distorted voltage containing the 5th or 11th harmonic is applied to a 3-phase motor, it will attempt to drive the motor in reverse, creating a negative torque.  In order to compensate for this negative torque, the motor must draw additional fundamental current.  This, in turn, can cause overheating and/or the tripping of over-current protection devices.  6-Pulse adjustable speed drives are a major source of the 5th, 7th and 11th harmonics.  12-Pulse drives are significantly more expensive, and are a source of the 11th and 13th harmonics, but through their design are able to eliminate the 5th and 7th.

In general, harmonics present on a distribution system can have the following deleterious effects:

    Overheating of transformers & rotating equipment
    Increased Hysteresis losses
    Decreased kVA capacity
    Neutral overloading
    Unacceptable neutral-to-ground voltages
    Distorted voltage and current waveforms
    Failed capacitor banks
    Breakers and fuses tripping
    Interference on phone and communications systems
    Unreliable operation of electronic equipment
    Erroneous register of electric meters
    Wasted energy/high electric bills - kW & kWh
    Wasted capacity - Inefficient distribution of power
    Increased maintenance of equipment and machinery


Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Steve M Smith on April 01, 2016, 02:32:04 AM
That graph also shows the way a Hammond organ combines waveforms!


Steve.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Mike Sokol on April 01, 2016, 07:06:26 AM
That graph also shows the way a Hammond organ combines waveforms!

Exactly...
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Kevin Graf on April 01, 2016, 08:17:13 AM
We had a computer game that looked like that graph. The game would display a compl;ex waveform and you would try to match it by setting the level of the fundamental and it's harmonics.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Mike Sokol on April 01, 2016, 08:20:29 AM
We had a computer game that looked like that graph. The game would display a compl;ex waveform and you would try to match it by setting the level of the fundamental and it's harmonics.

That's a little too nerdy, even for me... ;)
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Mike Sokol on April 01, 2016, 08:45:14 AM
Yeah, I was curious about that too.  These are all from our power distribution unit.  When I get a chance I'll process and upload a couple readings I got from regular wall outlets. They have even more junk at the higher frequencies.

Just musing on this a bit, if the harmonic content was high enough frequency, would a snap-on Ferrite Core filter on the power line cords short out some of it? Of course, this would do nothing for any audible frequency, but perhaps there's  RF hash on the line from other gear in the building using switched-mode power supplies.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on April 01, 2016, 11:10:55 AM
Just musing on this a bit, if the harmonic content was high enough frequency, would a snap-on Ferrite Core filter on the power line cords short out some of it? Of course, this would do nothing for any audible frequency, but perhaps there's  RF hash on the line from other gear in the building using switched-mode power supplies.

RF is relatively easy to get rid of. Audio frequency range interference would be much harder to damp out.

So the building in question is a performing arts facility built in probably the 1960's.  Still have lots of old dimmers down in the basement, and a pretty good-sized HVAC plant somewhere. No elevators or other big motors, though.

Based on the success of using a wall-wart transformer, I'm going to make an iPhone adapter so I can just carry the transformer and my phone around taking measurements. I tried using a guitar tuner cable adapter with an RTA app with no success, but that may be an intentional limitation of the demo app. At the worst I can make short audio recordings and then look at them in Audacity, which has an FFT function.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Mike Sokol on April 01, 2016, 11:26:45 AM
RF is relatively easy to get rid of. Audio frequency range interference would be much harder to damp out.

So the building in question is a performing arts facility built in probably the 1960's.  Still have lots of old dimmers down in the basement, and a pretty good-sized HVAC plant somewhere. No elevators or other big motors, though.

Based on the success of using a wall-wart transformer, I'm going to make an iPhone adapter so I can just carry the transformer and my phone around taking measurements. I tried using a guitar tuner cable adapter with an RTA app with no success, but that may be an intentional limitation of the demo app. At the worst I can make short audio recordings and then look at them in Audacity, which has an FFT function.

