K2, Audio Hiss

[Howard W. Ashcraft]

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




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[Able2fly]

 
In a message dated 3/17/2006 1:18:00 P.M. Eastern Standard Time,  
[hidden email] writes:

With RF gain off completely and AF gain fully
advanced, there is a  fair amount of hiss.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>



Howard and all,
 
To throw my observation into the mix, with AF gain full  clockwise and RF
full ccw, I hear NO hiss while sitting approx a foot and a half  away from K2
#3810. Its quiet even with earbuds. As I crack open the RF gain,  hiss happens
(as would be expected). I have plenty of RF gain as the K2's little  speaker can
be driven to uncomfortably loud SPL's. Maybe some LM380's are  noisier than
others?
 
           Bill  K3UJ
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[Ron D'Eau Claire-2]

Howard, W1WF 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.

------------------------------

My K2 makes a fair bit of noise with the AF gain all the way up too. I never
thought much about it because that's how all the receivers I've used
functioned.

I frequently do not use AGC. That's how I learned to tune in CW since, until
the more modern exotic AGC systems used today were developed, the BFO in a
receiver would trigger the AGC system. Hence, AGC was used only on AM
signals.

Operating a receiver in CW mode involved turning down the RF gain, then
advancing the AF gain to the point where the background noise was just
audible. The reason for that was, as you observed, any higher and the
background noise would become objectionable on weak signals. The AF gain is
kept below that point. Then the RF gain is advanced and used to control the
level of signals.

Operated that way, my K2 behaves like any other good CW receiver I've had.
There's plenty of gain, even with the AF gain backed down so the internal
noise isn't noticeable. The receiver is capable of rattling my phones with
band noise on even the narrowest filter settings.

Another reason for not running the K2 audio gain full up is that the K2's
"RF GAIN" is not really an RF gain control. It adjusts the gain of the
Intermediate Frequency (I.F.) amplifier instead. The K2's RF amplifier is
either switched in or out fixed at full gain to raise or lower the RF gain,
or an optional attenuator may be selected to reduce the RF gain below that
of the K2 with the RF amplifier (Preamplifier) switched out.

The range of level control of the K2's the so-called RF GAIN is rather
limited, with significant signal leakage through even at minimum. That's
typical for those integrated circuit amplifiers. Keeping the AF Gain below
maximum helps keep the signal leakage at minimum RF Gain setting from being
objectionable.

Something else that's come up here often over the past several years is that
there's apparently a fair bit of variation in the gain provided by the audio
amplifier in the K2. That's sure to make the apparent background level
higher in some K2's compared to others.

Ron AC7AC

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[Rob Locher W7GH]

Idiom Press makes an external SCAF-based audio filter that does an  
excellent job of eliminating hiss, among other things.  The web site  
http://www.idiompress.com has demo .wav files.

The Standard Disclaimer does NOT apply in this case...

73,
- Rob W7GH
K2 #5004


On Fri, 17 Mar 2006 10:02:45 -0800, Howard W. Ashcraft  
<[hidden email]> 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.

--
Idiom Press supplies the world's finest Morse Code keyers,
computerized antenna rotator controls, audio filters for serious hams,
books of interest to DX'ers and more.  http://www.idiompress.com

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[Geoffrey Mackenzie-Kennedy-2]

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









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