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I'm adding a 'mute switch' into my XLR cable.

To mute an XLR cable/Phantom powered microphone you temporarily connect pin 2 and 3 in the cable to short the connection. I have seen plenty of youtube videos and tutorials online of people doing this to add quick mute buttons and stuff into their podcast setups.

The difference with what i'm planning is instead of a momentary switch like a lot of people make, i'm going to add an SPST switch, so that I can flick a switch and mute my microphone for long periods of time, and only enable it when I want to.

Will there be any risk to my equipment when I mute the microphone and short pins 2 and 3 for long periods of time?

If it helps, the microphone and usb interface in my setup are

https://eu.audio-technica.com/AT2020

https://focusrite.com/en/usb-audio-interface/scarlett/scarlett-solo

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  • \$\begingroup\$ Is your phantom power circuit protected against indefinite short circuits? \$\endgroup\$ – Andy aka May 1 at 16:43
  • \$\begingroup\$ I've absolutely no idea sorry. I'm fairly new to electrical engineering, i'm just tinkering as a hobby. I know its the method that everyone uses to mute their microphones? If it helps i'll update my question to include my setup items \$\endgroup\$ – S_R May 1 at 16:46
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    \$\begingroup\$ @Andyaka phantom voltage is on both pins 2 and 3 so they have the bias voltage relative to pin 1 ground. Audio is balanced between pins 2 and 3. Basically, shorting pins 2 & 3 would only short the audio, not power. \$\endgroup\$ – Justme May 1 at 16:46
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    \$\begingroup\$ OK, I understand - yes in most (if not all cases) this wouldn't cause a problem I reckon. \$\endgroup\$ – Andy aka May 1 at 16:47
  • \$\begingroup\$ the voltage between pin 2 and pin 3 is so low that short circuiting it will not damage anything. and is unlikely to cause a measurable change to power consumption. \$\endgroup\$ – Jasen May 2 at 0:32
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For long-term muting, you could just turn the 48V phantom power off from the Focusrite. Or, make the switch disconnect pins 2&3 from the mic, to turn it off and save power.

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  • \$\begingroup\$ Would I have to connect both pin 2 and 3? What would happen if I disconnected only 1 of them? \$\endgroup\$ – S_R May 1 at 16:52
  • \$\begingroup\$ That would depend on the mic obviously, but it would only get phantom power via one wire only, and be able to output audio via one wire only. You can try what happens. \$\endgroup\$ – Justme May 1 at 16:53
  • \$\begingroup\$ Ok thanks for your help! \$\endgroup\$ – S_R May 1 at 16:56
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    \$\begingroup\$ Be prepared for a "pop" when you un-mute with this arrangement. \$\endgroup\$ – Transistor May 1 at 19:38
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This may be most easily understood by looking at an old-fashioned transformer balanced configuration.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. A phantom-powered microphone circuit using balanced / unbalanced transformers and a mute switch.

How it works:

  • The 48 V supply is fed in the centre-tap of XFMR2. Current splits both ways in the transformer with the result that the DC current is running opposite directions in each half of the winding so the flux in the core cancels out and the transformer doesn't saturate.
  • At the microphone end the reverse happens and the 48 V is collected, smoothed and used to power the microphone preamp.
  • Return current is through the screen or shield wire (pin 1 on an XLR).

Now have a look at your MUTE switch. It shorts out the audio but for the DC it is only connecting one 48 V line to the other. It does not load the power supply or short-circuit it in any way.

Also note that the preamp remains powered so unlike your other question where you proposed to cut the phantom power lines this circuit should not give an audible "pop" when un-muted.

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  • \$\begingroup\$ Whoa...nice one. Do you know which specific mic preamps used this configuration? \$\endgroup\$ – Tyler Stone May 1 at 17:44
  • \$\begingroup\$ As far as I know all the big pro names like Sennheiser, AKG, Shure, etc, did that in the 80s. There was a move to differential amplifiers then to avoid the cost of high-quality transformers which had have a wide, flat frequency response. \$\endgroup\$ – Transistor May 1 at 18:00

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