This nice little Neumann KM75 got hit by a drummer (I was using
it as a snare drum mic - duh!) and stopped working. Closer inspection revealed
that the glue inside the mic had dried out and the wires on a tiny DC-converter
(looks like a little transformer) had been trashed. I tried to re-solder,
and even rewind the converter, but no luck - it was too fragile & too
far gone. The DC converter was only held onto the circuit board by what
looks like nail polish (which had failed), and by the wires of the converter,
which are no thicker than hairs and got yanked out when the component moved.
[As preventative maintenance I would suggest carefully checking the glue
on these mics and regluing if necessary - the replacement part is getting
scarce and expensive, and is not easy to fit unless you are myopic with
a steady hand - of course, whatever you do is at your own risk].
About this time, Dave Royer had written about his tube mod for small -dipahragm mics, and I though this was a good opportunity to give it a shot. (n.b. the article can be downloaded from www.mojaveaudio.com). The nice thing about this circuit is its simplicity. It relies on tube grid leakage to bias the capsule, so the whole audio circuit consists of capsule, tube, cathode resistor, output cap and transformer - that's it!
I removed the existing circuit, slipped in a 5-pin XLR, and built a new circuit board from perfboard. You can see this in the picture here....
...the circuit is a cathode follower circuit based on a 5480 sub-miniature pentode (wired as triode), pretty much as described in Royer's article, except that I have added B+ and heater supply stabilisation caps inside the mic. The output transformer is inside the PSU - I used a Lundahl 5402 transformer wired in 2:1 mode. You can see the original neumann FET-based guts in the picture at the front.
Shortly after I'd finished this Neumann got in touch to say that they
could get me a replacement part to restore the mic. I haven't bothered
to do this. The mic sounds good - it has plenty of level & sensitivity,
(Updated 11 Feb 2005)