On 7/23/13 11:39 AM, Don Putnick wrote:
How much of the audio quality described in the recent threads can be attributed
to the audio circuit distortion, as reported by Rob Sherwood at this year's
Dayton Contest Academy? The K3 audio section starts around 41:40 into the video.
http://www.youtube.com/watch?v=SOf2OOGeGi8&feature=c4-overview-vl&list=PLRSwUN4qr1Loc58Ip6IB7d43IXkjMat8H
Don NA6Z
K3, KPA500, KAT500
Since that issue was addressed in 2008, I'm not sure they're related.
Here is one of Wayne's comments from that thread:
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
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
