[Elecraft] Sidetone mods

[Dan Barker]

I found the sidetone quality very annoying when my K2 first burped to life
last Spring. I tossed a question on the reflector, and got directed to the
"Sidetone Mod", which I performed. My sidetone got much better, but I still
hear trash on the signal. Our outputs are pure sine waves, shouldn't our
"inputs" be too?

Months later, I saw TF3MA's post, saying in part, "I have not been satisfied
with the sound of the current side-tone in my K2. I have done the side-tone
mod. [...] Looking at the circuit it is a High-Pass filter. [...] Why not
make a Low-Pass filter using the same choke?"

His reasoning sounds good to me, because the sidetone originates as a square
wave, and  a square wave presents all odd harmonics when treated as an audio
signal. The 1/3, 1/5, 1/9th etc. harmonics don't exist<g>, and we want to
filter the 3rd, 5th, 7th etc. out or the signal.

However, his description of the modifications to a modification to a
schematic from a PDF were more than I could follow. So, I decided to draw it
out.

<CONTEXT SHIFT>

Several days ago on this reflector, KO0B mentioned a PCB drawing program
that was pretty slick, and free. I downloaded it, and low-and-behold, the
PCB drawing program really works best if it has a schematic to go on (duh!).
The (FREE I say) package came with a schematic drawing program too! And at a
price I can afford, even this close to Christmas.

<CONTEXT anti-SHIFT>

So, I drew the Elecraft original circuit, copy/pasted it and applied the
Sidetone mod, copy/pasted it and applied the TF3MA changes, and now I can
see what's going on.

Wayne posted a note that Elecraft had not adopted this version in the
revision B K2, and I'm wondering why not. I'll wonder more after I do it
myself, of course (or maybe less - doubt it<g>).

If you'd like to see the result, click
http://www.visioncomm.net/images/sidetone.jpg and you can understand it too.
For those of you who downloaded the Schematic software, my drawing is also
available at http://www.visioncomm.net/images/K2sidetone.sch. For those of
you who didn't download the PCB software, DO IT NOW! IT'S FREE! IT'S WORTH
THE EFFORT. http://www.expresspcb.com/news.htm.

Dan Barker / WG4S / K2 #2456

ps: No, I haven't done the KO0B mod yet, but his reported 42 db improvement
sure beats the party-line 16 db improvement! I'll do it next!