I hooked this capsule up to a 5840 cathode follower circuit, and stuck it in the body of an old Reslo VMC microphone which I picked up at a car boot sometime ago (the reslo had an old BT telephone mouthpiece inside - the original element was long gone). In fact this is very similar to Dave Royer's 'country boy capacitor microphone' design. As this is a back electret, I wired the back of the capsule directly (i.e. without a capacitor) to the tube grid and the diaphragm to ground. This made sense at the time and it does work, but this will apply the grid leak voltage (about 60V) to the backplate. Time will tell if this will cause any long term problems!
I used the same PSU that I had built for my modified neumann tube mic, now with a sowter output tranny wired in 2:1 mode..... Behold! Frankenstein's Microphone (it lives!)....
There was quite a bit of hum at first, which I though might be due to interference getting into the tube, so I wrapped some copper foil around the tube and over the circuit to form a screen. This seemed to help a bit:
All back together again...
It sounds quite good actually - very 'vintage' sounding. I've been playing
my acoustic guitar into it and I feel like I'm sat in the deep south of
america in the 1920's (which is a bit odd for a british mic body, but hey!
I never said you could trust my ears).
This is an experimental design of a twin-capsule cathode follower
mic. MXL body, dual capsule courtesy of soundking. The design is based
on Royer's small diaphragm design, with a 5840 tube, 4u7 poly output cap,
and the biggest old Sowter that I could squeeze in there. Very simple &
sounds good. Noise level is low & it can take high sound pressure levels.
Tube heater is at 5.8V. Switch flips between omni and cardioid patterns.
The capsule is polarised via grid leakage, and the tube is biasing itself
by a similar process.
The construction is based around some thick copper wire out of some high current mains cable which I've bent & soldered to make the frame and ground for the circuit. Everything else kind of hangs off the frame.
Some pictures....
The schematic is as simple as it gets (heaters not shown):
NOTE: Transformer should be between 2:1 and 4:1 ratio.
Last update: 11 February 2005