[Elecraft] K3 Audio out artifacts

[Don Wilhelm]

Mike,

Contrary to popular myth --
There is no need for any kind of resistive loading at the headphone 
output.  A low impedance source (the amplifier in the K3) feeding a high 
impedance input (the soundcard) works as it should for spectral analysis.
.
If you were measuring available power output from the headphone jack, 
then load termination would become important.

73,
Don W3FPR

Bob Tellefsen wrote:
> Hi Scott
> Out of curiousity, on the headphone output, are
> you running it directly into a sound card to
> use SpectrumLab?  Or do you have some amount
> of resistive load on the audio output with measurement
> across the load?
> 73, Bob N6WG
>
> ----- Original Message ----- 
> From: "Mike Scott" <mike at paxsen.com>
> To: <Elecraft at mailman.qth.net>
> Sent: Monday, March 02, 2009 9:36 AM
> Subject: [Elecraft] K3 Audio out artifacts
>
>
>   
>> I have been investigating audio artifacts in connection with the K3 AGC 
>> hold
>> experimental firmware. As a reminder, I live in a high noise area.
>>
>> As part of the investigation I have been looking at both Line Out and
>> headphone audio using Spectrumlab on my PC.
>>
>> I am finding substantial 60-hz contamination on headphone audio that I do
>> not see on Line Out.
>>
>> On Line Out I see what looks like fairly clean audio with donor lines at
>> 2400 Hz and 3600 Hz outside the rx filter pass band (1 KHz DSP filtering). 
>> I
>> presume these are the 40th and 60th harmonics of 60 Hz. They are 50 dB 
>> below
>> noise background noise.
>>
>> With only a change of where I plug the PC audio cable (moving from Line 
>> Out
>> jack to rear headphone jack) the audio looks quite different.
>>
>> Now I see a whole array of 60 Hz artifacts. The 3rd and 5th harmonics are
>> about equal to comfortable listening level audio.  As I look higher in
>> frequency the harmonics generally reduce in level but only to about 20 dB
>> below comfortable audio listening level. There is also a broad artifact 
>> (300
>> Hz wide) at roughly 3300 Hz that is also 25 dB down.
>>
>> The artifacts do not go away with reduced volume control settings but 
>> remain
>> a constant feature of headphone audio out at any setting.
>>
>> Is there something about my test set up that I do not understand?
>> Does anybody else see contamination of headphone audio on their K3?
>> Could there be something wrong with my K3?
>> Could transformer coupling (Line Out) vs direct coupling (headphone) be
>> adding something to my test set up that I am not factoring into the 
>> problem?
>>
>>
>> I have never quite believed the audio of my K3 was like Wayne says it 
>> should
>> be but can make it better using slow AGC (separate post), I doubt that has
>> anything to do with the artifacts I see here.
>>
>>
>> Mike Scott - AE6WA
>> Tarzana, CA (DM04 / near LA)
>> NAQCC 3535
>> K3-100 #508 / KX1  #1311
>>
>>
>>
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>
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