[Elecraft] D-104 preamp ...

[Paul Christensen]

> Does anyone have a design for a FET preamplifier for a
> D-104 that will work off the K3 mic bias (rather than the
> separate +8V line) and the typical 3.3V mic bias in the
> current crop of laptops?

Joe,

I would try the circuit shown in the link below.

http://72.52.250.47/images/ECM.png

Referring to the diagram, note that in simplified form the resistor and 
capacitor as shown are integrated inside the K3 (and laptop PC).  Bias 
(shown as V+) is de/selected from the K3 menu.  The cap is inside the K3 and 
is used to couple and de-clamp the AC portion to the K3's mic circuit. 
Again, the R and C components shown in the above diagram are a simplified 
representation of what's in the K3 -- but it's close enough.

The common node between the cap and resistor is the K3 mic pin.  The K3 then 
supplies bias to your external nFET (e.g., MPF-102) and D104 element as 
shown in the dotted "capsule" area.  When bias is active on the K3 mic lead, 
audio is clamped and elevated to the DC bias potential.of ~ 3VDC to power 
the nFET.

The circuit shown is not a source follower (common drain) that generates 
less than unity gain, but it is a common source pre-amplifier with a similar 
attribute of very high input Z. Additional experimentation is needed to 
optimize the level between the nFET and K3, perhaps by varying the Rs source 
resistance to ground.  The diagram shows no Rs at all.  Rd (drain 
resistance) is the resistor going to V+ in the diagram and is fixed inside 
the K3.  However, the V(gain) expression for a common source FET amp shows 
that Rs (source resistance) is a denominator value.   So, by adding Rs, the 
circuit gain can be reduced.  Although the minimum value of Rd is fixed in 
the K3, it's possible to add additional Rd at the FET drain.  That 
resistance is also in series with the coupling cap in the K3.  By adding 
more Rd, a voltage divider is created that could impact the operation of the 
nFET with it's already low +3V at the drain.  For that reason, I would start 
affecting gain by adding Rs instead.

Paul, W9AC