[Elecraft] D-104, only slightly OT

[Paul Christensen]

> I know I could replace the element with one from Heil, but I would prefer 
> to
> keep it as original as possible (it does have the preamp in the G stand
> base).

Tom,

Odds are the crystal element is still in good shape, provided it has not 
been subjected to a high humidity environment over an extended period of 
time.

Also, anyone contemplating the use of the D-104 with the stock Astatic amp 
should consider an alternative buffer amp.  The input Z to Astatic's 2-stage 
preamp calculates to 470K-ohm -- way too low for a crystal cartridge.  The 
crystal cartridge should see a termination of no less than 5 megohm, and 
preferably greater than 10 megohm in order to preserve reasonable low-end 
response.

The series capacitance from a typical crystal cartridge is anywhere from 
800pF to 1600 pF.  Calculate the - 3dB turnover point into various 
terminating resistive values and you'll see why it's important to completely 
unload the crystal cartridge.

For simplicity and excellent performance, I favor a single transistor JFET, 
configured as a near unity-gain source-follower.  The web is full of FET and 
op-amp designs for the D-104, but nearly all of them suffer from inadequate 
design, particularly with respect to crystal cartridge loading.  Most 
designs I've seen attempt to bias the FET as the analog of a "grid leak" 
resistor used on a vacuum tube grid.  As a source follower, the FET is 
completely self-biased when the correct source resistor value is used.

The D-104 crystal cartridge can be attached right to the FET's gate with no 
other components necessary.  For isolation, I add a 5-meg resistor in series 
with the gate, but this is not strictly necessary.  If gate failure is a 
concern from static electricity, some ops have used a 10-megohm resistor 
from the gate to ground in an attempt to bleed static build-up..  I've never 
found this to be necessary.

If all of the above sounds like too much work, simply insert a 10-megohm 
resistor between the D-104 cartridge and Astatic's 2-stage preamp.  Signal 
to noise and distortion performance will suffer, but at least the cartridge 
will be unloaded.  The Astatic preamp can more than compensate for the loss 
in level owing to the use of the series resistor.

When the K3 is configured for ESSB transmit, the D-104 will sound nearly as 
good as an expensive studio condenser or dynamic mic.  As a final footnote, 
I recommend ESSB only for use on uncrowded bands, and when signal strength 
is high.  Otherwise, it's a waste of occupied bandwidth.

Paul, W9AC