[Elecraft] D-104 preamp ...

[Joe Subich, W4TV]

George,

 > If you aren't married to the original element you could change it out
 > to an electret  element wired directly to the K3 input with the bias
 > turned on. I did that than trying to find a replacement crystal
 > element.

If I were going to change the element I would change it to a Heil
HC5 or the new HC6 - I am no fan of electrets or bias on the mic
line.  However, the whole purpose of the FET buffer is to use the
outstanding, naturally pre-emphasized (rising) response of the
Astatic crystal element on both the K3 and computer soundcard without
the need for an external "battery box."

I suspect the solution lies in an MPF-102 as a common source amplifier
with a 470 Ohm resistor in the source lead, 680K to 1M as a gate leak
to set the low frequency roll of at around 150 Hz (my preference -
since I see absolutely no benefit in reproducing unnecessary and power
wasting low frequencies and shack noise) and connect the drain directly
to the K3 mic input.  The source resistor might need to be adjusted for 
use with a laptop that only provided 3.3V of bias.

Another solution is probably a common emitter NPN transistor with 330K - 
1M collector to base, a .47uF - 1uF blocking capacitor to the crystal
element with a shunt 1M resistor.  However, that would take a lot more
experimentation.

I was hoping that someone had already solved the problem and could
save me some cut and try ...


73,

    ... Joe, W4TV

On 7/21/2010 2:49 PM, George & Jan wrote:
> Joe,
> If you aren't married to the original element you could change it out to an
> electret  element wired directly to the K3 input with the bias turned on. I
> did that than trying to find a replacement crystal element.
> The electret element is available from many sources (including your local
> Radio Shack) for a couple of dollars. Make sure to get the 2 wire version.
> Otherwise you could create a "phantom" voltage source in the D104 using a
> blocking capacitor on the FET output and a diode to a pad-out resistor and
> filter capacitor.
> I get good reports on audio quality on my electret but suspect it is the K3
> not the microphone!
> George
> AI4VZ
> ---------------
> That circuit won't work with the K3's bias on mic pin (Joe's intended use).
>
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