First of all, look at the naked purpose of a preamp: to boost the level of a microphone to usable line level. Keep that in mind. Now, if that were the *only* purpose of a mic-pre, then there would not be around 3000 models out there—one company would simply design this simple circuit, patent it, and have a monopoly over the design. But each of those 3000 models, aside from achieving its basic purpose, provides something sonically different because each model contains slightly different circuitry. The variations can be as drastic as using tube vs. solid-state circuitry or class-A vs. class-B amplification, or as subtle as using transistors of different brands or materials. How does this affect the sound? Well I have been able to hear myself that different preamps can affect the frequency response of a microphone slightly, but what I didn't know is that preamps actually have a response time. I found it a little hard to believe that this could make a big difference. Sure enough, in most modern equipment it doesn't. It's very nice to scratch one spec off the list of importance.
The supposed "color" of a mic pre seems to determine most engineers' opinions, but I think (as do many others) that adding color to a mic pre defeats the purpose somewhat. If the studio were a kitchen, it would be like trying to get better-tasting cookies by mixing the batter in a bowl of a different brand. I'm sure it makes the slightest.....SLIGHTEST difference, but I would certainly try to focus on the batter itself first. I have no doubt that I could record a very professional-sounding record with talented musicians, good instruments, well-tuned acoustics, and detailed attention to mic placement.This being said, I have heard the difference between mic pres myself. Here is an excellent place to train your ears to the detail of listening required. The engineers at this site had musicians play/sing the same musical passages over and over again through different preamps or mics. It's all done as a very controlled experiment, which is very helpful.
Some of the facts/opinions I have read but have not had to chance to experiment myself:
- Class-A amplification delivers a clearer, less distorted sound than Class-B or Class-AB. The difference lies in the output stage (read more here). Class A mic preamps tend to produce a significant amount of heat due to their inefficiency
- You can find a diagram of the color character of well-known preamps here and of well-known mics here (both also from the listening sessions site)
- All-discrete circuitry (i.e., using individual transistors as opposed to integrated circuits) tends to be more trusted, although is almost always more expensive.