[Elecraft] KX1 low audio fix?

Thu Dec 4 14:02:01 2003

Check the schematic of the LM386 and you'll see how the R and C from pin =
1 to ground increases the gain. It decreases the current fed back from =
the output through the internal 15k ohm resistor. The 10uF cap from Pin =
1-8 bypasses the internal 1.35k ohm resistor thereby leaving only a 150 =
ohm resistor in the feedback path. Removing the 10uF will require that =
the impedance from pin 1 to GND be reduced.

    Dave WX7G

-----Original Message-----
From: Ron D'Eau Claire [mailto:[email protected]]
Sent: Thursday, December 04, 2003 11:32 AM
To: drcuthbert; [email protected]
Subject: RE: [Elecraft] KX1 low audio fix?

Removing the cap between pins 1 and 8 will REDUCE the amplification from =
200
to only 20.=20

I wasn't aware of the trick to connect a series cap/resistor from pin 1 =
to
ground. It's not discussed in the mfgr's data sheet. However, connecting =
a
series cap/resistor from pin 1 to the output at pin 5 is recommended by =
the
mfgr to boost the bass response! That might not be a good  idea because =
the
design counts on the bass response being rolled off to avoid "thumping" =
from
the AGC. (That's why Wayne doesn't recommend using hi-fi phones with it!
Good ones have too much bass response). =20

Ron AC7AC

-----Original Message-----
Gave it a quick try. New starting values: 220 ohms and 33 uF. Ups the =
audio
quite a bit. It oscillates with the R below 100 ohms, so it is marginal. =
It
needs some playing with, perhaps with the 10 uF removed from pin 1-8. =
Good
luck.

    Dave WX7G

-----Original Message-----
I did some testing on my KX1 and the audio level could be improved with =
more
gain between the RF input and the audio output. But, the LM386 is =
already
set to a gain of 200. I haven't tried this yet and perhaps one of you =
can
try it before I have time to do it. The gain can be further increased by
connecting a series R and C from pin 1 to ground. A starting place is 15
ohms at pin 1 connected to a 100uF cap to ground. The negative terminal =
of
the cap connects to ground. This should up the gain to 2000 (and maybe =
make
is oscillate nicely).

Alternatively, the 10 uF cap from pin 1 to 8 can be removed before =
trying
the mod.

   Dave WX7G