[Elecraft] D 104

[Ron D'Eau Claire]

All of the vacuum tube rigs from the 50's that I used with a D-104 had an
input impedance of between 500k Ohms and 1 Megohm (the grid resistor of the
first speech amplifier stage). Astatic does not list a specific load
impedance on the specifications sheet I have.  

The original D-104 used a Rochelle Salts crystal element which was very
sensitive to fracturing from a jolt (don't drop!) and was hydroscopic, which
made the life of the element dependent upon the hermetic seal and careful
handling. Many of those seals leaked over time which destroyed the element. 

Later Astatic switched to ceramic elements (like most other mic
manufacturers selling Rochelle Salts crystal elements). Ceramic elements are
not sensitive to moisture in the air, more resistant to mechanical abuse and
still have decent output and a fairly high impedance, which made them
popular with owners of rigs designed for crystal mics. 

One of the enduring features of the D-104 that was certainly responsible for
much of its popularity was its frequency response. It was carefully shaped
for optimum communications speech. That was critically important back in the
days before aggressive speech processing in Ham rigs. The frequency response
of even "high end" rigs was solely limited by the selection of coupling
capacitors in the speech amp and, perhaps, the modulation transformer, so a
well-designed response in the mic made a huge difference. 

The D-104 mic specifications show this frequency response:

100 Hz, -10 dB
200 Hz,  -3 dB
1 KHz,    0 dB
2 kHz,   +8 dB
3 KHz,  +12 dB
5 kHz,   -2 dB
6 kHz,  -11 dB
10 kHz, -18 dB

Our modern rigs have a frequency response that is almost completely defined
by the filters in the SSB generator - at least from a couple hundred Hz to
between 2 and 3 kHz. So you can throw away any values outside that frequency
range in the above table.

What's left is a rising frequency characteristic, which is normally good for
communications. If it rises too much when the D-104 is used with the K3, it
should be easy to bring it under control with the transmitter equalizer. 

Considering the K3's design to accommodate low-output mics, I'd be
astonished if an external preamp or impedance matching system was required.

Ron AC7AC