MXL 990S Mods
The MXL 990S is a great candidate for a capsule swap. Before we go into the details, though, we need to distinguish the MXL "990" from the "990S". The latter, with the "S" suffix, has two external switches, one for pad and one for a high-pass filter. It is this microphone that we describe below.
If you have an MXL 990, without external switches, see our MXL 990 mods page.
If you are shopping for a microphone to use for DIY mods, we would steer you toward the MXL 770 instead, as it offers more modification options than does the 990S. (See the DIY mods for MXL 770 page.)
Capsule Replacement for MXL 990S
As is true of nearly all transformerless condensers, most of the tone of the 990S comes from the capsule. Unfortunately, the 990S capsule leaves a lot to be desired; it is a noisy small-diaphragm design with sharp dips and boosts in its response.
The easiest, most cost-effective upgrade for this mic is to replace the stock capsule with a large-diaphragm K47 type -- either the RK-47 or RK7. Both make wonderful all-purpose microphones, with great detail in the mids, and a neutral to dark top end -- a welcome contrast to most over-bright condensers.
The RK-12 capsule is another option. It is subjectively brighter than the RK-47, but not as peaky or sharp sounding as are most large-diaphragm Chinese-made condensers. The RK-12 sounds great on acoustic guitars, drum overheads, and vocals.