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[Mike Sokol]

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? 

[Scott Helmke]

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.

[Scott Helmke]

Silly idea for attenuating noise from that venue - capacitor in series with a light bulb.

[John Roberts {JR}]

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

[Scott Helmke]

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.

[Frank Koenig]

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

[Scott Helmke]

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.

[Eric Vogel]

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....

[Jonathan Johnson]

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.

[John Roberts {JR}]

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   

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