XLR to TS grounding/shock concerns

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Re: XLR to TS grounding/shock concerns

Post by crochambeau »

FAP wrote: Sat Aug 20, 2022 7:58 am I think we need like a crochambeau bat signal or something :lol:
I was cramming for a noise show in the park a couple of counties over yesterday.

There's a lot to unpack here, and you'll have to forgive me - but I'm resistant to immersing myself into a project that is geared toward being a saleable product without compensation of some sort. But here is my understanding of phantom power (I'm not looking shit up to confirm, so this may be factually incorrect).

At your mixer, the signal pins should both be DC hot. This is because you do not want to run DC voltage "down" a transformer in an unbalanced manner and cook it.
FAP wrote: Sat Aug 20, 2022 5:15 pm
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
Where are these measurements taken? At the mixer/power supply, or some other instance of XLR?

Pin 3 is ground. Negative probe to pin 3 referencing any of the other two pins should read a positive voltage. I'm not seeing that there.

I'm not 100% clear where the unbalanced phone plug is coming into play. Generally speaking, you don't want DC across a phone plug.

I don't think it takes a lot of bias voltage to charge a carbon element. If you're dealing with vintage elements I'm assuming you've familiarized yourself a bit with their quirks.

Transformer:

https://www.tamuracorp.com/clientupload ... C-5017.pdf

Did that SINGLE PAGE .pdf take like a minute to load for anyone else? WTF?

Anyway, that is a 600:600 transformer. Transformers are rated at AC impedance, not DC resistance. The seller did not provide hardware under false pretences. That lower DC resistance called out on line 7 is why it is important that the DC fed into the circuit is balanced with respect to the iron, as transformers go funny really quick when you saturate the core.

Which brings me back to your DC readings and my misunderstanding of them.

FWIW, my eyeball take of the original circuit looked good. I still don't know where the XLR to TS is coming into play.
When in doubt, add resistance.

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Re: XLR to TS grounding/shock concerns

Post by FAP »

crochambeau wrote: Sun Aug 21, 2022 9:13 am I was cramming for a noise show in the park a couple of counties over yesterday.
Yes, yes, I was just taking the piss. I sincerely appreciate your time :)
crochambeau wrote: Sun Aug 21, 2022 9:13 am Where are these measurements taken? At the mixer/power supply, or some other instance of XLR?
From one of the XLR inputs on my mixer:
mixer.jpeg
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crochambeau wrote: Sun Aug 21, 2022 9:13 amPin 3 is ground. Negative probe to pin 3 referencing any of the other two pins should read a positive voltage. I'm not seeing that there.
Hmm, that is strange now that you mention it... in my case, negative probe to pin 3 only reads a negative voltage when referencing pin 1 (referencing pin 2 reads no voltage at all).
These measurements were all VDC, by the way; I tried measuring VAC for shits but couldn't get a good reading (doubtful it'd output VAC anyways).
crochambeau wrote: Sun Aug 21, 2022 9:13 amI'm not 100% clear where the unbalanced phone plug is coming into play. Generally speaking, you don't want DC across a phone plug.
Probably a miscommunication on my part: I was referencing a 'pro' XL microphone I had which came with an XLR female to 1/4" male mono cable, like so:
xlr-female-to-quarter-inch-male-mono.jpg
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On this cable, pins 1 & 3 are tied together (i.e. tying - and GND together for the 'downgrade' to 1/4"); I was just pointing out that if this company was tying 1 & 3 together as part of their standard product line, then I'd probably be safe doing the same for my much smaller scale DIY project.

