I've been using some small–diaphragm condenser mics to record acoustic drums in a small room; a pair of Shure SM81s and a pair of Rode NT55s. The room has been treated with bass traps, broadband absorption panels and diffusion. In cardioid mode, the NT55s seem to enhance the 2‑4 kHz region, which sounds a bit harsh. The SM81s sound more polished, but seem to push more around 6-8 kHz, which still sounds a bit brittle. On a recommendation I picked up a couple of (large-diaphragm) AKG C214s, and they sound much more natural. Though that solved my problem, I'm still curious: is there such a thing as a 'soft'-sounding small-diaphragm condenser? Is that brittle sound a characteristic of small-diaphragm condenser mics in general?
SOS contributor Mike Senior replies: This is a good question, with a few facets. The short answer is that SDCs (small-diaphragm condensers) do tend to sound 'harder'. The smoothest I've heard in project-studio environments are probably the classic Neumann KM83/84 and the DPA 2000 series, but the omni version of Oktava's more keenly priced MK012 also sounds very smooth. Some sE Electronics ones I've tried are really hard-sounding so I normally avoid those, but to be fair they've made a lot of models, so there may be a softer-sounding one I've not tried. AKG's C451 has a strong reputation for drum applications, but I'd not recommend it in your situation as it's pretty bright — great for recording to tape, but less suited to digital recording.
SDCs tend to have a better transient response than LDCs; larger diaphragms don't track transients in the same way, which is one reason they sound softer. For a similar reason, the high-frequency response of large-diaphragm condensers tends to roll off, whereas SDCs typically do this less, again making them sound subjectively more 'brittle'. But some LDCs have a really big on-axis high-frequency boost to compensate for their typically duller off-axis response in diffuse-field applications, so some can still sound fierce in the upper spectrum if you point them directly at your cymbals.
The AKG C214 (like the C414) is voiced without the HMF boost of so many other LDCs, making it sound smoother on drums. This characteristic makes it generally well-suited to instruments in vocal-led music, because it keeps the instruments from competing with the vocal in the upper-mids (which can otherwise become a bit of a battle zone in busy modern mixes).
A lot of people position overheads close-up, directly over the cymbals. You'll often see this on-stage, but it isn't usually good for studio work unless you're working to analogue tape.
The position of the mics also has a big impact on the sound. A lot of people seem to position overheads fairly close-up, directly over the cymbals. But while you'll often see this approach used on stage, it isn't usually very good for studio work unless you're working to analogue tape (which may benefit from the HF pre-emphasis) or using ribbon mics (which dull the sound by design), because the mics will tend to catch an overly bright sound. I find that SDCs usually work best if you put them over the centre of the kit or around about where the drummer's head is — this may mean that the cymbals are off-axis to the mics, but SDCs normally have a decent off-axis response so this shouldn't be a concern in practice.
Finally, I wouldn't rule out using your NT55s' omni capsules, as they may be voiced differently from the cardioids. The cardioid capsule of the Oktava MK012s that I mentioned earlier, for instance, sounds much harder than the omni one. Omni capsules might also help you get a more holistic kit sound in general, given that you appear to be working in a fairly well-damped room where the timbre won't be enriched that much by acoustic early reflections. If the room's very dead-sounding, you might also consider setting up a second overhead pair to supplement the tone of the first, as detailed in my SOS April 2013 Session Notes column (https://sosm.ag/sn-apr-2013).