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[Scott Helmke]

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.

[Mike Sokol]

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...

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?   

[Stephen Swaffer]

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.

[Scott Helmke]

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

[Art Welter]

Scott,

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

Art

[Mike Sokol]

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.   

[Scott Helmke]

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.

[Geri O'Neil]

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!

[Steve M Smith]

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.

[Mike Sokol]

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.

[ProSoundWeb Community]