410d0158   
   From: spam@spam.com   
      
   On Wed, 7 Sep 2011 15:58:48 -0700 (PDT), Engineer   
    wrote:   
      
      
   >>   
   >> 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.   
   >>   
   >> d   
   >   
   >Not a bodge... steering the Nyquist plot around the "minus 1" point   
   >for all frequencies is established practice in feedback control   
   >systems.  It's also perfectly acceptable here. BTW, the higher break   
   >point (of the two) for the VLF shelf is well below 100 Hz... around 15   
   >to 20 Hz is typical to stop a 1 Hz or so VLF oscillation.  I'll agree   
   >that the removal of one of the RC couplings in the forward path would   
   >be good, but hard to do when you have a separate driver stage after   
   >the phase splitter. There are direct-coupled DC-amplifier designs, but   
   >they are far more difficult to implement than a VLF shelf and, anyway,   
   >not needed.  Also, a massively larger primary inductance in the OPT   
   >primary would be next to godliness... but most of us (in NA, at least)   
   >cannot afford that so we use the best Hammond iron we can afford or   
   >find a decently large OPT from a recycled high grade amplifier!   
   >Cheers,   
   >Roger   
      
   Don't know where you got those numbers. It does this:   
      
   http://www.soundthoughts.co.uk/look/shelf.png   
      
   d   
      
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