Old 03-31-2017, 04:35 PM   #1
pipelineaudio
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Default 4 conductor TRRS Lapel mic to XLR

Is there any trick to doing this? The pinouts I have seen online show the sleeve as mic+ and the closest ring as ground, but though it works plugged into the iphone, making an adapter this way doesn't seem to work on a mic preamp. Is there a resistor I need to stick in there somewhere?
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Old 03-31-2017, 09:13 PM   #2
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Those mics typically require a small DC supply of something around 1.5 volts to work. It's known as plug-in power (as opposed to phantom power).
Phantom power will damage them but you could run them from it by using a resistive divider with a couple of series diodes across the output of the divider to regulate the DC supply.
You'd probably need to add a low pass filter to such a supply though (another resistor and capacitor across the supply to the mic) because such a diode arrangement can be quite noisy.

The easiest way would be to use a 1.5 volt battery for the mic power with a DC blocking capacitor in series with the input to the mic preamp.

Last edited by ReaDave; 03-31-2017 at 10:20 PM.
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Old 03-31-2017, 11:59 PM   #3
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eek, do you think they make a device for doing all that or even a preamp I can pass thru to the daw?
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Old 04-01-2017, 03:36 PM   #4
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Easy enough to make yourself, but here's a professional ready-made one:

http://www.making-sound.co.uk/product/mini-pip/

I'm not sure this is the same, but it sure smells like one:

https://www.gothamsound.com/product/pip
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Old 04-02-2017, 12:50 AM   #5
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Any ideas where I could find a schematic for doing it with a 1.5v battery?
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Old 04-02-2017, 01:01 AM   #6
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Quote:
Originally Posted by pipelineaudio View Post
Any ideas where I could find a schematic for doing it with a 1.5v battery?
Give me an hour or so. I'll put something together for you and post it here.
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Old 04-02-2017, 02:46 AM   #7
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Here you go. Hand drawn on the back of an old envelope.

Component values should not be critical but try to keep close to those values. The 10uF capacitor should give you a low frequency rolloff of around 15Hz with most low impedance mic preamps. If you don't get enough level, try experimenting with the value of R1 but I wouldn't go below 1K ohm or above 22K ohm.
R2 and C1 prevent DC appearing across the mic preamp input.

You may need a fair amount of gain to get a good level. That all depends on the output of the mic capsule.

This is an UNBALANCED circuit. Keep the cables as short as possible.

The XLR view is from the solder terminal view, not the pin view.

If you are using Apple earphones and just using the mic and not the earphones, leave the tip and ring 1 disconnected. You will possibly run into crosstalk issues (earphone signal leaking into mic preamp) if you try to use the earphones and the mic at the same time.
There's no harm in trying though. It may work well.

Do not use phantom power with this circuit!

Click image for high resolution

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Old 04-02-2017, 03:47 AM   #8
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I'm afraid I can't do that, Dave...

1,5 V is way to low for most electrets, if you count in that you loose a few hundred millivolts over the resistance and because battery voltage goes down after a while.

The 4 connector mini jack electrets all get 5V. You can go up to 9V without any problem. I use 12V a lot, because I've got some 12V T-power sources.

A higher voltage will result in better high SPL handling, lower noise and a bit more distortion. But it's the first two that count for me.



This is a schematic for an electret mic on phantom power. Should be easy enough to translate to a 3,5 mm quad jack. Maybe build this into a small box with a quad connector?
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Old 04-02-2017, 04:10 AM   #9
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Quote:
Originally Posted by cyrano View Post
I'm afraid I can't do that, Dave...

1,5 V is way to low for most electrets, if you count in that you loose a few hundred millivolts over the resistance and because battery voltage goes down after a while.
Be careful with that circuit on Apple headsets. Apple and Android headsets work on around 1.5 volts and higher voltages can harm them. My circuit above was specifically for Apple and Android headsets (that's what Pipe mentioned in his OP).
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Old 04-02-2017, 04:24 AM   #10
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Yep, Dave.

I was trying to jog my memory, as I could recall two types of 3,5 mm connections around.

