D-104 Preamp by N1PS

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Steve - K4HX:
Thanks to Pete for sending this along. The FET-follower circuit presents a 10 MegOhm load to the D-104 for best low frequency response (see the response curves from Astatic below).

DMOD:
Does anyone know if he used an NMOSFET or a junction FET and what was the part number?

Phil -AC0OB

ae7db:
The schematic symbol shows a MOSFET, but in the notes he lists acceptable FETS and they are all JFETS.

I use a similar circuit with a J310 myself.

Dean

n1ps:
Yes correct...a JFET. 

Here is a video too:  https://www.youtube.com/watch?v=N1qfq8cajbI&t=1s

peter

WBear2GCR:
JFET needed for the high input Z.
Also for the self bias aspect.

Almost any JFET will do.

The IDSS (quiescent current) can be selected, IF ur driving something of
a low-ish Z at the mic input. More current being = to lower output Z.

Otherwise, lower IDSS = less battery draw, IF ur powering off a battery.

There are a number of neat circuits out there.
K1DEU (sk) has one up somewhere, maybe here, with high/low tone control capabilities
built in...

                 _-_-bear

PS. the 47ufd cap at the B+ side is probably a bad idea, it will have to charge each time
and the draw off the jfet is very low... that will introduce a time constant in both turn on
and turn off. Skip that. The small cap is ok. The 180ohm resistor too is likely unnecessary
unless there is some turn on transient when keyed... which usually does not happen. No need
for current limiting, as the IDSS of the JFET and the 1kohm source resistor does that automatically.

The other thing is that the cap to ground at the gate, likely not going to do what is expected.
The xtal element looks like a capacitor, afaik - so that looks like a divider...  I'd consider another method
to make sure there is no RF getting into the JFET if there is any. A ferrite bead, or simply a 100-200ohm
gate resistor instead...

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