I haven't thought about my NTK in awhile, but I think this might give me a reason to bring it back out. I want to replace the capsule with an M7 from Geffel. Any thoughts or opinions? Thanks in advance.
i suppose it's theoretically possible (almost anything is, after all--with enough time & money). but remember that the capsule expects to see a certain impedence, voltage, etc. if the ntk is designed for different values of the above mentioned, then you'll find yourself basically rebuilding the guys of the ntk in the process. add to that the fact that a part of the way mics w/ the m7 sound is the head grille design. granted, that's not as big of the sonic signature as the capsule, but it's a factor.
another issue would be acquiring a donor m7. to my knowledge, gefell doesn't sell them, they only replace them in gefell mics. and if you're getting a vintage one from a donor mic, i'd say your money would be better spent fixing the donor mic.
that all said, if you're going to go through with it, i think a call to any or all of the following: david royer, klaus heine, tracy korby, and bill bradley.
Likely the NTK's capsule voltage, etc. are the same - it'll work. Many mics using K67-style capsules (underdamped) have (or should have) an EQ network built into the mic's electronics - that would have to get removed for the M7. Most LDC mic capsules are designed for a 60 volt polarization.
You may need to buy the whole head assembly off one of their mics with the replaceable capsule head, but that's a bit pricey. For about the same price (maybe a bit less) you could get a K47 capsule from Neumann, but I think in either case you will need to fabricate mounts for it.
The M7 doesn't always equal a great mic - nor anything resembling a U47 - the UM57 is a very different mic from the U47.