[Elecraft] K2, Audio Hiss

[Geoffrey Mackenzie-Kennedy]

Howard,

Although I follow your thinking, I speculate that the MC1350 IF amplifier 
and NE602 Product Detector in the K2, and possibly Q25, could be generating 
much of the nuisance audio hiss, even when the RF Gain is turned off. 
Unfortunately I cannot power up my K2 at the moment to check this out. 
Although it is possible to reduce the gain of an MC1350 by a minimum of 60db 
(specd) by changing the applied AGC voltage, I don't believe that the K2 
makes full use of this possible reduction in gain. If that is so the IF 
would not be completely "dead" when the RF Gain is off. Also since the 
output transistors inside the IC are not affected by the AGC voltage, they 
will contribute noise at any gain setting, as will the Product Detector, and 
perhaps Q25 contributes as well.

A thought for the pot.

73,
Geoff
GM4ESD


Howard W. Ashcraft wrote:


I was listening to 40 meters late last night (virtually no one there so
I had some time to fool around with the rig) and noticed the level of
audio hiss in the K2.  With RF gain off completely and AF gain fully
advanced, there is a fair amount of hiss.  This is also noticeable at
operating levels and is a distraction from listening to weak signals.
(obviously the operating af gain is set much lower and the hiss is also
less obvious.  Since I usually use a very good set of in ear buds, I
normally have the AF gain way down)

All audio amps will create hiss at some gain level because you are
amplifying noise that is inherent in the components.  But I'm wondering
if anyone has tried to mod the K2 to create a quieter audio amp?  This
appears to be an areas where some improvement is possible and it would
make the K2 more pleasurable to use and weak signals more easily heard.
I seem to recall from way back that we used to mod Dynaco stereo preamps
by replacing carbon resistors with low noise equivalents (usually wire
wound at the time) replacing potentiometers with gang switched low noise
resistors, etc.  Some semiconductors and ICs also have low noise
equivalents.

73
Howard Ashcraft
W1WF