[Elecraft] high level audio input to K2

[J Ellis]

Hi Folks,

For those of you who might want a high level
(0 dbm) input to the K2, I made the following
mod which requires no holes and is very easily
removed. It bypasses the internal speech amp
so it does not rely on padding the input. It also
provides DC isolation thru the use of a xfmr input.

It enables you to use, among other things, an
external voice processor such as a Symetrix
528E for really fantastic sounding audio. (while
these studio quality units are quite expensive new,
they are available quite reasonably on Ebay)

This mod does, however, use the 2nd RX antenna
jack as the input connector, so if you have the amp
and the 160M option, the "no-holes" part of this
mod won't apply. You will have to bore a hole...

Basically all this mod does is provide a DC isolated
transformer coupled auxiliary input to the balanced
modulator. The functions of the added resistor and
capacitor are identical to the existing R2 and C20
on the KSB2. You can use a 3 circuit stereo phone
plug to provide a 600 ohm balanced audio input
(shield grounded at the source, not at the K2)
or you can use a standard 2 circuit mono phone
plug for unbalanced input.

Remove R2 on the KSB2 (5.6K) and install another
5.6K at R2 but leave the lead on this resistor next
to the carrier balance pot about 1/4 inch long above
the board. This facilitates attaching a second 5.6K
resistor vertically to that junction point. The other end
of this second 5.6K will go to a .33 uf cap. The values
of the resistor and capacitor are not that critical.

Tie the free end of the capacitor to the center
conductor of a 7" piece of RG-174 and attach the
shield to the center pin of the 78LO6 regulator (gnd)
on the KSB2. Use small heat shrink tubing
or other suitable insulation on the shield of the
RG-174 to make sure it doesn't short to anything.

Obtain a 3 circuit 1/4" stereo jack, preferably with
a plastic sleeve. You might be able to get by with
one with a metal sleeve, but a plastic one will
give you additional protection against ground loops
or "rf in the audio" by providing a single point ground
connection to the audio input of the K2 right at the
KSB2 sideband adapter.

Fit the jack in the K2 and you will see that you will
have to file a flat spot on the mounting sleeve of the
jack to make it fit. Using a jack with a plastic sleeve
will ease the filing process and minimize the possibility
of damaging the mounting threads. Position the jack
squarely to avoid problems with toroidal xfmr T3.

To the tip and the ring of the jack, NOT THE
SLEEVE, attach the primary of a small 600 ohm
to 600 ohm xfmr. (Radio Shack model 273-1374).
This xfmr is small enough to mount on the jack.
To the other side of the transformer, attach the
free end of the RG-174 coax. I put a piece of heat
shrink over the jack and the xfmr to hold the xfmr
in place.

A few things to remember:

The speech amp, the compressor and the vox
circuitry in the K2 are bypassed when using this
input, but the regular microphone input is
unchanged. Make sure your mic is disconnected
before using the this input from a computer, TNC,
etc. as the mic might be live when you go into xmit,
even if you are using this auxiliary input.

PTT control is easily handled thru the key jack
on the back or thru the front panel connector.

You will need around a volt of audio. If this is a
problem, you might either need to use another xfmr
to step up the voltage or you could use an xfmr with
a sufficient step up ratio in place of the 600 ohm to
600 ohm unit as long as you can find one small enough!

You will need to be able to make audio level adjustments
from the device that you are using to drive the K2.
Since the internal speech amp and compressor are
not used when using this auxiliary input, you will have
no control over audio level control from the K2.

For what it is worth, I ran this mod past Wayne and
he blessed it. I have also installed it in two K2's, one
with the KPA-100. I experienced no problems with it,
even with the KPA-100 running full tilt.

Enjoy,

73,
John, NP2B
K2/2281, K2-100/2573


--
Jeanette and John Ellis (NP2C & NP2B)
P.O. Box 24492 Gallows Bay Station
Christiansted, St. Croix,
U.S. Virgin Islands 00824-0492
tel (340) 773-9643, fax (340) 773-6574
website   www.viaccess.net/~jellis
when responding to e-mail, please send to
jeanette@atthehelm.com or john@atthehelm.com