I want an A/B switching box to connect two microphones to one phantom-powered mic preamp. I need the signal to remain as clean as possible and have XLR ins and an XLR out. I've been trawling the Net for ages without success so any suggestions would be gratefully received.
SOS Forum post
Technical Editor Hugh Robjohns replies: If you just want to be able to switch between two separate mics, the usual way to do it is to use an XLR patchbay.
Alternatively, you can make up a simple switching box with two input XLRs, one output XLR and a double-pole, double-throw switch. The ground path (pin 1) can be connected to everything all the time (the common ground). Use the switch to route the hot and cold wires from each input to the output. Beware though — there will inherently be a loud splat when you throw the switch as the phantom power is redirected and the new mic boots up. This isn't a very nice or elegant way to do things, and you should pull the fader down before throwing the switch to protect your monitors. Personally, I'd use the XLR patchbay approach as it is a lot more flexible.
While we're on the subject, it's worth noting that you can't connect two mics to one mic preamp input at the same time, because it will interfere with the input impedances and could potentially overload the phantom power current limit.
If you are looking for a way of comparing mics directly, it is actually a lot better to do the switching after the preamp stage. This is because different mics have different sensitivities, and so need different gain settings to match levels. Switching after the preamp also avoids problems with phantom power, and it switches high-level signals instead of mic-level signals, so it will sound better!