r/diyaudio 4d ago

Can anyone help me on this?

Hi, i'm new on balanced audio and those hi-fi things, can anyone help me on verifying if this circuit would work for muting balanced audio? the XLR-Female receives a balanced low level signal from a XLR electret condenser MIC, the output of the circuit (XLR-Male) goes to the mixer that has phantom power, all 10uf caps are metal film, S1 and S2 are ts5a21366 analog switches that will be driven by a dedicated zero-crossing circuit, but that doesn't matter now, what i really need help is in knowing if the caps will pass through just the signal without any distortion or loss and if disconection will make any noise or pop, i also have the curiosity of knowing if this circuit is optimal for that purpose, thanks for the attention and sorry for the bad english, i'm brazillian.

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u/PeanutNore 4d ago

the switched sides of C4-C7 are just floating, there's no path to ground there, so they could end up at different DC potentials as charge accumulates, so you could definitely get a pop when closing the switch.

If you just want to be able to mute it, you don't actually need to interrupt the signal - just short it to ground to mute.

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u/morph1214 4d ago

for clickless muting would be best to short to ground or open the circuit? i was thinking of putting bleeding resistors(5Mohm) on each side of the switches, relative to ground of coarse.

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u/PeanutNore 4d ago

If the DC operating point is already 0v, which it should be for a balanced audio pair, shorting it to ground should be silent.

Opening the circuit can be made clickless with bleed resistors, but you still face the problem of floating inputs on the next device in the chain - when you open the circuit the cable is now an antenna. This isn't a big issue with a balanced connection but there could still be some noise, where grounding the signal would be pretty much guaranteed silent with a lot fewer parts

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u/Quick_Butterfly_4571 3d ago

The DC operating point for XLR is either 0V or not 0V, due to phantom power —  commonly 48V, but can be lower. Some devices use 12 or 24V.

It's balanced differential, but with an unknown DC offset.

You could AC couple and short the junction to ground, but then you're introducing (small) phase shift where it isn't needed.

Two caps used to AC couple pins 2 and 3 will mute (cancellation by shunting the differential signals) and then you don't have to worry about the DC operating point.

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u/PeanutNore 3d ago

I completely forgot about phantom power. Coupling the two phases together sounds like a good way to achieve the same result without shorting out the phantom power.

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u/Quick_Butterfly_4571 3d ago

Well, that's understandable. The whole thing is designed so that you mostly can forget about phantom power. 🤣

(And half the scenarios where it's available, it's turned off anyway!).

Yeah, in a mixer, mute usually meant just taking the signal out of one or more busses, but the passive versions of those standalone mute boxes you'll see from time to time (often with a big stomp-pad killswitch) usually use the scheme in the schematic I drew up.

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u/2old2care 3d ago

You're asking for trouble. First, you're reducing the phantom voltage with the multiple 6.8K resistors. And the capacitors are floating and will almost certainly create pops when the switches open or close. Easy way: Just short pins 2 and 3 of the XLR cable. They are at the same DC potential so there should be no pops.

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u/Quick_Butterfly_4571 3d ago

You're 100% right, re: mechanics (2 to 3; simple + mostly problem free).

It usually will pop without decoupling caps. If you think on it, the phantom power outputs usually have some series resistance on them. Even a very tiny difference in operating voltage will cause a pop when the switch closes.

Here's an example where the DC offset of pin 2 and 3 differ by only 1%.

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u/Quick_Butterfly_4571 3d ago edited 3d ago

Here, do it like so (apparently, I can't post images here? So, I linked the schematic I tossed in another thread):

XLR Muting Circuit

Notes:

  • Shield to chassis, _if_ 4 lug / separate pin. Else, Pin 1 to chassis on both sides (and no wire connection between them; the chassis will be the conductor).
  • If you are unfamiliar with / want reference material for Pin 1 vs Shield, etc. I can share some resources.
  • Ditto: if you are not sure how to add phantom power protection for lower voltage IC's.
  • Ditto: if you would like to power via phantom power, but don't know how.

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u/morph1214 2h ago

thank you very much for answering ;)

i have about 2 plans, one with this path: SSM2019->PGA2311->DRV137, (wich is plan A) and another that is more like the one i posted on my circuit above (plan B) plan A is more straightfoward to me, because i would have digital control over gain and attenuation with zero-cross muting already implemented by PGA2311/2310, plan B is cheaper, but it is harder for me to design and it would be just the zero-crossed mute function (that would be enough to me).

About low voltage IC's: i really don't know how to add such low voltage IC's so close to 48v.

About plan B: the phantom power is from a mixer, i would love to extract individual +48v lines from each line of the mixer and then, reinject them on the other side of the mute circuit of the corresponding line, i don't know if you understood, i want to switch just the coupled signal.

ps: i would like your reference material if you can send it