[Elecraft] D 104

[Jim Brown]

On 6/4/2011 12:16 PM, Grant Youngman wrote:
> That would be a COMPLETE AND TOTAL waste of time. The K3 mic input Z is 600 ohms nominal per spec.

Nominal is the operative word here. It's designed to have enough gain 
for the low impedance mics commonly used with ham gear. If you study the 
schematic, you see that the mic input stage of a K3 is NOT 600 ohms, 
it's an opamp, which is a fairly  high Z load. The input Z is at least 
10K, probably more like 15K -- there's a 10K resistor in series with the 
input!

> The D-104 element needs 500k minimum and better no less than 5M. Last I heard the input stage of a K3 was not a 12AU7/12AX7.

This whole issue of impedance matching in audio is widely misunderstood. 
In a passive mic (that is, one that is unamplified), it mainly affects 
gain and sensitivity. The high-Z D104 is a very hot mic, and the K3 has 
a LOT of available gain, so you can afford to lose a lot with the 
mismatch.  The second way that impedance CAN affect the sound of a 
passive mic is frequency response (due to stray L and C in the capsule 
or output network), but the K3 has VERY good and very flexible TXEQ, 
which is very likely to be able to correct for any response deviations.

> That's why Z transformation (preamp, xformer) IS required for this element

The mic input stage of the K3 is quite well designed so that it can work 
very with a wide range of mics. Calling it a 600 ohm input stage is 
technically WRONG and misleading (just as it misled you). Like I said, I 
would try the mic first.

73, Jim Brown K9YC