From: spam@spam.com   
      
   On Thu, 08 Sep 2011 09:31:41 -0500, John Byrns    
   wrote:   
      
   >In article <4e664bf7.8680753@news.eternal-september.org>,   
   > spam@spam.com (Don Pearce) wrote:   
   >   
   >> On Tue, 6 Sep 2011 09:03:54 -0700 (PDT), Engineer   
   >>  wrote:   
   >>   
   >> >On Sep 5, 1:46 am, s...@spam.com (Don Pearce) wrote:   
   >> >> On Sun, 4 Sep 2011 22:05:12 +0000, John L Stewart   
   >> >>   
   >> >>  wrote:   
   >> >>   
   >> >> >The drive requirements of the OP stage are not as critical as mentioned   
   >> >> >in earlier posts. The KT88, EL34 & 6550 all have G about double that   
   >> >> >found in the original Mullard circuits where we find 6L6, 5881, Etc.   
   >> >>   
   >> >> >A single dominant pole is a safe way to go in this cct. There are a   
   >> >> >couple of worked samples in RDH4 worth looking at.   
   >> >>   
   >> >> >Also strongly recommended by none other then Norman Crowhurst. You can   
   >> >> >read about how to implement all in a couple of his articles.   
   >> >>   
   >> >> >I like the cct proposed by Alex Pogossov recently. The attachment shows   
   >> >> >how it is connected in an SE Amp. The frequency selective network is in   
   >> >> >the cathode of the first stage rather than between stages.   
   >> >>   
   >> >> >My opinion, anyway, having done it.   
   >> >>   
   >> >> >Cheers, John Stewart   
   >> >>   
   >> >> >PS- The bicycle went more than 1000 KM in August (622 miles). Pedalling   
   >> >> >again today. Hey Patrick, hows it going in OZ?                J   
   >> >>   
   >> >> Why would you want a circuit that drops the gain by about 16dB at   
   >> >> 100Hz? What is its actual purpose?   
   >> >   
   >> >Ans: To get the loop gain (forward gain times NFB gain) well below   
   >> >unity at the low frequency where the phase lead becomes 180 degrees   
   >> >(due to two RC couplings and one OPT.)  Very low frequency oscillation   
   >> >in the range 0.5 to 2 Hz can result if there in no low-frequency shelf   
   >> >inside the loop.   
   >> >Cheers,   
   >> >Roger   
   >>   
   >> So it is a horribly bodged solution to a problem that has no business   
   >> being there in the first place? The idea of implementing negative   
   >> feedback and then defeating it by killing the open loop gain it   
   >> mediates is beyond ludicrous.   
   >   
   >On Sunday you suggested "a single pole that doesn't let up until beyond the   
   >unity gain frequency", isn't that equally ludicrous as it also "kills the open   
   >loop gain"?   
      
   Not at all. It operates as a local feedback loop within the overall   
   loop. At lower frequencies, where it really matters to have   
   particularly the output stage included, the loop stretches from   
   beginning to end. As the frequency rises the loop transfers steadily   
   to just the voltage amplifier. This does result in a slight rise in   
   harmonic distortion above about 10kHz, but frankly by then who cares?   
      
   And this pole is going to have to be present in some or other form   
   anyway because high frequency instability needs to be tamed - so why   
   not make it simple, predictable and controllable?   
      
   d   
      
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