Nah, you need a few more components in the capsule: the bias resistor (AKG used 10G), a coupling cap for the FET, a few resistors to divide the FET bias supply, and another 10G to bias the FET. Also, not too many FETs that you'd want to use as a capsule FET can tolerate 60V, so you need to divide that supply as well (or maybe bias the audio pins at a lower voltage, saves parts in the capsule). AKG's scheme is even a bit more involved than that; check their schematic for details--they have another FET as a cascode, and an IC, and some feedback . . . then the question becomes are people just going to be happy with the simplified version? I mean you can skip the FET bias, but that will degrade headroom. Or you could build a simpler circuit to run off of 48V phantom (~41V to the capsule), that will drop sensitivity a tad and degrade SNR (as in the CMRs). Then you have the question of whether people are gonna be happy with the epoxied capsules.
I can crank out a pre with a 60V bias supply, that is not a problem, it's the capsule end I'm not interested in tackling. Although there has to be demand for the 60V pre; it's expensive to bring these products to market. It's fairly cheap to do a circuit that fits in an XLR connector (as in my PFA) and does +41V, signal (biased if necessary), and ground. It wouldn't be anywhere near the full AKG circuit, but it would work.
Oops, yeah you're right about the capsule.
Revised part list (at the capsule):
- 1MR and 100nF, circuit to take +polarize and smooth it out
- 1GR bias resitor (could be 10G, but 1G is enough I think)
- FET (from Transsound TSB-120A capsule)
- 4.7kR source resistor for FET
- 47pF ceramic between capsule and FET gate
I used the FET from the Transsound capsule because it is "self-biased" so I don't need a dividing circuit.
In my prototype I cut then end of a pair of Studio Projects C4. This includes just the part to screw in to the capsule. I built the above components. The input is a standard (two-wire) battery box, plus +60V (polarization).
The prototype seemed to work (using a bunch of 9v batteries in series for the polarization), but I did not do any field testing yet. If you could build me a battery box with polarization (eg., for parts plus a small charge), I would be willing to continue on the project, and share my results with anyone who is interested. Oh yeah, running 40V polarization might be OK. That is equivalent to running a mic with a 10 or 20dB pad on, right? Ie., the noise floor increases, but it still handles loud signals, which is what *most* of us want. Also, remember that we need +polarize, but it is very low current, so it is not like a phantom supply.
As far as epoxied capsules, I would start with something cheap, like Studio projects C4. If this works well, go for AKG. These are cheap anyway. I wouldn't mess with Scheops, though. Besides we have the new (CMR?) collettes now, these do *exactly* what the above does, and if you've got money for Scheops, you can afford the CMR!
The fact is, I'm trying to get out of taping projects, but if someone else jumped in, I would probably slip back in
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Richard