I built my K2 #2024, the SSB adapter, 2nd ant and noise blanker, no
problems except I reversed one of the diodes in the SSB adapter.

Have used it mostly on SSB and it works well with a dynamic mic and
a mic preamp ( the one on the Elecraft homepage)

After doing the SSB filter mods I wanted to make a LP filter to
improve the audio by removing out of band hiss.

By using SPICE to model the circuit and filter I came up with
the following solution:

The filter is a passive
LP filter which uses 2 standard 150mH chokes with (Ri=60 Ohm), 2 6.8nF
caps and 2 820 Ohm resistors
and does not require
modification of the Control Board other than installing the sockets for 
the Audio filter and removing the jumpers R18 and R19.

The circuit is simple, instead of the jumpers R18 and R19 a circuit
consisting of 820 Ohm resistor in series with 150mH choke which is 
paralleled with 6.8nF capacitor is inserted for each jumper. 



  J1-1     ---- 820 Ohm ----- 150mH ------- J2-1
                          |            |
                          --- 6.8nF ---


  J1-2     ---- 820 Ohm ----- 150mH ------- J2-2
                          |            |
                          --- 6.8nF ---


The filter has a insertion loss of approx 3 dB and is flat to 2.2kHz.

Using spectrum analysis software on my computer I found that the filter
does a good job removing noise over 2.4kHz. Measured on the 10MHz band USB
using atmospheric noise

  Freq     level w.o. filter            level with LP filter

  1 kHz         0 dB (reference)                  0 dB (reference)
  2 kHz        +3 dB                             +3 dB
  2.4kHz       -2 dB                             -5 dB
  3.0kHz      -35 dB                            -45 dB
  4.0kHz      -45 dB                            -60 dB
  5.0kHz      -50 dB                           <-65 dB (measurement noise level)

The noise is reduced by 10 dB on 3 kHz and >15 dB on 4 kHz and above.

73 

TF3MA