> Then why not just put a 1 megohm or so resistor in series with the mic > element. I believe the K3 mic input has adequate gain to compensate for > it, and it will keep the crystal (or ceramic) mic element happy. > The KISS principle applies - a resistor is much more simple than an FET > or a transformer.
Plenty of folks have been doing that for decades with their D-104 mics. It's fine unless you want to optimize SNR as the source Z of the generator (D-104) increases by the amount of the added resistance . I've tried it both ways, and a JFET configured either as a source-follower or common-source amp with a J201 JFET has always been noticeably quieter when compared to the addition of a single resistor after mic element. For the K3, it's easy to optimize the D-104 with a JFET by using exactly two parts: a pair of resistors. That's just two more parts than one 1 meg-ohm resistor. In the K3 menu, one simply activates mic bias. When using a JFET as a common-source amplifier, the drain resistor is supplied by the K3 (R89 = 5.6K). A 1uF cap also internal to the K3 (C28) isolates audio from the DC bias injection. That leaves only a source resistor and gate leak resistor. Sure, not quite as economical as a single 1 meg resistor after the mic element, but with only three total parts, I find that the added performance more than offsets the addition of a two more parts. In one of my D-104 mics, the JFET and two resistors are mounted with adhesive tape right on the back of the D-104 element. It doesn't get much simpler. The JFET is functioning only as an impedance transformation device -- it is not matching impedance, nor does it need to for the reasons cited by you and K9YC. Paul, W9AC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