The schematics above work with a Chinese Apple lookalike. And that one doesn't work at all on 1.2 V (rechargeable AAA). Have you ever blown an Apple headphone/mic set? Or an Android one, for that matter?

The current Macbooks with quad connectors provide 5V, as far as I can tell. I'll pull up the schematic of one later.

Most electret pip powers supplies, especially the ones on batteries, us at least 3V for that reason.


EDIT: the jack on Macbooks supplies 3,3 V.
EDIT 2: The iphone is an entirely different beast. It has a way of detecting mic type and setting the bias voltage. Presumably, it can provide either 1,8 or 3,3V.
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Old 04-02-2017, 04:52 AM   #11
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I guess it all comes down to experimentation. The circuit I posted is about as basic as it gets and I was aiming for the simplest solution based on measurements from my iPhone 3GS and Samsung S6 (around 1.5v) which both work with the Apple headset I got with my original 3G iPhone.

It looks like you've done somewhat more experimentation with those mics and I have no reason to doubt your findings and research. After all, the original Radio Shack PZM mics ran from a 1.5 volt battery but most of us who owned them (I still have two of them) pretty quickly found out that they ran fine on 12 volts and had better S/N and more headroom at that voltage. It's probably the same for the Apple headsets.

I was being cautious with my Apple headset recommendation because I didn't want to be responsible for providing a circuit that caused damage!
If you've ran them from higher voltages though, it's all good.
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Old 04-02-2017, 06:06 AM   #12
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Well, I can't think of a case that would ruin an electret with 9V or less...

Now you make me wreck my brain

It just depends on what Pipeline wants. If it's just for one try, anything will work, even with one AA battery. If he wants to use it regularly, I'd go for a 9V block battery, or phantom feed.
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Old 04-02-2017, 09:10 AM   #13
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Don't get me wrong here. Your circuit is definitely a more eloquent solution being fully balanced and powered from phantom power. It will have much better performance in noisy environments and with longer cable runs.

I was just aiming for the simplest solution I could think of that would do the job.

If I was to use your circuit with an Apple headset though, I'd be inclined to replace the zener diode with a 5.1 volt one (1N4733) or even 4.7 volt (1N4732) just to play it safe.

Last edited by ReaDave; 04-02-2017 at 09:19 AM.
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Old 04-02-2017, 10:33 AM   #14
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I just need to run it once on a real mic pre, after that they run on phones or whatever they use.

Thank you guys, I will try this in a bit and see what happens!
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Old 04-02-2017, 05:13 PM   #15
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Cyrano, how many volts does the design you drew end up being?
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Old 04-03-2017, 04:22 AM   #16
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There's one zener in it. That sets the bias voltage. I've used it with 5,6 V zeners, as these are reputed to be the quietest. But I can 't hear any difference with 9,6 or 12 V zeners, so I've stopped being anal about it

5,6 V zeners are the quietest, but the gain in S/R is probably undone by a rise in temperature or so...
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Old 04-03-2017, 11:11 AM   #17
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Quote:
Originally Posted by cyrano View Post
5,6 V zeners are the quietest, but the gain in S/R is probably undone by a rise in temperature or so...
True that. The biggest source of noise though I suspect would be the mic capsules. Your circuit already contains plenty of filtering for the zener so I'd expect that to be insignificant compared to the capsule noise.
Mobile phone headsets aren't typically known for their low self noise!

My suggestion with the 5.1 or 4.7 volt zener was more to err on the side of caution with the supply voltage to the Apple headset.
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Old 04-03-2017, 02:16 PM   #18
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FWIW, Dave, that's not "my" circuit. It is exactly what I came too use after a lot of experimenting, but with added filtering.

And I can't tell if the filtering is needed, as I didn't have any problems with noise before...
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Old 04-09-2017, 08:57 AM   #19
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Just bumping this.
Pipeline, I'm curious to know how you went.
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Old 04-19-2017, 08:08 AM   #20
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Maybe I'm not understanding the original question, but does the mic not take a battery? I went through this recently and, while it took going through every adapter around to pass the signal to the other end properly, going stereo mini to stereo 1/4 and then stereo 1/4 to XLR worked. But preamp won't power the law mic this way, have to use the battery.
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