From: info@turneraudio.com.au   
      
   John Stewart reminds us that............   
      
   "A paper by Anthony New published in Electronics World makes a very good    
   argument that THD testing is a waste of time."   
      
   But with most tube amps used for hi-fi and which operate in class A1 for   
   nearly all the power required for home hi-fi listening, measuring THD is   
   actually not such a bad way to assess or compare amps. RDH4 gives approximate   
   factors for estimating IMD    
   once the THD has been measured. This assumes there are only 2 test frequencies   
   applied at the amp input with say 1Vrms of 80Hz and 0.25Vrms of say 5kHz. The   
   80Hz may then be fairly easily filtered out of the sample voltage at amp   
   output, leaving just    
   5kHz which may be examined on a CRO to estimate the percentage of amplitude   
   modulation caused by presence of 80Hz.   
   The 5kHz wave may be passed through a diode & R+C detector circuit to measure   
   low levels of AM which may be difficult to see on a CRO if it is below 1%.   
   This saves the home constructor the effort needed to build a special filter   
   capable of rejecting 80Hz AND 5kHz, and just giving the sidebands of 4,920Hz   
   and 5,080Hz. Nobody would bother now to make such an effort because software   
   and a PC is +90dB easier    
   to do, even though much is learnt by building one's own analog gear which was   
   used before PCs and digitalia came along.   
   In fact I made a tunable BPF for between 1.4kHz and 11kHz and with Q = 50 to   
   pick out all F between 2kHz and 10kHz, including IMD products. Only 3 op-amps   
   are needed with some overlapping FB paths plus a good pot. Some schematics are   
   online, or in old    
   books.     
   It was assumed that the "standard" IMD test with 4:1 ratio of LF to HF was the   
   best test for any tube amp because the OPT iron behavior and tubes and RC   
   couplings caused their worst levels of nonlinearity at bass frequencies, ie, a   
   large bass signal of    
   80Hz or worse, say 30Hz, will upset the clean production of 5kHz more than say   
   having two signals of say 1kHz and 5kHz, even if the 1kHz is 4 times the level   
   of 5 kHz.    
      
      
   So, at the end of the day, if the tube amp is used at levels where nothing   
   ever clips, and the THD < 0.1% based on 1 kHz THD testing, and the open loop   
   bandwidth ( with no global NFB ) is say 30Hz to 40kHz, then music is usually   
   judged as subjectivley    
   pleasing to most ppl. If one then tries to compare by using a Halcro SS amp   
   capable of THD levels at 1 Watt which are undetectable, then few would hear   
   any difference in a blind A-B test where they are asked to identify which has   
   less distortion. If    
   asked which sounds best, maybe the class A triode amp will be chosen more   
   often, but this depends on source and recodings etc, and the 1,001 other   
   things that have been endlessly discussed at rec.audio.hi-end, with moderators   
   having to weed out flamers.    
      
   I myself have conducted comparison blind tests between amps making THD of 0.1%   
   and 0.0002% and found the listeners concluded that "the same bastard designed   
   and built both amps, which were both OK". And indeed the 2 x 300W mosfet amp   
   with 6 mosfets and    
   class AB was undetectable when substituted by a "mystery switch" with 2 x 65W   
   clas AB tube amp with 4 x 6550 per channel.   
      
   So once I was able to routinely make tube amps with 0.1% THD at 1 kHz at   
   50Watts, and with wide open loop BW, I didn't need to check IMD. The IMD was   
   always low enough. Other factors such as noise, Rout, BW and stability were   
   always important.   
      
   I also found that someone could make the worlds best amp in a back shed, and   
   nobody really wanted to pay the world price, they would always go to some   
   brandname, which so often was a technical POS, but with percieved "investment   
   value", so I cncluded    
   many people access the sound of the music by the price tag of the amp, or the   
   bling factor, and just as likely hardly know what real music is.    
      
   I'm now happily retired and I won't be an underpaid volanteer expert any more.   
   Patrick Turner.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)