Q: What is the function of the mic preamp "loading" switch?
A: In older tube and solid-state gear, the 600-ohm output transformer was designed to terminate into a 600-ohm load. When some vintage preamps were removed from their original consoles and wired into more modern systems, which nearly always had high input impedances (10K-ohm and above), the result was a very slight rise in high-frequency response. Because some engineers prefer that sound, we made the output transformer 600-ohm load resistor easy to switch in or out of the circuit.
Q: Does the mic preamp 'impedance' switch make any difference in the sound?
A: Yes and no – with condenser microphones, the input impedance switch (300 or 1200 ohms) will make almost no difference due to the buffer amplifiers in the mics. With dynamic mics and especially with passive ribbon mics, the preamp input impedance will affect the sound, often quite a bit. A good rule of thumb is the mic preamp input impedance should be approximately five to six times the microphone’s rated output impedance (a 50-ohm ribbon mic feeding a 300-ohm input impedance, for example).
There is no danger to the mic if you load it down however, so if it sounds good, don’t be afraid to use it. One example would be a modern ribbon mic with a 300-ohm output impedance on a guitar cabinet. Maybe it’s a bit muddy or tubby sounding; by lowering the input impedance of the preamp, you’ll filter out some of the low end, maybe enough to make it sound right. Maybe not…