There's no XLR-to-1/4" conversion going on in my project: it'll always be an XLR out, just with pins 1 & 3 tied together.
crochambeau wrote: Sun Aug 21, 2022 9:13 amI don't think it takes a lot of bias voltage to charge a carbon element.
Apparently, it does!
You can run an unbalanced 1/4" mono cable across the earpiece element and have a working microphone no problem, but the same can't be done for the mouthpiece.
Given that [from my research] phone lines get ~48VDC, it would stand to reason, then, that the mouthpiece is being amplified with ~48VDC somewhere along the line (could very well be at the base level, since the handset circuitry by itself isn't active).
articles.jpg
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crochambeau wrote: Sun Aug 21, 2022 9:13 amFWIW, my eyeball take of the original circuit looked good.
It looked good to me, too: no idea why it's not working.
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Re: XLR to TS grounding/shock concerns

Post by FAP »

If it helps, this is basically what I'm trying to accomplish:
tldr.jpg
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Re: XLR to TS grounding/shock concerns

Post by crochambeau »

FAP wrote: Sun Aug 21, 2022 10:53 am
crochambeau wrote: Sun Aug 21, 2022 9:13 am I was cramming for a noise show in the park a couple of counties over yesterday.
Yes, yes, I was just taking the piss. I sincerely appreciate your time :)
Hahaha, no worries.

XLR to 1/4" cables exist, and are often a lossy compromise.

We're speaking of phantom power, which is delivered on an XLR INPUT at the mixer.

The aforementioned XLR to 1/4" cable, which has titled this thread so I assume it is important, does not connect into that system.

Is it actually important, or just some random cable that came with a microphone somehow related to the project?

I presume you see my confusion here, so I'll stop hammering on it.
FAP wrote: Sun Aug 21, 2022 10:53 am
crochambeau wrote: Sun Aug 21, 2022 9:13 amI don't think it takes a lot of bias voltage to charge a carbon element.
Apparently, it does!
You can run an unbalanced 1/4" mono cable across the earpiece element and have a working microphone no problem, but the same can't be done for the mouthpiece.
Given that [from my research] phone lines get ~48VDC, it would stand to reason, then, that the mouthpiece is being amplified with ~48VDC somewhere along the line (could very well be at the base level, since the handset circuitry by itself isn't active).
articles.jpg
1/4" high z (impedance) phone plugs are not typically used in DC applications (they are a terrible connector for DC), so yes - plugging a microphone capsule that requires DC in order to operate into a high z line is going to yield poor or non-existent results.

That telephones are connected to a 48 volt power supply does not mean that the actual voltage across a carbon capsule needs to be 48 volts. Early telephones did not contain amplifiers, so the signal at the microphone needed to be strong enough to drive miles of wire.

In looking at the project you're describing, I assume with phantom power in the mix you're dealing with a quite capable amplification system, so the huge charge right out of the gate is not needed.

How many volts will it take? I would expect 1-2 volts should activate a carbon microphone element - but that is a wild ass guess, and if you're using OLD stock (new or used) the variances of what a given microphone needs to produce sound will probably change from one mic to another.

With respect to reading voltages, I suggest always knowing where your common/ground is. when reading voltages simply connect the COM (typically black) probe to common and measure the area of interest with the other probe. That way we don't have to decipher stuff like this:

1 & 2 ≈ -48v

1 & 3 ≈ -48v
2 & 1 ≈ +48v
2 & 3 ≈ 0v
3 & 1 ≈ +48v
3 & 2 ≈ 0v

As those colored pairs are actually saying the same thing, but it introduces a sort of option paralysis that befuddles comprehension. At least, it does to me - and I'm trying to help. ;)
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Re: XLR to TS grounding/shock concerns

Post by crochambeau »

FAP wrote: Sun Aug 21, 2022 11:10 am If it helps, this is basically what I'm trying to accomplish:
Image
Okay, this is a cool idea.

Feeding the telephone power via phantom power/XLR to your mixer, and then also running 1/4" external ins that can be manipulated by the phone hardware?

That's cool as fuck, you should build it.

DC block via capacitor anything going away from the phone on 1/4" branches. You can probably treat the "speaker" on the handset as a moving coil microphone, meaning that part does not need to dick around with DC anything. I may be wrong, they made phones in so many different configurations it can spin the head.

The switches in the handset cradle and the switches that are in the rotary dial will be connected to the power supply in a stock phone, as power interruption was how these output signals to operators. My guess is that you do NOT need that connected. Power only needs to go to the microphone element, everything else can treat the switches as signal interrupts.

Anyway, good luck. I can put more engineering behind this (trust me, you don't need or want me to) but I'll ask for a built telephone in return. ;)
When in doubt, add resistance.

