A simple XLR momentary A/B - will this work?

[spargo]

I'm trying to design an A/B box for an XLR connection.  Ideally, this would work with dynamic and 48V phantom powered microphones.  Any input on this design?

It uses a momentary switch, and the idea is that it alternates shorting pins 2 and 3 on each output.  The caps will actually be non-polar, I just couldn't get the right symbol in Eagle.

[spargo]

Well, guess I'll order some parts and give it a try.  The caps would be 50V to handle the 48V phantom power.

Does anyone know how to use the phantom power supplied to power an LED?

[Mike Burgundy]

I think shorting 2 and 3 might work as mute, but I doubt this will work problem-free.
Which in/output connects to a microphone?
What's the intended mics impedance (typically 150-300Ohms)? What's the intended mic preamp impedance (typically 1-3k)? You're bridging the "active" connection with just 660Ohms - that'll cost quite some signal.
also switch bounce might very well produce a godawful bang when using phantom, or even without.

[spargo]

From the research I've done, plainly splitting the signal via a switch will get you pops.  But this is a different method...Essentially both signals are always active, but when the switch is engaged one of the channels shorts with pins 2/3 and the other channel that was previously shorted unshorts and engages.  Supposedly this should work pop-free even with phantom power.  OUTB can be sent phantom power to support a condenser mic plugged into the input.  The musician's mic goes to IN, and OUTA and OUTB can go to any two different places.

[.Mike]

When I tried to make an XLR muter, I used the wiring diagram from the ProCo Cough Drop.

For me, it didn't fully mute-- there was still some signal when the mic was muted. I tried removing the cap and resistor, and shorting pins 2 and 3 directly, and it still had some bleed through.

I set it aside, 2 years ago. I should really get back to it... heh.

Mike

[spargo]

I've prototyped this and it seems to work great with dynamic mics - no pops.

When trying it with a phantom powered mic, however, there is a slight pop or click when switching channels (there's a bit more of a pop when shorting vs. unshorting).  Any ideas?  This couldn't be as simple as something like a pulldown resistor we use in stompboxes, could it?

[harmonic]

Spargo: did you get any further with this? I'm going to be attempting to build the exact same thing very soon. :-)

[spargo]

Yes, I am near done.  The circuit is complete but I ran into multiple issues with switching, trying a momentary 3PDT, JFET switching, and CD4053 switching.  None worked well.  I'm going to give one final go with relays and I will keep you posted.  If this works I'm going to have a few PCBs made and may have some extras if you're interested.

[harmonic]

Interested to hear how the 3pdt momentary switch didn't work. What was the problem with it?

Yes, very interested to hear how things progress and any layouts, etc. that are designed. :-)

Cheers!
h.

[bluesman1218]

Sounds like a project that RG should weigh in on. BTW, I would use 'lytics rated for more than 50v. That's cutting it awfully close, don't you think?

[spargo]

RG has helped out a bit in other threads for this, in regard to the switching aspect.  What are you talking about for the 50V, the capacitor ratings?  It's difficult to find bigger since they need to be non-polar.  Besides, most phantom power, although it's called 48V, is much lower.  Many mics only need as little as 12V from phantom power.  The 50V non-polar caps I got are huge.

[bluesman1218]

Oops, I forgot what you said about the caps in the schem.  I haven't researched it, but I seem to remember higher voltage caps in smaller form factor. You may just have to look at different styles. As far as the actual voltage goes, I was considering the voltage supplied by the circuit, not the draw by the mics. If the voltage is there, that determines the safety margin you want.

[spargo]

The circuit itself is passive.  Due to the switching difficulties though, it will likely require 9V to switch a relay and power an LED - but this power won't be involved in the circuit itself.

[bluesman1218]

Got it. You're not providing the Phantom Power. Now I'm up to speed.

[spargo]

It's basically a much simpler clone of a ProCo Panic Button, for using to talk amongst a team and sound teach via in-ears.

[harmonic]

Spargo: what was up with the 3pdt momentary switch solution?

[spargo]

I could only find one 3PDT momentary switch and they were from Mammoth.  After getting them, they just seemed like something I could see breaking easily, and they needed a bit too much force.  They still had that light "click".  They need to be very easy to press.  If the relays work well, I'll use those soft touch SPST momentary switches from Small Bear.  Those are AWESOME.  I use them for tap tempos, etc.  No click at all.

[harmonic]

Ah, I see. Just to be clear, the click you describe is the mechanical click of the switch or a thump on the audio signal?

[spargo]

Mechanical click.  I wanted a switch that was smooth and easy to depress.  Not great if you're playing live and don't push the switch down far enough to change channels.    I've gotten the design to have no pops in the audio path with dynamic or condenser mics.

[harmonic]

Right, cool. I can see the 'quality' aspect being higher on a soft-touch switch. On the other hand, the more defined 'click' of the Mammoth part might help avoid accidentals. :-)

So the circuit as posted at the top of this thread works without pops for phantom-powered condensers when using the Mammoth 3pdt?