[Elecraft] D104 mic

[Paul Christensen]

> Use a 100k to 1 meg ohm resistor from gate to ground. Put a
> 560 ohm source resistor to ground, and couple the source to the K3 through 
> a
> .47 uF cap.

I agree with comments from Tom and Don.  However, the "leak" resistor from 
the FET's gate to ground usually isn't necessary as the crystal cartridge 
itself often provides ample leak to bias the FET.

The source resistor can be calculated to set mid-point bias a bit more 
precisely by taking the reciprocal of the FET's transconductance (gm or gfs) 
value.  For example, the Fairchild MPF-102 is specified with a gm value 
between 2000 uS and 5000 uS.  The geometric mean of those values is about 
3,000 uS. The reciprocal value is 333.  So, a 330-ohm source resistor to 
ground is a good choice when the actual gm value is unknown.

I would stay away from op-amp solutions for a number of reasons:  First, 
unless a bi-polar supply is used, a voltage divider must normally be set up 
to bias the op-amp's non-inverting input, and consequently, the op-amp 
output to half the supply voltage.  However, some op-amps are designed to 
operate from single supplies, contain internal dividers, and require no 
external divider components.  Second, the parts count can get to be quite a 
bit more than a single FET.  Finally, unless a JFET-input op-amp or biFET 
op-amp is used (e.g., Texas Instruments TL071), obtaining stable operation 
and achieving a high input Z into the megohm range is pretty difficult.  The 
biFET designs can more easily achieve this.

As Don pointed out, some folks use a 1 to 5 meg resistor between the D-104 
cartridge and the low-Z input of a transceiver.  That re-establishes balance 
in frequency response from the cartridge but it also sets up a very 
asymmetric voltage divider, the equivalent of which is a pot with its wiper 
arm nearly at ground potential.  Often, the result is low level and poor 
audio SNR.

All things considered (RF immunity, single supply, low parts count, low 
current, super high input Z, and very low output Z), it's pretty hard to 
beat a single FET when used as a source follower with crystal mics.

Although I do own a condenser mic, my primary mic is ca. 1935 Shure 70H 
crystal type with a JFET interface.  With that combination, response is 
reasonably flat to about 80 Hz.

Paul, W9AC