Many of the high end mic pres (such as the Vipre)
have variable impedance inputs that can be switch selected to match various microphones or simply give another flavour to the sound.
It usually means selecting different transformer taps on the primary or microphone side of the input transformer.
They are all low impedance inputs.
Most will still have input level controls and/or trim pots as well as output level controls.
Old RCA tube preamps had multi tap transformers on the mic input to handle 37.5, 150 or 600 ohm impedance mics.
Hope this helps!

If Mic and input stage have closely matched impedences ,bottom end response and detail improves dramatically. :eek:

Usually the extremities of the mic's frequency response suffers if matching is too far out.Also ,total mic output may be compromised.

Transformers, of good quality and used properly, are the best thing to happen to electric sound since the invention of the tube. Unfortunately, because of the cost, size, and weight, and other practical limitations, engineers have been designing them out of circuits since the invention of the transistor. Most microphones have some kind of transformer, but a top quality transformer can add 100 to 300 dollars or more to the cost of a mic. Ofcourse, the cheaper the mic, the cheaper the transformer, and even some of the more expensive mics have no transformer, supposedly to keep the signal chain, and sound, less cluttered. I don't agree with that since in my 35+ years tinkering with audio circuits I have never found a circumstance where a transformer hurt the sound. I always check with signal generator, voltmeter, oscilliscope, distortion analyzer, and my ears. Again, high quality manufacture and proper application can't be over emphasized. It's always possible to totally screw things up with a bad transformer in a bad place, but Stephen Paul has said on his forum that the sound of almost any mic can be improved by adding a transformer, or replacing the crappy stock one with a Jensen or other quality make and model.

Now, regarding preamps: most solid state and some tube preamps have no transformers. This saves cost, weight and space, and may be desireable for measurement applications or anybody who is worried about the flaws inherent in poorly designed and manufactured transformers. But, I think of transformers as audio problem-solvers. Especially if you're running long wires between devices, changing a single-ended output or input to balanced, and ofcourse, most simply, if you need isolation between devices. In these cases, a transformer may be the most cost effective answer. And reliability should be mentioned too, because you don't need a power supply and fifty other parts to make it do it's job, and will still work even after problems have burnt out every other part in the circuit.

Impedence matching is one of the primary uses for transformers in preamplifiers, and can provide additional gain to the input stage if matched closely to the output impedence of the sending device. One benefit of using transformers in microphones (or power amps) is a stable or relatively constant impedence presented to the next device in the chain, thus allowing more precise matching at the input stage (or to the speakers). An output device with no transformer can have either a very low impedence, which is normally desireable, or impedences which vary widely with frequency, normally undesireable in an audio chain.

Preamps with input impedence selectors provide the audio engineer not only with the opportunity to closely match the impedence of a mic to the preamp input stage for maximum gain, signa-to-noise, frequency response, etc., but also the ability to un-match the mic for a different sound, all other things being equal. This is where ears become more important than meters, and an experienced listener will be able to find a setting that brings out a pleasant sonic quality in a mic not possible with transformerless preamps. Electrically speaking, mis-matching will cause some loss of gain, but usually only about 3 to 6 db or so, not of much use as a gain control by itself, and some loss of high frequencies and transients. What usually happens in the low-end and mid response is some reduction of the lowest bass frequencies, but more interestingly, 'bumps' and 'dips' will appear at some low and mid freqs. These anomalies can 'soften' or 'mellow' the sound, and works great for vintage mics to get that 'rich', 'warm' effect on vocals and instruments without using equalizers and other gadgets that really do clutter up the signal chain. There also may some subtle compression effects, mainly from the tubes which have to work harder with mis-matched transformers.

NOTE: These special functions of mis-matching may have little or no influence on transformerless mics. The output impedence can be so low in these mics that it doesn't matter what impedence you plug it into, it won't change the mic output. And, conversely, if the output impedence is too high, anything other than a high input impedence j-fet preamp (or directly into the tube grid) will load down the mic signal to nothing.

So, for my money, when good transformers are properly used, engineer becomes artist, and artist now can make art. :D

Brian

Wow ! thanks for the great info guys, I'll remember this next time I buy a mic or a pre..