[Elecraft] A Rob Sherwood's talk in Dayton

[Hisashi T Fujinaka]

I misheard the audio and thought it was in the TX chain. I wondered why
he wasn't more upset about it. This all makes much more sense.

On Fri, 7 Jun 2013, Wayne Burdick wrote:

> 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
>
>
> On Jun 7, 2013, at 3:55 AM, Bill W4ZV <btippett at alum.mit.edu> wrote:
>
>> Hisashi T Fujinaka wrote
>>> Can anyone point out more about the K3 transmit audio fixes that
>>> Sherwood talked about?
>>
>> I thought most of the comments were about receive audio distortion.  There
>> have been several changes, both hardware (DSP upgrade) and firmware (AGC
>> Decay) and possibly others which affect this.
>>
>> 73,  Bill  W4ZV
>>
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-- 
Hisashi T Fujinaka - htodd at twofifty.com
BSEE(6/86) + BSChem(3/95) + BAEnglish(8/95) + MSCS(8/03) + $2.50 = latte