[Elecraft] MCU 4.51 NB

[Don Wilhelm]

I fail to see any advantage to turning on the preamp when it is not 
needed.  If you can hear the atmospheric noise without the preamp, leave 
it off (if the noise level increases when the antenna is attached, you 
have enough gain).  By extension, if you can still hear the atmospheric 
noise with the attenuator in, leave it in - using more front end gain 
than is necessary for the band and antenna conditions will only reduce 
the dynamic range of the receiver.  If you do not know what that means 
in terms of operating, let me put it simply - you may not hear that weak 
one that you would have heard if you had set the preamp and attenuator 
properly.

Technical discussion - the band noise is S-3 with the preamp off.  The 
K3 will start to overload on a signal that is S9+70, and will hear 
signals that are greater than the S-3 band noise level.  Turn the preamp 
on, and the band noise raises to about S-5, but the K3 overloads on that 
signal that is now S9+70, so you can now only hear between S5 and S9+70 
where before you could hear between S3 and S9+70.  You have reduced the 
dynamic range by turning on the preamp when it was not needed.
BTW - The S9+70 overload point I used was just an example off the top of 
my head - I do not have the overload point for the K3 at handy 
reference, so use that as a "for instance" rather than as an absolute 
data point.  (also BTW - my "overload" term is technically referred to 
as the "compression" point).

73,
Don W3FPR

On 5/12/2012 6:56 PM, David Cutter wrote:
> Would it help the tests to engage the pre-amp? ie make things more
> challenging.
>
> David
> G3UNA
>
> On 12/05/2012 17:22, Joe Subich, W4TV wrote:
>> On 5/12/2012 12:17 PM, Ted Bryant wrote:
>>    >   Was v4.51 sent to the 4.50 testers?
>>
>> Apparently not all (I had to request it).
>>
>> I've been doing some crude testing ... with 4.51 AGC THR = 15 is
>> approximately -73 dBm/S9 (to the best of my ability to measure with
>> the XG3 and step attenuator).
>>
>> With high values for AGC THR it is important to use higher values of
>> AGC SLP otherwise a sudden very strong signal will drive the audio
>> amp and/or headphone amp into severe distortion - particularly if the
>> AF Gain is high.
>>
>> I'm finding AGC THR in the 12/13 range and SLP in the 8/10 range is
>> very comfortable and seems to "open up" the K3 receiver.  The only
>> problem is that my poor antennas don't given me enough strong signals
>> to really evaluate strong signal pile-up with off air signals at the
>> higher thresholds.
>>
>> 73,
>>
>>       ... Joe, W4TV
>>
>>