“RK-99” Large Electret capsule


A look at an unusual large diameter electret.

Working from left to right of picture, front to back of capsule, we have:

Outer brass shell.

Diaphragm made of aluminum foil instead of mylar. The aluminum can’t be stretched as tight as mylar, so the diaphragm is tuned lower than normal.

2 thin plastic spacer rings set the distance between diaphragm & backplate.

Backplate drilled in a K67 pattern of alternate blind and thru holes, covered with silvery electret plastic film.

Backplate and 3 pieces of paper which occupy acoustic low pass filter chamber between backplate and delay plate. 

Setscrew threads through delay plate to set thickness (tuning) of the air chamber.

Clear plastic delay plate with brass resonator on back. These seem to be molded or glued together.

The ring which screws into the shell and holds it all together.

Two things caught my attention:

  1. 1.It’s an electret. We had been treating it as an externally polarized capsule. WRONG!

  2. 2.The delay response / tuning is adjustable.

So these got installed in the modified E-1000 “lollipop” bodies. The setscrew was turned all the way out and the outer ring screwed in tight to compress the paper in the delay chamber, hoping to increase damping on the diaphragm and reduce the peaky top end, or at least broaden the peak. Then the setscrew was turned in enough to re-establish electrical contact with the backplate.

The net result is a very sensitive mic with scooped mids, a presence rise, and a slight very high end rise. It may need a pad for loud sources. Bass is quite extended, flat to 20Hz @ 2 meters from the source, rolled off electrically below 20, but still there in subsonics. This is one cardioid you could use on a pipe organ. It needs a pop shield for vocals or placement above or to the side. It’s a bit like a U47 with a pronounced proximity effect which can be good with a skilled voice talent, bad with an amateur who moves around a lot. I suspect some 2nd harmonic “enhancement”. I kept the K596 FET operating unbiased in its “pentode” region.

So far, this is a usable mic with a pleasing character. It’s not super flat, it’s got a “toob-ish” sound, and it’s probably got too much gain for close miking loud sources. It’s good enough that I purchased a second capsule. Should be fun to experiment further.

Matt has a few more of these capsules, but hasn’t decided what to do with them. If you want one, email him and good luck!