A/B Switch 2 mic pair in / 1 mic pair out (must carry phantom power

mlynnh

Member
2020-02-13 9:13 pm
The challenge:
Imagine a music room in a school that has microphone pairs on opposite sides of the room. One pair or the other pair will be used for recording purposes, but never both pairs at the same time.

The microphone (pair) selector switch must carry phantom power to the pair of microphones in use, with no phantom power delivered to the unused microphones. This switch must select between pair A or pair B silently.

Inputs XLR:
I want to select pair A or pair B (condenser microphones w/Phantom power) applied to the pair in use.

Output XLR:
I want to output via one pair of XLR cables to an analog to digital convertor, which will feed a computer used for recording.

This switch must silently switch between microphone pairs.

Perhaps a simple DPST switch would do the job, but as I'm not confident with phantom power (48VDC), I thought that I had better ask this group.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> with no phantom power delivered to the unused microphones. This switch must select between pair A or pair B silently.

Can't be done. Phantom power mikes thump, more or less, at power up and power down. So when you switch to B, B mike thumps and wakes-up the sleepers in the back of the class. Switch to A, thump again. Ideally, on the better Phantom mikes, the thump is common-mode and tends to cancel in a good mike preamp. But 24V thumps in a 10mV signal demands a LOT of CMRR. And some mikes power-up very asymmetrically.

The answer is two mike preamps, leave both pairs hot, and switch at line level.

Or turn-down before switching.

Or accept thump.

Oh, I suppose we "could" devise a multi-step switch which makes the power, waits, and connects the signal. More to go wrong IMHO.
 

jaddie

Member
2011-04-01 12:06 am
> AC couple them through the A/B switch. Should thump very little

Phantom is 48V through 3.4k. Some mikes draw 1.4mA, some draw 8mA. The mike lines can sit at 21V or 43V of DC. Even with high CMRR, that's major thump in a mV circuit.

Hence my suggestion. Keep them all powered, block the DC with a capon each line, switch would have zero phantom on it. No thumps.