[Elecraft] K3 Harmonic Distortion

[Joe Subich, W4TV]

> 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. 

Reducing the DSP bandwidth will not resolve the harmonics 
generated in the speaker amplifier and/or Line Out circuits. 

The solution is simply not pushing the output gain and keeping 
the output under 1V peak and use high impedance devices.  With 
the speaker amplifier 800 mW into a 16 Ohm speaker shows less
than .01% THD+N - if more audio is necessary 1V peak will easily 
drive a good quality 100 watt per channel stereo system.  With 
Line Out, 1V peak is plenty for good s/n soundcard operation. 

73, 

   ... Joe, W4TV 
   




> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of alsopb
> Sent: Wednesday, September 03, 2008 9:39 AM
> To: elecraft at mailman.qth.net
> Subject: RE: [Elecraft] K3 Harmonic Distortion
> 
> 
> 
> 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
> >   
> >