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Background:

I have a condenser microphone, which requires +48V phantom power from my mixing desk. The microphone has a source impedance of "<200 Ohm" as per manufacturer. As you may recall, phantom power is delivered over both the "+" and "-" wires in the XLR connector, put on the wire with a 6.8 kOhm resistor pull-up on each wire. This means the maximum amount of current that can be drawn is about 14 mA (into a short) and more realistically < 10 mA. Some of this power is used by the built-in pre-amp in the microphone data sheet says 2.5 mA.

I've built an inline mute switch using a DPDT switch (rated for +48V DC) where the "mute state" switches the phantom power over to a blinking LED, reminding me I'm on mute.

Unfortunately, because this is a mechanical switch, AND because of the phantom power, switching on/off makes for loud pops. Given that I want the blinky-LED to consume some current when in the "mute" configuration, I probably need to switch off the pre-amp in the microphone to have enough current available -- and, anyway, I do need to turn off the signal from the microphone for it to be "muted."

With that background, I believe that I need to build some kind of active/solid state circuitry to mute/un-mute the microphone, turn on a blinking LED when it's muted, and still pass phantom power to the microphone. It's the last part that has me somewhat confused. Because the original source of the phantom power is already resistively protected with high impedance, how do I best re-buffer the signal, yet carry the phantom power forward without too much power loss?

I've considered extracting the common mode power with a center-tap transformer, and then using my own opamp to re-buffer the signal (which I can then turn off when needed) but this feels quite inelegant as I add another couple of high-resistance resistors in the path of the power. Yet, I don't know how to pass the power without also passing the signal any other way.

Here's an example sketch of what I have so far:

enter image description here

Microphone on the left, mixing desk on the right.

I'm looking for suggestions on how to better accomplish the separation of signal vs phantom power, ideally in a way that doesn't require adding more inline high-resistance resistors.

Jon Watte
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    So if your DPDT switch disconnects phantom power from mic and reconnects it to LED, why not simply rewire your switch so that it does not disconnect phantom power to mic but simply connects both + and - wires together and to the LED? – Justme Apr 21 '21 at 04:22
  • If I understand your proposal right, that would keep the microphone pre-amp powered, and have it output its signal into a dead short. I don't think that would be all that great for the microphone? But I don't actually know if it's correctly protected or not. There's also still the problem that if the switch makes connection with the LED before shorting out the wires, it would still pop. Audio is super finicky to mechanical switching, which is why I think solid state would be the only "good enough" solution. – Jon Watte Apr 21 '21 at 04:43
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    That's why sometimes you put resistors in series so it's not dead short. There are a lot of schematics how to do XLR mute switches on the net, even schematics of how commercial units or switches inside mics work. Definitely not solid state and still good enough. – Justme Apr 21 '21 at 05:01
  • I'd suggest re-thinking to apply Mute somewhere further down the signal chain, on line level without 48VDC present. –  Apr 21 '21 at 11:05
  • @user_1818839 You apparently have never run an actual phantom powered microphone setup, or have never had a need to provide a physical mute in such a setup. Which isn't very surprising, as that's a bit niche, but your advise here is less than stellar. – Jon Watte May 25 '21 at 22:54
  • @Justme I've read those schematics, and they all either use a mechanical switch (which causes significant pop) or they can't extract the phantom power or otherwise power intermediate loads (such as a big rude blinking LED saying "YOU'RE MUTED, DUMMY!") – Jon Watte May 25 '21 at 22:56
  • An update: I tried a DPDT switch approach, and it didn't work that well -- very loud pops. Also, I tried using a relay, powering it from a separate battery, but that actually makes noise while it's allegedly "muted;" presumably the relay contacts aren't 100% solid, or there's some EMI from the coil picked up out the mixer output side. Next idea: high-side MOSFET drivers to turn on a capacitor shunt. – Jon Watte May 25 '21 at 22:56
  • @JonWatte Curious what you mean. I have used, designed and built phantom powered systems and made my own capacitor (not electret) microphones (phantom powered), though my own capsules were a failure. Didn't even contemplate trying to switch phantom power on or off silently, or mute at mic level, so I'm probably missing your point. –  May 26 '21 at 14:34
  • I don't *want* to switch off the phantom power, but I do need the ability to mute the microphone. Specifically, whoever is in front of the microphone needs to be able to mute it, while the mixing board and/or transmission equipment may be out of reach. (This is obviously more important for live situations than recording situations.) So hearing "why would you want to do that" seems a bit unhelpful :-) The 48 V comes from a computer audio interface -- I'm not about to go hacking into the low-latency USB sound driver chain! – Jon Watte Jun 05 '21 at 19:38
  • @JonWatte - Hi, Your [new question today](https://electronics.stackexchange.com/questions/569656) seems to be the next step in your attempt to solve this problem. Is that correct? If so, to avoid duplication of effort, it would seem best for me to close this question (but *not* delete it), to avoid new readers posting comments & questions *here*, when things have moved on and their attention should be on that new question. Is there any reason for me *not* to do that? Thanks. – SamGibson Jun 05 '21 at 20:17
  • Ah yes, mute operation from the mic position has its uses. I stand by my original comment that muting is better applied at line level, (such as within the desk) and would suggest looking whether the desk has any inputs for remote control purposes. In a previous job we designed desks with such capabilities, precisely for announcer operation but these were special purpose products. Your alternative; a local micamp, with mute capabilities, is good (and allows line level output). Matchamp modules used to be good and cheap, leaving only the muting to do (can't remember if they had mute capability) –  Jun 05 '21 at 20:45
  • Matchamp still available. https://www.alice.co.uk/product/alice-mic-matchamp-xtx131-mkii/ No mute input but it has a gain port which could give you maybe 60dB of your 120dB attenuation, and quietly if they did their job as well as I think they did. (short cct = highest gain, will be obvious why if you've designed low noise balanced input stages) –  Jun 05 '21 at 21:02
  • @user_1818839 I'd love to have a desk, and an engineer to drive it! Currently there's not even a desk involved. The circuit is "microphone and screen and keyboard/mouse inputs in one place; cable run to a different location where the noisy computer/fan equipment is, where a USB interface goes straight into broadcast software." Muting in the broadcast software is less convenient because the software in question might be full-screen and have input focus on the computer side. Think "zoom presenting full screen slides" or "game streaming to twitch" as use cases. – Jon Watte Jun 06 '21 at 20:36
  • Now, as someone who wants to learn audio-level precision analog electronics, I'd be much more interested in the circuitry ("how to fish") rather than the built solution ("one serving of fish and chips!") Also, the match-amp doesn't do phantom power, so then another box would be needed for that ... – Jon Watte Jun 06 '21 at 20:37
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    @SamGibson yes, this question can be closed, as it didn't gather particularly helpful replies. – Jon Watte Jun 06 '21 at 20:49

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