Op-amp; switching between non-inverting and differential amplifiers on the fly

[Jhonyrod]

Hello! I want to build a microphone preamp, mainly for my computer, but I also want to be able to use it with other equipment if I need to. I'm not talking about those cheap PC mics, I'm talking about proper mics with a balanced differential signal output.

I've been working on the design for a while now, but I've got stuck in a couple of places. The matter at hand is that I want to use a single OPA2134 (dual audio op-amp) to either amplify "both" signals (hot and cold) or recombine them as a mono output, selecting between those two options with a flick of a switch. I plan on using analogue multiplexers for that effect but I'm not sure how I need to wire the op-amp to be able to switch between modes.

I also want to add later a phantom power option (I was fiddling with that, that's why you see a 60V input) and maybe even a USB ADC, but first things first.

Here's an screenshot of what I've got so far:



S1 on that schematic switches the cold input between the op-amps and S2 grounds the non-inverting input of U1A when it's not in use.

I haven't added any AC coupling or filters yet, I'm not even sure that I'll need them given the source, but I think I want to build a physical mockup first and adjust as needed.

Thanks for your help!

[Hideki]

That is a strange device you have invented

Unfortunately it doesn't work like it should in any of the modes. As drawn it looks a bit like the first part of an instrumentation amplifier, but there are no resistors on the non-inverting inputs to ground, so you have almost infinite input impedance. For a mic preamp, those resistors would be rather small in value (closer to 1k than gigaohms), or the mic will be completely unloaded. Gain is usually set by a variable resistor linking the two halves (R4 and R5 in your case). This is followed by a normal single-op-amp differential amp.

With S1 and S2 in the other position I don't see how it will "recombine them as a mono output". Note that the microphone signal IS already mono. It's just balanced (or differential) - not left and right.
You end up with one op amp doing nothing and the other being a non-inverting amplifier - again with a sky-high input impedance.

[Quarlo Klobrigney]

KISS. How about just putting a phase reversal switch on the input?