[Elecraft] Improved sidetone

[Marteinn Sverrisson]

Improved Sidetone in K2...

I have not been satisfied with the sound of the current
side-tone in my K2. I have done the side-tone mod, using a 47mH choke.

Looking at the circuit it is a High-Pass filter with some 
peaking in the 600Hz region but it does not attenuate the
higher order harmonics of the side-tone.

Why not make a Low-Pass filter using the same choke?
I moved the 47mH choke (rs=180 Ohm) in series with 
C33 (the 2.2uF cap) and put a new 2.2uF cap across Rp5 pins 7 and 10, 
making a genuine LP filter. 
The internal resistance rs of the choke controls the Q of the filter.
This makes a huge improvement of the side-tone sound, now it is soft 
and sweet :-).

To make the mod I had to cut the trace from C33 leading to U10,
I cut it next to C33 and inserted the choke across the gap, 
soldered it to the C33 pad and the pad for C35, 
this is done on the underside of the Control Board. Then solder
a small 2.2uF cap between pins 7 and 10 on Rp5.

Here are measurements of the side-tone harmonics before and after.
The harsh sidetone is now gone and instead a softer tone is now 
generated, almost a pure sinewave.

Side-tone at F0=600Hz, reference level 0dB.

                Before          After

600Hz           F0=0dB          F0=0db
1200Hz          F1=-16dB        F1=-42dB
1800Hz          F2=-27dB        F2=-42dB
2400Hz          F3=-54dB        F3=-70dB
3000Hz          F4=-47dB        F4=-70dB

Higher order harmonics not visible on the spectrum, with the new LP filter.
Measurements done with "baudline" program running on Linux.

73's
TF3MA