[Elecraft] D-104 preamp ...

[Paul Christensen]

> 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.

The K3 and the laptop will effectively provide their own drain resistor in a 
common source configuration.  For example, in the K3, bias appears to be 
turned on/off through Q8 and injected to the mic through R89 on the FP 
Board.  R89 would become the FET's drain resistor (Rd) at 5K, so no new Rd 
is needed.  From R89, audio is coupled through a 1 uF cap at C28 on the Main 
DSP Board.  As a common source amp, a source resistor (Rs) is optional but 
helps set stage gain.  The K3's 5K and 1 uF values are a really good choice 
since the FET's output Z is roughly equal to the 5K drain resistor. 
Thevenizing from C28, the cap sees 10K in parallel with 5K, leaving a load 
of about 3.3K.  The -3dB turnover point should be about 48 Hz.

When using the K3, it looks like the whole D104 interface can be done with 
just one external part -- a single MPF-102.  That's even simpler than the 
already simple source follower circuit that's become popular.  In your case, 
one extra gate shunt resistor can be used to optionally limit the low-end 
response and provide for gate static protection.  I would bet the laptop's 
circuit is close enough if it also provides a switched pull-up resistor (to 
function as Rd) so that one homebrewed buffer could be built and 
inter-changed between the laptop and K3.

Great concept, Joe!  I'll try this the next time I work on an old D104.

Paul, W9AC