[Elecraft] Re: Sidetone mods

[Wayne Burdick]

Dan,

The sidetone mod we used works very well, assuming you add both the resistor and
the RF choke. Check the schematic in the revision D K2 manual. Many of those who
have made the mod or who have used a rev B K2 have commented that it sounds very
good, and it certainly is a huge improvement over the original minimal-parts
audio filtering, which was provided by the op-amp.

The new circuit functions like an L-C tank that resonates in the range of about
400 Hz. It looks like a high-pass filter at first glance. But since the op-amp's
output impedance is quite low, the L and C function approximately as if they
were in parallel. The signal is coupled into the tank at a low-Z point (the
output of the op-amp), and the signal is picked off at a hi-Z point (the high
side of the inductor) for injection into the AF amp.

The low resonant frequency reflects our choice of inductor (82 mH, same value we
use in the KAF2). It is very effective at rolling off high harmonics over the
entire sidetone pitch range, although the amplitude will vary several dB
depending on your selected pitch. 

73,
Wayne
N6KR