Short Circuit
Shortening the circuit
Monday, June 24, 2013
A discussion of noise sources at the input of a condenser mic got me wondering just how directly I could connect the diaphragm and impedance converter. For a two-pattern mic, I think this is about it - 3/4” of wire. Dump the input capacitor. Put the bias on the backplates. That gets the input down to the capsule itself, a 1 gigohm gate bias resistor, and the FET. The FET bias resistors are placed on a tiny scrap of perfboard with the FET and wired into the Schoeps-style printed circuit with a couple inches of wire, but those wires carry a much lower impedance signal which is much less susceptible to noise. Props to Johnny Zhivago and Matt McGlynn for inspiration.
For a cardioid-only capsule, the FET could be attached directly to the capsule. ( No switch. )
So here are pictures for the curious. For the adventurous, remember this was done by a professional engineer in a closed laboratory. If you decide to hack at your mics, it’s your responsibility to take proper precautions, and if you screw up, well... This isn’t a tutorial. I’m just telling y’all what I did.
AKG thought it was worth installing the tube upside down in the ELA M-251 to shorten the capsule wire and associated capacitance.
Here's a VERY short circuit from capsule to impedance converter (FET).
So here’s the circuit inside the head basket:
The parts identifiers refer to the original MXL part labels on the circuit board. The 1 G resistor comes from MXL. The other two resistors are new. The new FET is a type J305 for lower input capacitance. Backplate bias comes from the hot end of C12.
All the parts associated with the input stage are removed from the PCB except R7. The input capacitor, the two 1G resistors R1 and R9, the FET, R6, R13, and R14.
While I’m in here, I changed C3 and C4 to 1uF, bypassed the electrolytics, and changed D3 to a 12V zener.
Front view and Side view. The most important upgrade - an RK-12 capsule from Microphone-Parts.com
Inside view.
The switch.
Cardioid or Omni.
Shiny bits.
The mic with its pants on.