Also, if you can get an o'scope function running on your phone or tablet, that will often tell you as much as a spectrum analyzer will.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Steve M Smith on April 01, 2016, 12:27:16 PM
That's a little too nerdy, even for me... ;)

You say that in public but I bet you would have a go. I would!


Steve.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Mike Sokol on April 01, 2016, 01:14:59 PM
You say that in public but I bet you would have a go. I would!

If I had a few beers in me you can bet your sweet bippie I would. Is this a drinking game?  8)
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on April 02, 2016, 05:27:48 PM
Moving on to the iPhone edition!  Fooling around with an audio spectrum analyzer app (3 bucks from Black Cat Systems) and a guitar interface cable (the Peterson tuner cable), I get a reasonable display of the first few harmonics. That Peterson cable is a little weird, though - seems to have some active stuff in there. So the next step (not today at this point) is to use a TRRS plug and build my own interface cable.  That should be fairly simple, once I figure out the appropriate voltage divider resistors from the wall wart's 9-10 volts. I was thinking I'd need a capacitor to block the DC bias from the mic input, but actually I just need to have it see a load that isn't too low a resistance. I don't think the wall wart will care about a volt or two DC.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on April 05, 2016, 08:53:40 AM
Silly idea for attenuating noise from that venue - capacitor in series with a light bulb.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: John Roberts {JR} on April 05, 2016, 10:17:25 AM
Moving on to the iPhone edition!  Fooling around with an audio spectrum analyzer app (3 bucks from Black Cat Systems) and a guitar interface cable (the Peterson tuner cable), I get a reasonable display of the first few harmonics. That Peterson cable is a little weird, though - seems to have some active stuff in there. So the next step (not today at this point) is to use a TRRS plug and build my own interface cable.  That should be fairly simple, once I figure out the appropriate voltage divider resistors from the wall wart's 9-10 volts. I was thinking I'd need a capacitor to block the DC bias from the mic input, but actually I just need to have it see a load that isn't too low a resistance. I don't think the wall wart will care about a volt or two DC.
The wall wart will not care about a few mA of phantom current .

JR
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on April 05, 2016, 12:53:42 PM
The wall wart will not care about a few mA of phantom current.

After I thought about it a bit... I'll need to have a fairly steep attenuation from wall wart to iPhone mic jack - meaning that the mic power, which is only a volt or two, will be going through probably 100k ohms. We're talking even less than milliamps.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Frank Koenig on April 05, 2016, 01:40:46 PM
Scott,

I have nothing really to add here but love what you're doing and am following with interest. Nothing better than using some existing, cheap technology to solve a problem in a new way.

Best,

--Frank
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on April 05, 2016, 05:09:31 PM
Scott,

I have nothing really to add here but love what you're doing and am following with interest. Nothing better than using some existing, cheap technology to solve a problem in a new way.

Best,

--Frank

Thanks! Things are kind of dragging right now because I've got other stuff happening, but within a few days I should be able to publish a parts list and schematic for an iPhone/Android test circuit based on an AC wall wart for safety.  Probably $20 or less for parts plus a $2.99 app that already exists.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Eric Vogel on April 05, 2016, 06:44:03 PM
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.

Here's a circuit that I made years ago to test HIz 100/70 volt, and LOWz amplifier circuits.
The H pad divider has -40db of gain and feeds a high quality DI transformer.  This should work well with your idea.

120vac (44dbu) in > 1.2vac (4dbu) out

Balanced H pad - 680k inputZ, 6k8 outputZ, feeding Jensen DI xfmr JT-DB-E
.01% 1/4w metal film for best balance

L------(340k)-------(2k7)------->yellow lead of JT-DB-E
                    |
                    |
                 (1k37)
                    |
                    |
N------(340k)-------(2k7)------->orange lead of JT-DB-E

do not connect black lead of JT-DB-E.

this arrangement will provide full galvanic and electrostatic isolation between primary and secondary coils.

follow schematic for secondary terminations:
http://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-db-e.pdf

You could build all this in a nylon/plastic nema box with a powercon in/out (so you could monitor the circuit open and loaded with load of your choosing) and have a male xlr feeding mic level out.  Build whatever adapter cables to feed iThings etc....
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Jonathan Johnson on April 06, 2016, 01:39:44 AM
You could build all this in a nylon/plastic nema box with a powercon in/out (so you could monitor the circuit open and loaded with load of your choosing) and have a male xlr feeding mic level out.  Build whatever adapter cables to feed iThings etc....

