From: mynamespacedbydots@xs4all.nl   
      
   "Phil Allison"  wrote in message   
   news:afcl8aFc81iU1@mid.individual.net...   
   >   
   > "GRe"   
   > "Phil Allison"   
   >>   
   >>> You need a THD meter to make sense of what matters and what does not.   
   >>>   
   >>   
   >> Input f/V/THD       Output THD (600ohm terminated)   
   >>   
   >> 1kHz/1.0V/0.075%    0.082%   
   >> 1kHz/3.6V/0.086%    0.101%   
   >>   
   >> 3.6V is the max. output for the audio generator.   
   >   
   >   
   > ** You need to use a much lower frequency.   
   >   
   >   Transformer non linearity is concentrated  at the low end of the audio   
   > band.   
      
   I used the function generator HV-output(600ohm) for this test.   
      
   Input f/V/THD       Output THD (600ohm terminated)   
      
   200Hz/10V/0.036%    0.049%   
   100Hz/10V/0.037%    0.077%   
    50Hz/10V/0.037%    0.170%   
    25Hz/10V/0.036%    0.442%   
      
   So, per "rough approximation" about 0.4% added distortion at 25Hz/10V.   
   Scoping the distortionmeter's output it looks like mainly 3H, but   
   seems quite acceptable to me for that frequency and level.   
      
   >> Can one simply substract THD Out/In numbers to gain THD for the   
   >> transformer only?   
   >   
   >   
   > ** To a rough approximation only.   
   >   
   > The figures quoted above suggest the tranny is adding almost nothing.   
   >   
   > PLUS it is crucial the source impedance be low - like 50 or 100 ohms.   
      
   In this case it does'nt seem to make a difference using the function   
   generator 50ohm or 600ohm output, see below, so I used the latter for   
   the tests as the max. Vout is about 14V vs. about 7V(unterminated).   
      
   At 25Hz/5V/600ohm output, THDout - THDin = 0.307 - 0.034 = 0.273%   
   At 25Hz/5V/ 50ohm output, THDout - THDin = 0.721 - 0.444 = 0.277%   
      
      
   > Easy way is to use one channel of a good hi-fi amplifier to buffer the   
   > output of the generator and drop the impedance to near zero.   
      
   Only handy in the living room, not on the bench.   
      
   Gio.   
      
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