[Elecraft] Added LIN OUT measurements

[Joe Subich, W4TV]

> Maybe I wasn't clear what I was asking. I was asking for a 
> carrier to be injected that produced an audio tone of less 
> than 900Hz so that the third harmonic of that was within the 
> passband. I'm interested in the DSP and if that is 
> introducing the effect I'm hearing

As has been stated a dozen times in a dozen different ways, what 
you are hearing is simply the effect of overdriving the audio 
amplifiers.   I posted the following table showing harmonic 
distortion products for the speaker amplifier the other day: 

> The following table shows harmonic levels relative to a 500 Hz 
> reference tone at the given voltage across an 8 Ohm speaker.  
> 0 dB is 1V Peak (.7V 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). 

Until the level is increased to the point that the amplifier is 
in compression and/or clipping, harmonic levels are extremely 
low.  

Here is a similar set of data points for the headphone output: 

8 Ohm (old Yaesu) headset 

Reference    2nd   3rd   4th   5th   6th   7th   8th   9th 
============================================================
 -6 dB       -60   -36   -74   -37   -73   -39   -75   -44
 -7 dB       -60   -69   -79   -75   -85   -77   -87   -81
-10 dB       -61   -69   -82   -60   -73   -66   -87   -80
-20 dB       -72   -60   -73   -66   -74   -70   -75   -74  
-30 dB       -61   -56   -63   -64   -65   -67   -66    nf  


32 Ohm (computer) headset 

Reference    2nd   3rd   4th   5th   6th   7th   8th   9th 
============================================================
 +1 dB       -62   -20   -68   -28   -82   -50   -70   -46
  0 dB       -62   -73   -89   -79   -90   -84   -94   -86
-10 dB       -70   -66   -80   -73   -85   -80   -86   -81
-20 dB       -72   -60   -73   -66   -74   -70   -75   -74  
-30 dB       -52   -52   -52    nf   -54   -60   -55    nf  


200 Ohm (Heil Proset) headset 

Reference    2nd   3rd   4th   5th   6th   7th   8th   9th 
============================================================
 +4 dB       -64   -48   -90   -47   -84   -50   -80   -53
 +3 dB       -65   -79   -95   -86   -98   -93   -97   -93
  0 dB       -68   -79   -82   -84   -86   -80   -84   -83
-10 dB       -76   -75   -82   -86   -84    nf   -86    nf
-20 dB       -72   -80   -73    nf   -75    nf   -76    nf  

0 dB is 770 mV RMS (1V Peak) measured for the reference tone 
across the headphone element - total audio level will be 
somewhat higher with distortion/noise.  Note: in every case 
harmonic levels are extremely low, generally down more than 
60 dB right to the point that the headphone amplifier goes 
into compression/clipping.  "nf" indicares harmonic product 
not measurable (in the noise floor). 

The table shows the harmonic levels at maximum "linear" 
output and again with the reference tone raised just 1 dB 
above the clipping point.  

Now my ears are not "young" and have some loss from more 
than 30 years around control room monitors that were always 
"too loud" and the many hours around the big pumps/blowers 
in the TV transmitter sites.  Still, with all of the tested 
headphones the audio level is nearly painful at least 3 dB 
before the amplifier went into clipping and the harmonic 
products exceed -60 dB.  

Based on these and previous tests, anyone who can not get 
adequate headphone levels without distortion should be look-
ing for more efficient headphones and/or loudspeakers. 

73, 

   ... Joe, W4TV 
 





> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Paul Fletcher
> Sent: Thursday, September 04, 2008 12:37 AM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] Added LIN OUT measurements
> 
> 
> 
> Jim,
> 
> Maybe I wasn't clear what I was asking. I was asking for a 
> carrier to be injected that produced an audio tone of less 
> than 900Hz so that the third harmonic of that was within the 
> passband. I'm interested in the DSP and if that is 
> introducing the effect I'm hearing. As for the difference 
> between impedance and resistance your point is duly noted and 
> the only way this could be achieved is to use a set of Heil 
> cans as the load (or something with very similar characteristics).
> 
> 73 Paul.