[Elecraft] K3 Harmonic Distortion

[alsopb]

Clearly I must be missing something.

In my view, all the reported distortion numbers are beyond expectations.
After all you are buying a $2K radio, not a many kilobuck audio system.

The "problems" people are having with "harmonics" when they listen to CW
with a 2.7KHz filter/DSP setting seem to be easily solved by just reducing
the DSP bandwidth.  The last resort solution is to get a few years older and
you won't hear them.

Workarounds for other problems seem to given in the postings.

I personally hear and see nothing objectional given the above perspective.


73 de Brian/K3KO



Joe Subich, W4TV-3 wrote:
> 
> 
> VP8NO writes: 
> 
>> Perhaps someone suitably qualified and tooled up might like to try and
>> [repeat] and comment upon the view presented by Rob Sherwood at the 
>> Dayton convention.  According to him the K3 audio amp stage has 
>> serious shortcomings.
> 
> I had to listen to the audio of Sherwood's talk before looking 
> in the right area.  First, Rob doesn't say the K3 audio has 
> serious shortcomings, he complains about distortion on the 
> speaker.  Sherwood also admits that using a high impedance 
> speaker (e.g., amplified computer speakers) or high impedance 
> headphones resolves the issue!
> 
> My measurements did not show that distortion but I was measuring 
> at the headphone output with a high impedance load - the lowest 
> impedance headphones I use are 32 Ohms.  As Jack Smith points 
> out, the headphones and the speakers have separate audio 
> amplifiers but share a common DAC.
> 
> Since Sherwood's comments were based on the speaker output and 
> a LOW impedance load, I dragged out an old Optimus XTX25 speaker 
> - an unpowered 8 Ohm speaker I've used with other radios for many 
> years.  With the 8 Ohm speaker is was easy to generate harmonic 
> distortion - particularly if the K3 were set to SPKRS = 2 with 
> only one speaker connected - simply by turning up the volume 
> until the amplifier was forced into clipping/saturation!  
> 
> With the 8 Ohm speaker, 1 to 2V peak represents a comfortable 
> to loud listening level for me.  At those levels, the harmonics 
> are all down more than 65 dB and any other distortion is very 
> acceptable.  
> 
> The following table shows harmonic levels relative to a 500 Hz 
> reference tone at the given voltage across the speaker.  0 dB 
> is 1V Peak (.707V RMS).  +10 dB represents 2.25V RMS (signal + 
> distortion).  +13 dB is 3.2V RMS (the speaker amplifier is in 
> the compression region at the threshold of clipping) and +15 
> dB is 4V RMS (the speaker amplifier is in hard clipping). 
> 
> Reference    2nd   3rd   4th   5th   6th   7th   8th   9th 
> ============================================================
> +15 dB       -37   -15   -45   -26   -60   -43   -51   -37
> +13 dB       -49   -31   -51   -35   -57   -39   -63   -49
> +10 dB       -70   -68   -95   -67   -85   -69   -85   -73
>   0 dB       -66   -74   -70   -77   -73   -76   -77   -80
> -10 dB       -54   -66   -63   -90   -73   -82   -80   -87
> -20 dB       -50   -78   -68   -74   -73   -74   -74   -78     
> 
> In order to drive the audio amplifier hard enough to create 
> the distortion, it was necessary to disable the AGC or set 
> AGC SLP to low values (reduce the AGC reduction for strong 
> signals).  
> 
> Again, as in the case of the Line Out, if the audio gain,  
> AGC and RF gain are operated to keep the speaker amplifier 
> out of the clipping range - or away from the compression 
> region just before clipping - the distortion products are 
> entirely acceptable.  For those with noisy shacks, impaired 
> hearing or using the internal speaker it may be difficult 
> to find a comfortable listening level without entering the 
> compression region.  
> 
> For those who wish to operate without AGC and with maximum 
> RF gain, I would suggest setting the AF gain to a moderate 
> level and using powered (computer) speakers or a 100 watt 
> per channel stereo amplifier capable of outputs greater 
> than the 1 watt per channel at .1% THD+N specified for the 
> LM4950 with an 8 Ohm load!  Note, simply going from 8 to 
> 16 Ohm speakers will improve THD by 8 dB. 
> 
> 73, 
> 
>    ... Joe, W4TV 
>   
> 
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