One thing to keep in mind is that UL requires physical separation of high- and low-voltage portions of the circuitry. I don't know the actual specs, but this often takes the form of a gap in the circuit board (or between circuit boards) or, in some cases, a physical barrier between the high- and low-voltage wiring.

This test device may never darken UL's door, but it's probably a good idea to observe these clearances anyway.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: John Roberts {JR} on April 06, 2016, 10:21:40 AM
One thing to keep in mind is that UL requires physical separation of high- and low-voltage portions of the circuitry. I don't know the actual specs, but this often takes the form of a gap in the circuit board (or between circuit boards) or, in some cases, a physical barrier between the high- and low-voltage wiring.

This test device may never darken UL's door, but it's probably a good idea to observe these clearances anyway.
The Jensen transformer is rated for 250VAC breakdown voltage so "should" be OK (While even they warn not to depend on that insulation for human safety applications).
======
The wall-wart if UL approved will be double-insulated. If one insulation layer fails, there will be a second back up layer of protection.

Caveat, different wall wart-transformers can use different internal transformer construction, so audio frequency response could vary significantly between different lumps.

You might be able to benchmark the wall-wart frequency response driving it with a power amp. This shouldn't hurt the power amp but confirm the amp is happy with the unusual load. You shouldn't have to drive it to full 115VAC to characterize response.   

JR   
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on April 06, 2016, 01:52:06 PM
 
Caveat, different wall wart-transformers can use different internal transformer construction, so audio frequency response could vary significantly between different lumps.

You might be able to benchmark the wall-wart frequency response driving it with a power amp. This shouldn't hurt the power amp but confirm the amp is happy with the unusual load. You shouldn't have to drive it to full 115VAC to characterize response. 

Planning on doing just that, actually.  Last night I ordered a nice generic 9v wall wart from Jameco, so that I can come up with a good-enough design that doesn't depend on my junk box wall wart of unknown origin.  I don't think I'd need a power amp just to get enough input voltage, but I do want something with a very low output impedance to make the test as accurate as possible.

Back a couple years I was using SMAART with a headphone amp to test various audio isolation transformers. Very handy for looking at distortion at different frequencies and signal levels.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on April 09, 2016, 10:50:32 PM
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).

Way back at the start... and JR wins... something. Cup of coffee and a muffin, given the budget here?  :)

I tested both my existing 9v wall wart and a brand new Jameco model 100061 wall wart transformer ($8.95) for frequency and phase response, using SMAART and a moderate-sized power amp. Probably not running at a full 120 volts, maybe about half that.  The result? Quite surprisingly flat.

Next step is interfacing to an iPhone mic input. Jameco sells an appropriate TRRS plug, part number 2200954 ($1.95). I made up a two resistor voltage divider - 1Mohm and 1Kohm, for a rather drastic reduction of the 9 volts down to something like 9mV. The iPhone circuitry uses some kind of active load sensing to see if there's a mic attached (sleeve is signal, second ring is ground), and the Internet seems to agree that a resistance of around 1Kohm takes care of that. So, 1K resistor between sleeve and second ring, one side of 9v wall wart output to second ring, and other side of wall wart output through 1M resistor to sleeve. 

The RTA display is an app, "Audio Spectrum Analyzer" by Black Cat Systems ($2.99). Settings I used include Logarithmic display, Hanning window, FFT Length = 131072.

Once I figure out how to fit the resistors into that tiny connector I'll have my pocket-sized $20 power quality meter.

Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on April 09, 2016, 10:57:35 PM
So that's the hardware and software. I still want to make up a complementary solution to measure neutral to ground.

The rest is interpretation and then solution.

Interpretation is going to be a matter of practice. Measure power in a bunch of locations, take screenshots maybe, and note any issues with power at any of those locations. I've gotten pretty good at reading a SMAART transfer function from a noisy room while tuning speaker systems, so I expect that with practice I'll be able to interpret this stuff as well.

Solution is of course a much bigger issue.  There's not a lot you can do about the power in a lot of situations... though there are maybe some cheats. Maybe only one or two pieces of gear will actually cause problems from noisy/dirty power, such that some filtering would be a reasonable thing to carry around. "Outlet shopping" will be feasible with a portable metering solution, allowing users to find and avoid specific outlets that are noisy.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Mike Sokol on April 09, 2016, 11:37:31 PM
So that's the hardware and software. I still want to make up a complementary solution to measure neutral to ground.

Neutral to ground voltages are typically around 1/2 of the voltage drop in the branch circuit. So if you start with 120 volts at the panel, and its dropped to 110 volts at your amp rack while under load, you can have up to 5 volts between the neutral and ground. Of course, this voltage will be modulated by any current draw on that branch circuit. You've also got to watch for swapped Hot-Neutral polarity in the receptacle, since that will cause 120 volts potential between the neutral and ground.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on April 10, 2016, 08:29:53 AM
Neutral to ground voltages are typically around 1/2 of the voltage drop in the branch circuit. So if you start with 120 volts at the panel, and its dropped to 110 volts at your amp rack while under load, you can have up to 5 volts between the neutral and ground. Of course, this voltage will be modulated by any current draw on that branch circuit. You've also got to watch for swapped Hot-Neutral polarity in the receptacle, since that will cause 120 volts potential between the neutral and ground.

Yes, this will actually be the more complex part because the voltage range is more variable. I'm thinking some kind of variable attenuator along with some basic overvolt protection on the output, twiddle the knob until the readings make sense.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Mike Sokol on April 10, 2016, 08:32:27 AM
Yes, this will actually be the more complex part because the voltage range is more variable. I'm thinking some kind of variable attenuator along with some basic overvolt protection on the output, twiddle the knob until the readings make sense.
I would suggest series limiting resistors with clamping diodes
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on April 10, 2016, 12:56:27 PM
I would suggest series limiting resistors with clamping diodes

That would keep it below about half a volt, though I've been toying with the idea of using clamping LEDs before attenuating the signal the rest of the way down to iphone mic level. That way you know there's a problem right from the start.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Mike Sokol on April 10, 2016, 01:09:56 PM
That would keep it below about half a volt, though I've been toying with the idea of using clamping LEDs before attenuating the signal the rest of the way down to iphone mic level. That way you know there's a problem right from the start.

Also remember to calculate when happens if you accidentally plug into a 120-volt receptacle mis-wired with 240-volts. I blew up a nice Sennheiser RF receiver one time by plugging into a "special" outlet they wired for the janitor's floor polisher in the gym. Yup, this was a brand new NEMA 5-15 receptacle in the wall wired up with 240 volts instead of 120 volts. I've seen this several times at gas stations, contractors compressors, and some of the PA systems I run for a British sound company in the area.   
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on April 10, 2016, 01:17:18 PM
Also remember to calculate when happens if you accidentally plug into a 120-volt receptacle mis-wired with 240-volts. I blew up a nice Sennheiser RF receiver one time by plugging into a "special" outlet they wired for the janitor's floor polisher in the gym. Yup, this was a brand new NEMA 5-15 receptacle in the wall wired up with 240 volts instead of 120 volts. I've seen this several times at gas stations, contractors compressors, and some of the PA systems I run for a British sound company in the area.