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Re: XLR to TS grounding/shock concerns

Post by crochambeau »

crochambeau wrote: Sun Aug 21, 2022 11:26 am [

1 & 2 ≈ -48v

1 & 3 ≈ -48v
2 & 1 ≈ +48v
2 & 3 ≈ 0v
3 & 1 ≈ +48v
3 & 2 ≈ 0v
Is this just a pin 1-3 reversal? I'm hoping it user error and not wrong voltages being produced at the mixer - as those can cause damage.
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Re: XLR to TS grounding/shock concerns

Post by NoiseWiki »

FAP wrote: Sun Aug 21, 2022 10:53 am
Probably a miscommunication on my part: I was referencing a 'pro' XL microphone I had which came with an XLR female to 1/4" male mono cable, like so:

Image

On this cable, pins 1 & 3 are tied together (i.e. tying - and GND together for the 'downgrade' to 1/4"); I was just pointing out that if this company was tying 1 & 3 together as part of their standard product line, then I'd probably be safe doing the same for my much smaller scale DIY project.
Okay well that's not helpful for what you want to do because it can't be plugged into a jack with phantom power. In this case it's fine to connect those two lines because it's unbalanced
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Re: XLR to TS grounding/shock concerns

Post by FAP »

Okay, let's just say fuck it to the XLR-to-1/4" cable analogy, because it's clearly muddying the waters.
fuqit.jpg
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Let's reference this instead:
simpler.jpg
To answer the other questions:
crochambeau wrote: Sun Aug 21, 2022 11:40 am Is this just a pin 1-3 reversal? I'm hoping it user error and not wrong voltages being produced at the mixer - as those can cause damage.
Let's keep it simple: measuring with the negative probe at pin 3, I get -48v with the positive probe at pin 1. With the negative probe at pin 3, I get 0v with the positive probe at pin 2.
crochambeau wrote: Sun Aug 21, 2022 11:36 am DC block via capacitor anything going away from the phone on 1/4" branches.
What would you recommend for a farad value for the blocking cap?
Would putting a cap in series with the positive lead (pin 2) be sufficient?
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Re: XLR to TS grounding/shock concerns

Post by crochambeau »

Pin 1 is ground/common in XLR, not pin 3. Hey, I looked something up!
FAP wrote: Sun Aug 21, 2022 3:49 pm
crochambeau wrote: Sun Aug 21, 2022 11:36 am DC block via capacitor anything going away from the phone on 1/4" branches.
What would you recommend for a farad value for the blocking cap?
Would putting a cap in series with the positive lead (pin 2) be sufficient?
I use 1 uF a lot in line level/middle impedance (like 10K) stuff, but if this is feeding a low impedance microphone input increasing that will allow lows through.

Phantom power comes in on two lines. While it's true that blocking one line will interrupt a DC circuit on those two lines alone, there are three lines with XLR, and any pin 1-3 interconnectivity will result in voltage coming through.

So, you obviously want a DC blocker on the HOT microphone signal, feeding the +48 volt phantom power signal lead (1 or 2 is inconsequential, as this is balanced line selection only determines output polarity).

Since the COLD side of the microphone capsule needs to see COMMON in order for the +48 to mean anything you'll want a DC blocker there as well.

Look here:
carbontreat.JPG
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The resistors in yellow, you want those two resistors to be tightly matched.

You could also get this set-up with a transformer, though you want the side being fed the DC voltage to have a center-tap so you can grab the DC from a balanced point in center. All of this balancing just means that the negative and positive signal excursions that are riding along the DC on pin 2 & 3 cancel each other out when fed to the microphone element, otherwise it'll load the signal and losses will occur.

I feel like I'm working on this thing now, is that your intent?
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Re: XLR to TS grounding/shock concerns

Post by FAP »

uhh... I can't really afford to send one of these things free, so no, not really :oops:

Thanks for the explanations. I think I'll just have to R&D this thing some more.

EDIT: I think it's clear somewhere along the line I must've confused 1 & 3's functions with each other. I'm going to re-wire the original circuit and post my findings later.
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