3-pattern RK-12 Mics
3-pattern RK-12 Mics
3-Pattern Mics
Monday, November 4, 2013
Following up on some of the ideas in previous microphones, I built another pair of RK-12 mics with the FET in the head basket attached directly to the capsule. This time, a dual-polarity capsule bias supply permits generating all three basic pickup patterns, omnidirectional, cardioid, and figure eight.
To recap the design philosophy, we attempt to keep the stray capacitive load on the capsule as light as possible in order to maximize the capsule’s signal output, thereby getting the best sensitivity and noise figure we can from it. No processing, EQ, or sweetening is employed. The raw sound of the capsule itself is captured, and processing is left to other devices after the microphone, if it is desired. The design can be described as KISS, “Keep It Simple, Stupid.”
Wherever a part can be eliminated, it’s gone. So there is no input capacitor, the capsule itself is used for that. It’s a capacitor, after all. Positive bias is applied to the front diaphragm, which is grounded for audio. Signal is taken from the backplate. The rear diaphragm is either disconnected which yields a cardioid pattern, connected to positive bias giving an omni pattern, or connected to negative bias for a figure-8 pattern.
Another pair of mics is built on the same type chassis as before. This time, they have omni, cardioid, and fig-8 patterns and are built around RK-12 capsules.
The bias supply is a little Hartley oscillator with voltage doublers on the output, generating + & - 65 volts. A SPDT On-Off-On switch connects the rear diaphragm to it.
The J305 low-capacity FET, the 1 gigohm gate resistor, and the two source bias resistors are on a teensy scrap of PC board next to the capsule with the gate lead of the FET soldered directly to the tag on the backplate.
If you try this, be careful not to let flux or vapor reach the diaphragm.
This is for expert solderers only.
The rest of the circuit is very much as described in the last article. The circuit board of the karaoke mic which furnished a chassis is cut down to only the output stage, a few necessary parts are swapped and added, and it’s all buttoned up with red paint this time.
More snapshots from the workbench:
Measurements will be forthcoming. I will say that I noticed low level details deep within the music which I haven’t heard with any other mics. My initial reaction is “WOW!”