Panasonic Electret

The original Panasonic WM-63 was used as is, with the standard 2.2K drain resistor.
This enabled it to be used for most purposes, but lacked some flair for performance.
Along came n article in Speaker Builder magazine 6/90, describing the Mitey Mike system. It used
the WM-63 capsule using the standard hookup, but used a low power buffer amp to satbilize
the input voltage and relative output, producing a calibrated SPL output. It also buffered
the output with a low Z driver. In 4/97 the Mitey Mike II was introduced. It used different
active components, and used another mod to an updated WM-60 capsule. As discussed in the article,
the WM-60 comes with an integrated 75 ohm source resistor, but the capsule showed SPL induced
distortion as low as 100dB. The source resistance was increased to 150 ohms, and the drain resistor
was increased to 10K. This made it possible to use at 125dB, with least distortion. By using selected
capsules, the response could be flat to 1 dB up to 20kHz. These came with the kit.
Another variation shown on the Siegfried Linkwitz pages, has a different modification using a common
source resistor only of 10K. There is no drain resistor.
The mods to the capsule require cutting the fet source connection to case ground. It can then be
used in different combinations of both types of mods. The case must be kept separate, but
must be stabilized by some type of ground connection. In the Linkwitz hookup, it is actualy tied
to the drain, which is a stabilized common ground, fed with positive voltage.
I have some different views by folks at the DIY Mike pages at Yahoo. I have heard there might be an
internal diode or such, to bias the fet gate. In the MMII, the source resistance was increased to increase
gate bias, enabling higher SPL's to be used. On the mod using a 10K source resistor, this might seem to increase
bias greatly, but may not if actual circuitry inside the fet prevents it so. I don't know the answer.
In any case, I think using a 10K source resistor is too much. I myself might try a standard 2.2K source resistor
instead.
As shown in the Picture, I took apart an old WM-63 I had
in the box. Seems basic, with no other visable components, resistors, etc. There is an air space in front and in back
of the polrized back plate. 3 little holes in the plate allow it to breath into the back chamber, which is
normally sealed off to any rear exit. A rubber plate glued to the back board seals off the connections and may also
dampen rear resonance. Sealing off the back, and adding some mass is probably a good thing to do. Ataching the capsule
firmly to a non resonant wand accomplishes that.
It has come to my attention, that some have reported extrems of response from the WM-61, much above what has
been commonly reported. Has Panasonic been producing garbage lately?? I'll try to keep up on this.
All this stuff on these mics is useless if you can't buy quality parts. That was the whole basis of my interest in the Panasonic line.
There are other alternatives out there, and I'll try to comment on those as well.
DIAGRAMS
Some references here.............
Linkwitz system test.
MicDIYers
Bang Campbell Associates
Eric Wallin
Sound First
Also get informatiom in the alt.science.physics.acoustics newsgroup, as well as rec.audio.tech

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