This is way above my head - but I was planning to ask a question about this because I am very interested to add this to my future mic pres..
Here is some info I have gathered if it is of use to anybody.. especially use newbies... all via Hugh Robjohns of the SoundonSound mag
(I Understood the first one if that's any help)
"The input impedance of a mic amp has a direct bearing on the captured sound quality from most (but not all) microphones. In general, the higher the impedance, the better, and a lot of high-end mic preamps have input impedances of around 5k(omega) instead of the more normal 1.2k(omega). However, some microphones ? notably ribbon mics ? prefer a much lower input impedance too, say 600(omega) or less.
Increasing the impedance means that the microphone has to supply less current, which can help it to produce a greater signal level. More level from the source means less amplifier gain is required in the preamp, which means less noise overall. A higher impedance also means there will be less HF loss from the inherently capacitive cable. This translates as a brighter, clearer sound, often with a slightly more apparent room acoustic.
Reducing the input impedance places greater demands on the microphone to supply current, and this can cause all sorts of odd effects with some mics. In general, dynamic mics will respond to a lower input impedance by producing a more uneven frequency response, as resonances in the electro-mechanical system become more emphasised. This can be thought of as a kind of 'free EQ', though its effects are rather unpredictable and not always useful!
In any case, all of these effects are fairly subtle. They're obvious enough when making direct comparisons, but you won't usually be aware of them in a complete mix. Dynamic mics will almost always show the greatest effects, while good-quality transformerless mics will usually show the least change."
Before the advent of the practical silicon transistor amplifier around 40 years ago, it was custom and practice in the audio industry to use impedance matching when interfacing equipment ? this was the era of the 600(omega) termination. In the case of microphones and microphone amplifiers, it was important to transfer as much power between source and destination as possible, and that meant matched impedances. Consequently, microphones (generally ribbons and dynamics) were designed typically with 300(omega) output impedance and the mic amplifiers of the day offered 300(omega) input impedances. However, as technology advanced it became fashionable to use voltage coupling, where the amplifier input impedance was at least ten times higher than the source impedance. Consequently, mic amp inputs gradually rose to something like 1.5k(omega) or even 3k(omega).
Presonus Eureka..
"The third rotary control is actually a switch to change the input impedance in five steps from 50(omega) to 2500(omega). Many preamps offering variable input impedance provide a higher maximum figure than this ? typically 5k(omega) or so ? but the range offered here is sufficient to match a wide selection of mics, including most ribbons, and affords useful tone-shaping characteristics which are far more subtle than most EQs can manage."
ISA 428The button cycles through four options: Low (600(omega)); ISA110 (1400(omega)); Medium (2400(omega)); and High (6800(omega)). These different impedance values are obtained by selecting different taps on the microphone input transformer.
(my emphasis)
VIPRE
It can be routed either via a custom transformer (with selectable impedance matching via one of the large rotary controls)
