3. Nov 2001. Modified TF3MA LP filter for K2.

After doing the Audio Gain Control modification, I redesigned the LP filter 
and now my K2's SSB sound quality is improved. 

The load on the filter is now constant 5kOhm, regardless of VOL position. 
Before the Audio Gain Control mod, the audio frequency responce 
was affected by the VOL posision, promoting high frequencies
on low VOL.

The TF3MA LP filter uses 2 ea standard 147(or 150)mH chokes with (Ri=60 Ohm), 
2 ea 6.8nF caps, 2 ea 1800 Ohm resistors and 1 ea 22nF cap.
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 LP circuit  consists of 1800 Ohm resistor in series 
with 147mH choke which is paralleled with 6.8nF capacitor 
and a 22nF cap across the output terminals. 



  J1-1     ---- 1.8 kOhm ----- 147mH ------- J2-1
                          |            |  |
                          --- 6.8nF ---   |
                                         22nF
                                          |
  J1-2     ---- 1.8 kOhm ----- 147mH ------- J2-2
                          |            |
                          --- 6.8nF ---


The filter has approx 4dB extra insertion loss ( rel to original K2 ) 
and is flat +-3dB from 100Hz to  2.2kHz. It has more than 40dB
attenuation from 3.5kHz up.
The frequency responce follows closely the frequency responce of the
KAF2 filter, but with somewhat higher attenuation above 2.5 kHz.

Audio spectrum analysis confirm that the TF3MA LP filter works as expected.

73 

TF3MA