[Elecraft] [K3] K3 Distorted Sound while using AM

[Guy Olinger K2AV]

I have utterly clear AM reception. And am so far unable to reproduce
your symptoms.

I'm not sure what firmware you are using but with last couple beta
versions it is possible to drive the DAC circuits into overrange
"blast" protection muting.

It is possible to accentuate the lows on AM to the point that it
overdrives the range of the DAC, regardless of what the AGC is doing.
When this happens the DSP programs goes into mute mode to keep from
creating garbage numbers in the program that blow your ears off. This
feature has been very strongly requested by K3 owners (not a
"mistake") and is decidedly on purpose. (See the very long threads
related to "blast".)

Set all of your equalizer settings to flat. Make sure that you are not
receiving the carrier at the edge of your passband, as full carrier is
needed to demodulate without distortion on regular AM signals (not a
K3 requirement, just physics). Even with synchronous AM, the carrier
must still be in the passband to participate in the continuous
sychronizing process.

On synchronous AM the SHIFT control only displays USB or LSB to let
you know which sideband is being used plus the carrier for synchronous
demodulation.

On non-synchronous, the SHIFT control is fairly useless as all it does
is shift the passband at a minor rate versus the tuning knob.  On AM
the sideband demodulation is provided by the carrier, so the SSB shift
effect does not work.  If it is moving the audio into distortion then
the AM carrier is being moved out of the passband and its level being
reduced below that of the sidebands.  Standard AM demodulation
requires that the carrier be a greater level than the sidebands.

Since the carrier has a fixed relationship to its sidebands, the
typical SSB shift effects for low or high audio do not apply for
standard AM demodulation, only if demodulating it as SSB. In order to
use shift to reduce lows in standard AM demodulation you would first
need to tune through one sideband's low audio, leaving the low on the
other side of carrier still providing lows. Then through the carrier
and only then through the other lows.  However since you tuned out the
carrier, the demodulation will go into distortion before you can slice
the lows on the other side of carrier.

The only audio range you can tune out in standard AM demodulation
without distortion is highs, and that requires reducing bandwidth with
the carrier in the center of the passband.

On one level your issues sound like trying to demodulate AM using a
2.7 or 2.8 SSB filter, which really sounds pretty awful. Just for
jollies make sure with the current K3 utility that you have the proper
filter chosen in the filter configuration menu, and that there is not
a mistake in the slot being used for your filters.

If you do not have a 6.0 or 13 kHz filter, the only good sounding
demodulation of an AM signal is SSB on one or the other sideband.
This is surprisingly good sounding, and I use it for listening to BC
that is down in the noise (e.g afternoon Yankee games on WCBS 880). I
actually use as narrow as 1.8 kHz SSB with the shift and width
adjusted for best voice versus noise. The NR usually has a setting
which improves it further.

This is the most attention given to AM by hams that I've seen in 40
years.  I've gone back to listening to AM broadcasts on my K3 (BC and
HF) while I'm workbenching.  There is some high grade stuff out there
to be found, and for me the K3's rendition is superior. At my age I
don't hear the 60 dB down 8 kHz buzzy artifact stuff that some people
complain about. IMHO that's something that needs to be fixed in the
analog by reducing the analog bandwidth above 4 or 5 kHz. Or fixed by
providing an actual hi-fi demod/audio board as an option for those who
want such.

73, Guy.

On Mon, Dec 7, 2009 at 1:34 AM, Richard Jones - KJ5QY <rick at prismnet.com> wrote:
>
> I think I found at least one of the problems.
>
> If you go into the "RX EQ" (by clicking the "MENU" button,  then turning the
> knob until you see the "RX EQ 1").  Next,  turn the lowest base control
> (Number 1) up from the "0" EQ position until you here the distortion on AM.
> It will not take much.  On my rig,  just taking it a few levels (Db) above
> the "reference" the distortion on AM is there in a vengeance.
>
> The following is the most important as I have found:  If you have/get the
> distortion and you raise the "Shift Control" (While in AM or AM Sync) from
> "*150" to "155",  the distortion completely goes away!!!  I am not sure what
> is going on here.
>
> I hope this,  and the posts above will give some additional information on
> what I am experiencing, and hopefully an answer.
>
> Thanks,
>
> Rick
>
>
>
> Richard Jones - KJ5QY wrote:
>>
>> Hello,
>>
>> I am having a problem when I go into AM, or AM sync.  Example:  I go to an
>> AM Shortwave station.  If I turn the AF gain past approximately 3/4 or
>> farther the radio starts to "Stutter, Distort, and finally the audio
>> disappears.  I cannot make out what is coming out from my speaker.  The
>> audio is completely distorted or gone.  I noticed I could rotate the Af
>> gain (Counter Clockwise) and it would go back to less distortion.  I did
>> check the external speaker jack and also unplugged it only to find the
>> same problem.
>>
>> The AF knob feels like it was one of those "old potentiometers" that we
>> are all aware of (Get scratchy after use and loose).  From my
>> understanding Elecraft used extremely fine pots with a very long life
>> (They do have a bad wobble/looseness in them though).
>>
>> Upon further investigation I found that if I rotated the RF gain control
>> from the fully clockwise position (all the way open) to the full counter
>> clockwise position (all the way closed) that when you almost get to the
>> end of travel the radio audio goes completely dead.  I have tried this
>> multiples times and it is very repeatable.
>>
>> After more investigation I found that if I rotated the "shift" (While in
>> AM or Sync AM) control from its standard (Normal) value of 1.50 to 1.55
>> this horrible distortion would disappear?  I am really confused as to what
>> is going on here.
>>
>> This happens while in the "Normal" AM mode as well as the "Sync" AM mode.
>>
>> I have tried to explain this in away that I hope most people will
>> understand my problem (I am not an EE).
>>
>> Could others please let me know if it is my unit or a wider problem?
>>
>> I am using 3.59 and the serial number is 283?.  This unit was fully loaded
>> and built by Elecraft.
>>
>> Thanks,
>>
>> Rick
>>
>
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