From: info@turneraudio.com.au   
      
   On Tuesday, 6 November 2012 02:46:34 UTC+11, John L Stewart wrote:   
      
   snip for brevity,   
      
    The THD may go down but what about the higher order harmonics & IM? It has   
   been shown by many tests that these are the objectionable components in the   
   reproduced sound. Cheers to all, John    
      
   Well, you are possibly about correct about IMD being worse sounding than THD.   
   All music generates many harmonics of fundemental F produced by instruments.   
   An amplifier  or speaker adds to the THD but is not all that bad itself. If a   
   saxaphone's 3H is    
   increased by 1% then the tone won't seem to have changed much. But IMD   
   products, ie, the sum and difference between various F are often quite   
   an-harmonic, and their presence can have a far greater effect.    
      
   If you have an OP tube which makes 5% of 2H near clipping, then for 2H   
   cancellation you need the driver to make 5% of 2H. That 5% cancels the OP 5%,   
   but in fact the sum and difference between the driver 2H and fundemental are   
   produced, ie, 1H and 3H, ie,    
   more fundemenmtal plus 3H, and so you can get more 3H than was present to   
   begin with even if you had a perfect driver. But 2H + 5H yields 3H and 7H, and   
   suddenly you have lots more H than you started with although their levels tend   
   to be lowish, and    
   which are called second order. AFAIK, there is no need to worry about their   
   production where the amp had low THD without any FB, as is the case with most   
   well made class A amps used where they are meant to be used at average power <   
   1/10 clipping power.    
   One can always analyse to find just how many H a single 6SN7 might produce   
   even when set up optimally. One may find H extending to 10th H. So what? We   
   still must use devices to amplify sound, and you can't avoid the H production   
   by one weay or another.    
   RDH4 gives a guide to IMD significance and cites that IMD is around about 3   
   times the % of THD where you have the standard old IMD test where there the   
   output V0 has say 4V of 80Hz and say 1V of 5kHz. So if THD was say 1% at 4V of   
   80Hz, then expect 3%    
   IMD relative to 5kHz levels. One can display the 5kHz tone on a CRO, and   
   without any 80Hz by means of using a high pass filter, and then you can see   
   what % depth of amplitude modulation exists, ie, presence of sidebands at   
   4,920Hz and 5,080Hz. Neither of    
   these F are harmonious with 5,000Hz, and one may think they'd sound bad, but   
   IMD also is generated within instruments, and our ears seem used to such   
   slight amplitude modulation of high F tones by the usually much larger   
   amplitudes of bass tones.   
      
   Anyway, All I know is that if you have an SE amp which measures well without   
   using much GNFB, it usually sounds just fine at levels for which it has been   
   designed for. I have found that the best way use multigrid tubes to get basic   
   linearity is to employ    
   CFB windings on the OPT, and not to rely solely on the triode connection for   
   FB via the screen, which is of course the same as the triode internal NFB in a   
   real triode such as 300B. With say 20% CFB with a multigrid and SE amp, THD   
   just under clipping    
   can be routinely less 2% with correspondingly low IMD.The CFB converts the OP   
   stage to have quite low voltage gain like as if it was a triode. So the driver   
   stage which is usually a signal triode then has to make a substantial amount   
   of drive voltage,    
   say 60Vrms and so it also naturally generates around 2% THD even if fairly   
   well set up. So some substantial 2H cancelling occurs inevitably. Usually an   
   OP triode tube at near clipping may make far more 2H than the driver tube   
   could ever make unless    
   someone has set up the driver with a low value of anode load to provoke   
   production of 2H.    
      
   I've long since given up being too concerned about the unavoidable issues of   
   THD/IMD in tube amps. All one can do is minimise them "legitimately", and I   
   find 99.999% of everyone likes the sound. And if they change to something   
   solid state and THD becomes    
   100 times less because an extra 40dB NFB is used, they don't hear the   
   slightest betterment, and in fact are likely to dislike the sound.    
      
   As I said previously, I working on drastically reforming a pair of Jadis 300B   
   SE amps. The owner is currently using Atmosphere OTL with sensitive speakers   
   and it will be interesting to see if he finds the Reformed Turnerized Jadis to   
   be better. Perhaps    
   he'll find it different, rather than better/worse, but my main function has   
   been to make the Jadis work properly, quit smoking, and stop being a sulking   
   and silent and expensive French loungeroom companion. I have thought of   
   sending the original Jadis    
   OPTs to Jadis in France but methinks they'd take no notice at all, and pretend   
   i didn't exist, and probably never admit they ever could have forgotten to use   
   an air gap. I'm trying not to be unkind. Anyway, Jadis are free to take my   
   advice in future when    
   I post up my reforms for all to read. BTW, the Jadis have CLC filtering for   
   main 300B B+. The choke used is a 20mm stack of 20T E&I, and C are 2 x   
   470uF/500V. Nice caps, but shame about the choke. The choke air gap was 10   
   times too large, reducing L    
   value to a negligibly effective amount. I was able to re-gap the little choke   
   to give nearly 2H, and thus it is now useful. PSU resonance is now 5Hz, Q not   
   too high, and just passable.    
      
   I could perhaps use the Jadis OPT for a future parafeed amp because available   
   wasteless winding Z ratios are 888:1 and 222:1. So 888:1 gives 5k0 : 5.6r, and   
   Lp is high enough with no air gap at 88H, and RwP = 76 ohms only, so losses   
   are low enough. The    
   OPT would them be useful for an 845. But with 222:1 I get 1.25k : 5.6, OK for   
   3 x KT88/6550 or 4 x EL34 in triode for about 30Watts. Using 2 x KT120 in   
   triode also comes to mind, because one may idle the KT120 at Pda at 35W and   
   possibly 40W because Pda    
   rating = 60W. A single KT120 is potentially able to do what 2 x 6CA7/EL34 can   
   do in SE.    
      
   But KT120 triode Ra is not really all that low to allow a low RLa value, and   
   so RLa must be high with probable Ea at +500Vdc, and Ia up to say 70mA, with   
   load for one at 5k0, giving PO = 12 Watts. Theroetically 2 x KT120 in parallel   
   should give 25Watts    
   into RLa = 2k5, just right for a Hammond SEA1627, but not that easy with the   
   old Jadis OPT because there are just not enough sensible and available load   
   matches.     
   Patrick Turner.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)