[Elecraft] K3 - D104 Buffer Schematic

[Paul Christensen]

Last night, I built up the following D104 buffer circuit as discussed by 
Joe, W4TV.

http://72.52.250.47/images/D104.jpg

Joe's idea works very well.  The difference between this circuit and other 
D104 buffer circuits as seen on the Internet is that it is powered directly 
from the K3's mic lead when the bias menu is activated.  One benefit is that 
the circuit can be ported to a PC sound card.  Another benefit for most 
builders:  Only one part is required -- either a J201 or MPF-102 nFET 
transistor.  When using the PC input configuration shown, I also had good 
results with a Samsung NC10 netbook.

I tried the MPF-102 and J201, and each works well, although the J201 FET can 
be operated with an Ids current down to 0.5 mA.  R89 in the K3 limits the 
Ids current.  The minimum Ids spec for the MPF-102 is 2.0 mA but seems to 
perform the same, albeit with slightly less gain as expected.   The D104 
cartridge level is relatively high anyway and I found that in either 
configuration, the K3's mic menu could be set for the "low gain" setting. 
Also note the inclusion of the optional Rg and Rs resistors.  My 
breadboarded pre-amp is stable without those components and the gate biases 
just fine.  I'm also not too concerned about static here in Florida.  At 
15-cents ea., the FET is easy and cheap enough to change.

Finally, if RFI becomes an issue, it may be helpful to experiment with a 
bifilar-wound, common-mode choke at the shielded cable entry, close to the 
FET.  I would try that before attempting differential-mode abatement 
techniques (e.g, bypass caps to ground).  Similarly, the mic cable at the K3 
connector end can be wound through a #31 core, if necessary.

Paul, W9AC