Rule of thumb for mic impedance

Which ever setting sounds the best to suit the track!

R

I wish it was that simple. It's really easy when the system is well defined, like CATV. 75R source, 75R load for all frequencies in the specified range. You are looking for optimal power transfer, so you'll lose 1/2 of your input signal voltage but you'll not get standing waves or reflections.

With audio gear, there is no set impedance system so it can vary wildly across frequency, so while you can match power at a singlular frequency, other points might be very different, and might change as the signal changes too.  A moving target if you will.

Speakers do this, and it's why you always want lower driving impedance than the speaker you are driving, although there isn't a hard rule of thumb here either. Maybe at least 2x the rated impedance of the load vs. the source, I.E., 2 ohm head driving 4 ohm speaker..

But IMHO, you want a fairly low driving impedance to overcome any potential issue with interfacing to gear.

Other points of interest is that a lower driving impedance in conjunction with the cable(or other parasitic) capacitance gives you a higher cutoff point.

I'll give you the rule of thumb....if it's vintage design preamp--use the original impedance from it being mounted in the console. If it's a new one with no transformers or tubes...you also won't likely have an adjustment, but higher is better.

It's not really a matching thing with the mic. You CAN certainly use it that way for color...but, IME, vintage designs with 1200+ impedance tacked on get weird sounding--it's not JUST more highs and lows. If you want a vintage sound, the muted top/bottom from loading is part of that...if you don't...don't use a vintage design. Adding variability to an old design just ends up in some weird NEW territory, IME.

Anyway--that's my rule of thumb for preamp impedance.