From time to time I have done microphone recordings over the last decade. I used a simple Shure studio mic with a Behringer MIC100 external pre-amplifier. The pre-amp provides you with a great dynamic range that you can adjust.
This setup works just perfectly. I use a mic stand and adjust the microphone to stuck into the horn just where bell begins (talking of a 24" brass horn).
If you go further into the horn, the sound might get distorted especially because of the reflections caused by the microphone's body. The sound still needs space to escape properly.
However, if your microphone is positioned in front of the horn or even with greater distance, you will record a great portion of the room reflections instead of the machine itself. The sound will become quite tinny.
A different way, that I started to use recently, is a field recorder with a good built-in microphone. That way, you'll have the mic, amplifier, and the recording device in one piece. I now use it for digitizing cylinder electrically but it also works wonders with live recordings at 24 bits. There are many different models and brands and I decided for the Tascam DR-2d which came on the market this March and now only sells for about $220-250. I capture everything at the same time and later synchronize the camera's video with the better sound track.
Whatever you do, just be sure to use a professional microphone, it does make a world of difference:
[youtubehq]
http://www.youtube.com/watch?v=6YVRlAgx7yk[/youtubehq]
Too bad, that YouTube still does not offer a sufficient sound quality encoding. They have HD videos but still 126 Kbit/s AAC encoding. My ears remain insulted.
