A potentially novel pre-amp for electret mic capsules which have their internal FETs Drain and Source connections available separately

  
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This circuit might also be of interest to people making pre-amps for electret and externally polarized condenser microphones which have no internal FET. (If a single FET`s noise is the limiting factor, perhaps two or four could be run in parallel. ) Some `two terminal` electret microphones can be modified to provide separate access to the Source an
A potentially novel pre-amp for electret mic capsules which have their internal FETs Drain and Source connections available separately - schematic

d Drain of the internal FET. This, together with a new arrangement for biasing the modified capsule, is known as the "Linkwitz modification" (Linkwitz mod) and was first developed, or at least first made widely known, by Siegfried Linkwitz : The test arrangements are not necessarily very accurate in terms of absolute sound levels - I was using a cheap sound meter. I guess they are accurate within a few dB SPL. The tests were purely of 1kHz at sound levels between 110 dB SPL and 140 dB SPL. I have not yet measured frequency response or background noise. Nor have I yet done any listening tests. They are 14mm in diameter. They are a "back electret" design with a layer of charged fluoropolymer (I assume) on the front of the backplate. The diaphragm is 3 um thick aluminized mylar (I assume) with the aluminization layer, which is grounded, on the front. At the back we have Ground, Source and Drain. For the capsule at the lower right of the above photo, the Ground is the gold-plated tracks, the Drain is to the upper left and the Source is to the lower right. The only access to the backplate we have is through the gate of the FET - so we have to do everything via the FET`s Source and Drain. We need to get to know this device as well as possible. This does not appear to be one of the FETs in current production at NEC. The current ones have rather similar specifications. Maybe this pre-amp design will work just as well with FETs...



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