“I just bought a used Sennheiser MD421 for recording toms. When I tested it last night I couldn’t stand the huge amount of lows. Is it some kind of proximity effect? Mic was too close? Or the room? Drum itself? I found that there are 4 different types of bass roll off on a switch (I’m guessing). It says S, I, II, M, but it doesn’t say what these settings do. Help.”
The 421 is an “old standard” for recording drums, particularly toms and kick. However, back in those “old” days the 421 was more than that: it was just a great mic – a mic that was used for most anything. Check out some old videos (from the late ’60′s, early ’70′s) and you’ll eventually see 421′s being used as vocal mics. (If you’ve never tried one of these on voice you should – they sound remarkably good. They mostly fell out of favor because of their electric razor-like appearance.)
421′s do have a lot of proximity effect. This is one reason why so many engineers have liked them for drums over the years. But it also has a really nice, smooth rise in upper midrange response, which provides clarity in a mix (nice attack when used on drums). This type of characteristic has influenced the design of many modern mics.
The rotary switch by the connector is a bass roll off switch. The “M” allegedly stands for Music and the “S” for Singing (keep in mind how old this design is). The “S” setting has the most roll off. The I and II positions are less severe. The M position is no roll off. According to frequency response charts I’ve seen in the past, the roll off is very smooth (not steep – maybe 6 dB per octave) and appears to begin in the 400 – 500 Hz range.
The excess bass you’re reporting could be just about any of the things you’ve mentioned, and may even be a malfunctioning mic. I would suggest testing it on vocal or acoustic guitar just to confirm that it can produce a normal sound. If it seems to be working correctly then you can go back to work in the studio getting the sound you want.