From: phil_a@tpg.com.au   
      
   "Phil Allison"   
   >   In order to find the " open loop " THD of a Quad II output stage, I   
   > removed the two EF86s and connected an output tranny primary to   
   > the + supply and both plate pins.   
   >   
   > The secondary was then driven by my bench oscillator, which has   
   > very low THD -  circa 0.002%.   
   >   
   > This gave 52Vrms from grid to grid on the output valves - enough to   
   > drive the stage to 10 W at 8 ohms and 1kHz.   
   >   
   > Results:  THD =  0.76%,   DF = 1.   
      
   ** Correction, the above THD figure is in error.   
      
   Magnetising current in the output tranny caused far more 3H than I had   
   hunched for the low level and 1kHz frequency I was using.   
      
   So I repeated the test with a 100W mosfet amp as a buffer between the   
   bench oscillator and the tranny - the THD from this set up was checked   
   at 0.04% at 52V rms.   
      
   The corrected figure is a staggering 0.18% THD.   
      
   I was so surprised at this result, I repeated the measurement in various   
   ways several more times.   
      
   Peter J. Walker sure knew what he was doing when he went with cathode   
   feedback for that output stage rather than UL or similar.   
      
      
   ....  Phil   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)