b6049552   
   From: apogosso@tpg.com.au   
      
   "Patrick Turner"  wrote in message   
   news:976e0f6d-2ec7-4889-b097-aafabfe60fca@28g2000pry.googlegroups.com...   
      
   OK, the 0.47 + 1M strapped with 0.047 + 220k acts like this :-   
   At 1 kHz, all C have low Z so the phase shift is low and the driver,   
   ie concertina or whatever you have "sees" a load of 220k. As F is   
   reduced, there is a pole between 0.047 and 220 at 15.4Hz which is   
   usually above the F at which the amp may want to oscillate. Sometimes   
   I have used 0.022uF, so pole is at 32Hz. phase shift caused is less   
   than 90 degrees. As F is further reduced, the network response tends   
   to flatten to a shelf formed by 1M and 220k, ie, signal flattens at   
   roughly -15db, 0.18 times the 1kHz level. The 0.047 has gone to an   
   open circuit by 2Hz with little effect. But the 0.47 the rolls the   
   response off at the pole between 0.47uF and 1.22M ohms, ie at 0.27Hz,   
   below which there is an ultimate phase shift of 90d, but at such a low   
   F as to not cause bothers because its well below the poles of all   
   other sages.   
      
   Alex:   
   Shelving network does a good job, but it has a drawback. It is an attenuator   
   for low frequencies. If low frequency signal or DC step gets applied to the   
   input of the amp, the first stage will or might overload, while the shelving   
   circuit will protect the next stage from the overloading.   
      
   To prevent the front stage from overloading at VLF it is better to apply the   
   shelving as a local feedback, i.e. in the cathode circuit of the 1-st stage.   
   Imagine the cathode of the 1-st stage is not directly connected to a   
   feedback divider (say 1K/100R) but through a RC circuit of paralleled 10K   
   and 5uF (approx.) This cathode degeneration at LF will act similar to the   
   attenuator shelving, but will prevent overloading of the first stage by VLF.   
   Then you do not need 1M||0.05uF interstage attenuator.   
      
   Of course in this cathode shelving the grid DC bias voltage has to be   
   elevated, so the grid can not be directly connected to the volume control.   
   However it is not a big deal. there are three known ways of elevating grid   
   bias voltage:   
   - fixed divider from upply voltage;   
   - divider cathode-grid-ground;   
   - split cathode resistor and from the tap thus formed throw 1M to the grid.   
      
   Regards,   
   Alex   
      
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