John, KD2BD wrote:
There may be more than just two...
http://www.qsl.net/kd2bd/ksb2.html
It looks like many of us have been down the same road.
----------------------------------------------------
As I understand the evolution (having followed it since I built S/N 1289 in
2000):
1) The K2 was designed with a minimum-bandwidth SSB filter. The Filter
provided an audio bandwidth of barely 2 KHz. The idea was that narrower
bandwidths provided the best signal/noise ratio - a very important issue for
QRP operation especially.
2) It was discovered that a number of early SSB filters were significantly
narrower than 2kHz, severely impacting their performance on SSB. The problem
was traced to too much variation in the characteristics of the crystals
being received from the Elecraft vendor. Elecraft changed their
specifications and matching process to ensure adequate consistency. They
also replaced crystals in anyone's K2 that exhibited poor bandwidth.
3) John, KI6WX, developed several different mods for the OPT1 filter which
allowed widening the bandwidth of the filters to more closely approximate
what other Ham SSB rigs use. This was especially popular after the KPA100
was released, allowing higher power for the non-QRP operators. These mod
kits consist of different sets of coupling capacitors that are used with the
original Elecraft crystals (it's important to use Elecraft crystals that
were shipped after the specifications were tightened - see 2 above). The
information is on the Elecraft web site. Kits of capacitors are also
available from Elecraft if the builder doesn't have the right ones handy.
4) The issue of audio gain has come up from time to time since the K2 was
released. The gain is actually a composite of audio and RF gain to produce
10 watts or 100 watts PEP (it is not recommended to exceed those power
levels on SSB in any event because the harmonic distortion levels increase
quickly). That's why a QRP K2 often shows just barely a full 10 watts output
on 10 meters, while it easily produces more than 10 watts on, say, 40
meters. Monitoring the transmit ALC action one sees that often only one bar
barely flickers on 10 meters while several bars flash on 40. What's
happening is that the transmitter RF strip has lower gain on 10, so more
output from the balanced modulator is needed there. Some mics don't drive
the audio chip hard enough to produce full output on the bands were the
transmitter strip RF gain is lower. Again, John, KI6WX, and others came up
with mods to improve the levels for full output with mics that were marginal
when used with the K2. Other operators used external amplifiers with
lower-level mics. Those mods are also documented on the Elecraft web site.
5) Now it appears that there are some questions about the audio QUALITY
produced by the audio portion of the K2 that Jim, K9YC, is exploring.
Elecraft has always encouraged builders to tinker and improve on the K2.
Many of the improvements to the K2 design that have been made over the years
have come from just such efforts, such as the filter mods by John KI6WX.
(Actually John has been responsible for several other significant mods as
well.)
What Jim, K9YC, and others are doing pursuing better audio quality is in the
best tradition of Elecraft builders and tinkerers.
Ron AC7AC
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