XLR to TS grounding/shock concerns

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FAP
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XLR to TS grounding/shock concerns

Post by FAP »

I’ll elaborate later but in short, I’m working on a project that necessitates the use of XLR jacks with pins 1 & 3 (negative & ground) tied together. I’m just concerned about the possible risk of shock when using phantom power, something that will also be necessary. Everything I’ve read so far indicates this likely won’t be an issue—plus my enclosure is plastic—but it’s the first time I’ve built something with the intent to sell it to professional musicians who likely deal with different grounds/grounding issues every day.
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Re: XLR to TS grounding/shock concerns

Post by NoiseWiki »

That is intriguing... well I can't suggest much but I did find this interesting article

https://www.tangible-technology.com/pow ... nnect.html
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Re: XLR to TS grounding/shock concerns

Post by FAP »

A bit more info:
- I'm trying to make a microphone out of an old rotary phone handset
- I want to add a switch to toggle between the earpiece speaker and the mouthpiece speaker
- I was lead to believe the mouthpiece speaker needs +48v in order to operate (I did a lot of research here but in short, +48v was a standard for old telephone systems)
- Phantom power, of course, provides said +48v; I get phantom power from a Behringer xenyx 802 mixer (i.e. not directly into an amp)
- I tried the schematic below ("before") but it didn't work: I made some alterations ("after") that do work
- Interestingly, with my "after" schematic, the mouthpiece works even with or without phantom power: phantom power just boosts it.

TL;DR I just want to double-check if there'd be any issues with my "after" schematic i.e. do I risk blowing stuff up with how I have it wired?

I should also note the transformer I was using turned out to be 60Ω:60Ω, not 600Ω:600Ω as the "before" schematic specifies (gonna have to get a hold of Digi-Key on that one: I got gypped!): I'm curious to know if that may have been why the "before" schematic didn't work.
EDIT: here's the transformer I have:
https://www.tamuracorp.com/clientupload ... C-5017.pdf

phonecircuit01beforejpg.jpg
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phonecircuit02after.jpg
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Last edited by FAP on Sat Aug 20, 2022 8:04 am, edited 1 time in total.
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Re: XLR to TS grounding/shock concerns

Post by FAP »

I think we need like a crochambeau bat signal or something :lol:
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Re: XLR to TS grounding/shock concerns

Post by NoiseWiki »

So you are using both the mouthpiece and the earpiece as microphones?
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Re: XLR to TS grounding/shock concerns

Post by NoiseWiki »

Seems ok.. one thing I noticed in that link is the author mentioned that some phantom power supplies don't actually go up to 48v but I imagine this system will work with less than 48.. in fact the voltage rotary phones use is -48 not +48 so your previous attempt made have had the wrong polarity?
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Re: XLR to TS grounding/shock concerns

Post by FAP »

NoiseWiki wrote: Sat Aug 20, 2022 8:09 am So you are using both the mouthpiece and the earpiece as microphones?
Yep! And an interesting not on polarity, but I tried swapping leads already so I don’t think that’s the culprit. There doesn’t seem to be a hard rule about which side is + or - when working with either mouth or ear piece, at least as far as I’ve experienced.

Also, fun fact: rotary telephone bells chime at 90vac at 20hz. This explains why they sounded so weird when I tried just plugging them into the wall (120vac at 60hz).
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Re: XLR to TS grounding/shock concerns

Post by NoiseWiki »

FAP wrote: Sat Aug 20, 2022 8:37 am
Also, fun fact: rotary telephone bells chime at 90vac at 20hz. This explains why they sounded so weird when I tried just plugging them into the wall (120vac at 60hz).
Actually my first thought was analog phone lines are 90v so I was wondering where the 48v cane from but it appears that the phone receives a ring tone at 90v ac and then when in use is 6 to 12v DC. It's kinda crazy but obviously worked

Anywho.... I guess I'm a little concerned about tying the 1 and 2 together. I'm spitballing here because I don't really know but my guess as to why it works is because it's canceling out the polarity creating an inverted signal -48v

But if it hasn't fried the mixer then I suppose it's fine but I suppose there's a chance it's sending 48 to the ground which doesn't sound like a good thing
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Re: XLR to TS grounding/shock concerns

Post by FAP »

I ran some more tests and am preserving my findings here for posterity:

First, to establish the terminology I'll be using:
1 = XLR pin 1 (typically - or "cold")
2 = XLR pin 2 (typically + or "hot")
3 = XLR pin 3 (typically ground/shield)

Reading XLR voltages with my DMM, where the first number is the positive probe and the second number is the negative probe:
1 & 2 ≈ -48v
1 & 3 ≈ -48v
2 & 1 ≈ +48v
2 & 3 ≈ 0v
3 & 1 ≈ +48v
3 & 2 ≈ 0v

Reading XLR current with my DMM (order of probes doesn't matter):
1 & 2 tied together, cut (DMM), then to 3 ≈ 13.6ma
1 & 3 tied together, cut (DMM), then to 2 ≈ 6.8ma
2 & 3 tied together, cut (DMM), then to 2 ≈ 6.8ma

Both mouthpiece and earpiece have no polarity i.e. either side could be set as "+" or "-"
So when I say "+" or "-" here, I'm talking about the mouth/ear piece terminals, but just keep in mind "+" and "-" can swap places without consequence.

For the earpiece:
1 & 2 tied together as "+" with 3 as "-" yields no response, with or without phantom power
All other combinations, whether leads are tied together or not, work.

For the mouthpiece:
1 & 2 tied together as "+" with 3 as "-" yields a quiet, muffled response with phantom power, and no response without phantom power.
1 & 3 tied together as "+" with 2 as "-" yields a working microphone, with or without phantom power (phantom power will still boost the signal).
2 & 3 tied together as "+" with 1 as "-" yields a working microphone, with or without phantom power (phantom power will still boost the signal).
Interestingly, if 1 or 2 is set to "+" while 3 is set to "-" (without connecting 3 to either pin 1 or 2), the mouthpiece will only work as a mic if phantom power is on.

But the smoking gun is an XLR-to-1/4" mono cable that comes with a mic I got god know when: tracing the leads on this cable, pins 1 & 3 are tied together. I'm not familiar with the Gemini brand of microphones, but I figure if it's good enough for a company with large-scale manufacturing capabilities, it's probably good enough for me.
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Re: XLR to TS grounding/shock concerns

Post by Soloman Tump »

This is all very interesting and something j cannot help with at all.

Good luck
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