That's where I'm hoping JR's awesome outlet tester finds a manufacturer.   8)
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: John Roberts {JR} on April 10, 2016, 03:21:20 PM
That's where I'm hoping JR's awesome outlet tester finds a manufacturer.   8)
I dropped the 240V feature from my final cut of PCB because it took up too much room. Right now the OD-1 PCB fits neatly inside a standard off the shelf plug.

Just for chuckles I tested my 240v detector circuit design anyhow, and shared it with the most recent company to gong me..

The picture with the neon bulb glowing is across 240V, the second picture across only 120V is dark. Circuit is dirt simple, only two resistors forming a simple divider such that 120V divided is below neon threshold and 240 divided isn't.

The neon lamp is too thick so doesn't fit inside between the plug housing and PCB, and 200+V SMD resistors need to be 2012. I could almost cut a hole in the PCB for some extra clearance for the bulb thickness but the larger resistors to handle high voltage take up all my PCB real estate already.

JR

PS: I have my final cut PCB and parts waiting on my bench but I am forcing myself to do my taxes first...  and I hate doing taxes so this could take some time with me procrastinating. 
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on April 10, 2016, 04:54:39 PM
The picture with the neon bulb glowing is across 240V, the second picture across only 120V is dark. Circuit is dirt simple, only two resistors forming a simple divider such that 120V divided is below neon threshold and 240 divided isn't.

Nice trick - makes perfect sense, and that neon bulb isn't going to draw enough current to make things difficult.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: John Roberts {JR} on April 10, 2016, 07:06:21 PM
Nice trick - makes perfect sense, and that neon bulb isn't going to draw enough current to make things difficult.
In fact you want to current limit neon lamps above their breakdown voltage, but the resistor divider values can be sized to accomplish that at the same time.

KISS.

JR
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on April 12, 2016, 10:13:47 AM
Here's a scan I took last night - small church, this particular outlet too noisy for their acoustic guitar amp. They plug the bulk of their gear into Surge-X boxes nowadays.

Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Frank DeWitt on April 12, 2016, 11:02:52 AM

PS: I have my final cut PCB and parts waiting on my bench but I am forcing myself to do my taxes first...  and I hate doing taxes so this could take some time with me procrastinating.

I hear you.  I would get red in the face angry doing taxes.  I finally discovered a good account.  If you or anyone does anything more complicated then one job all year and no other income or giving then an accountant can probably save you more then he costs.   Example,  I have my own business doing Mechanical Design When my son was in college for ME I hired him for some extra help during his breaks, My account spotted it and asked if he could do about 2 hrs more a week.  He said If he can then he qualify as full time and he can get benefits.  I assume you give all full time employees free college tuition.  Bam  Just like that, his tuition became a PRE TAX expense.   That's real money.  That is one example, there have probably been 20 other savings I would not have thought of.
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Scott Helmke on April 12, 2016, 12:18:03 PM
Here's an outlet on one of their Surge-X strips.  Probably the bulk of the filtering is going on well above 20kHz.

Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: John Roberts {JR} on April 12, 2016, 01:54:28 PM
I am unsure what if anything to draw from those plots.

The product's power supply will ignore most of this.

It will be interesting over time if you can correlate what you see with what you hear.

JR 
Title: Re: Cost effective (even for weekend warriors) ways to measure power quality
Post by: Mike Sokol on April 12, 2016, 04:29:29 PM
It will be interesting over time if you can correlate what you see with what you hear.

I've found that you can learn a lot if you can create a way to make noise and inject it into your AC line while you have a guitar amp with a guitar plugged into it, simulating your real-world conditions. One simple way is an old-school Triac light dimmer with a big tungsten bulb. You should be able to watch your harmonics display and guitar amp noise modulate as you change the dimming percentage. Then see if you can borrow a neon beer sign from the local bar. Turning it on and off should generate a different type of noise in your guitar amp and spectrum display. By cataloging what you see on your display with what you hear you may be able to learn how to predict what you see on your display with an actual instigating factor. Then maybe you can chase it down and do something about it.