[Elecraft] A Rob Sherwood's talk in Dayton

[Fred Smith]

The most understood presentation I have ever heard on a complex mater, that
was easy for even me to understand.


73,
Fred/N0AZZ
K3 Ser #'s 6730/5299--KX3 # 2573--K2/100--KAT100
P3/SVGA--KPA500--KAT500--W2




-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Rick Bates
Sent: Friday, June 07, 2013 11:01 AM
To: 'Wayne Burdick'; elecraft at mailman.qth.net
Subject: Re: [Elecraft] A Rob Sherwood's talk in Dayton

This policy is so clearly stated that I have to comment that it's a nice
change to actually hear from the actual designer AND user that doesn't speak
in 'engineer-ese'.  To 'hear' clear, concise and accurate information
without having to translate is wonderful.  I actually understood it!

Bravo!

Rick wa6nhc

-----Original Message-----
From: Wayne Burdick

Rob helped us identify the K3's audio output distortion issue, which we
subsequently cleaned up with a hardware change (five years ago) and new
firmware (about three years ago). 

If you have an older K3, see:

 
http://www.elecraft.com/K3/mods/K3_AF_Stage_Upgrade_Instructions_Rev_B.pdf

This mod, long since incorporated into production, adds a large RF choke in
series with the AF amplifier stage's DC supply. As Rob's plots show, this
dramatically knocked down the IMD products; most are now down 70-80 dB. I
have pretty good ears, and once we made this change, I couldn't hear any
difference between the internal amplifier and external powered speakers.

Rob is correct that it would be possible to further reduce low-level
distortion products. However, many of these fall outside the hearing range
of the average user because they're related to an image of the 12-kHz CODEC
sampling rate. What's left within hearing range could be reduced with a
change in the AF amp IC, though there are diminishing returns and pragmatic
considerations. The stereo audio amplifier IC we use is capable of driving
two 4-ohm speakers at up to a few watts apiece, and it generates no RFI
because it is a classic analog device running class AB. We could get higher
drive power using a beefier analog device with much higher quiescent
current, or by using a class-D or higher switching-style device. The former
would increase the radio's current drain substantially, while the latter
would require additional shielding and decoupling. 

As always, our goal is to continuously improve the K3 in a way that is
applicable to units in the field. If we find a practical way to achieve a
further significant improvement in the audio channel, we'll certainly make
it available. 

73,
Wayne
N6KR


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