[Elecraft] Fwd: D 104

[Paul Christensen]

> 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