41616b57   
   From: apogosso@tpg.com.au   
      
   "Patrick Turner"  wrote in message   
   news:4339858a-099e-4d81-9bb0-2643f7942c64@x38g2000pri.googlegroups.com...   
      
   Patrick:   
   But what of the phase shift of that C? Isn't it better to have R&C in   
   parallel inserted from FB take off at OPT sec to the feedback R? Say   
   you have 1k0 and 100r as the normal FB divider so that 1/11 of the OPT   
   signal is applied to V1 cathode. Say one adds 3k3 so you then have   
   3k3, 1k0, then 100r at k to 0V at V1. Then ß becomes 0.022, much less   
   than 0.09, and at very low F there is no phase shift, so with less NFB   
   its probably going to be stable. But there isn't enough FB at higher F   
   so you shunt the 3k3 with say 6.8uF. So at 100Hz the 6.8uF = 233 ohms   
   reactance and 3k3 is well shunted.   
      
   Peaking still has to be checked in output and in error signal.   
      
   Alex:   
   I suggested something quite opposite. The NFB divider looks like:   
   - 1K from the speaker terminal to cathode of the driver stage;   
   - 100R from the cathode to 100uF capacitor;   
   - the other end of the 100uF capacitor is tied to GND.   
      
   Thus the feedback "beta" increases (!) at low frequencies (below 10Hz in   
   this case), giving -90deg phase lag in the loop below 10Hz. This is in   
   effect turning the amp into a anti-rumble filter.   
      
   From the first glance it might sound crazy to increase the loop gain at VLF   
   where we want an overal reduction of yje loop gain, but consider this:   
   - OPT typically gives +90deg lead below 15...20Hz;   
   - interstage coupling (simple RC with no shelving, 220K and 0.033uF) is   
   calculated to give -3dB corner at say 15...20Hz and also gives +90 deg lead   
   below;   
   - but this "funny" NFB with a 100R and 100uF gives -90deg LAG below 10Hz!   
   And this lag maintains down until 1Hz!   
      
   At LF one lag subtracts from two leads and in combination we have only   
   +90deg below 10Hz. Therefore, 0dB line at about 3Hz (typically) will be   
   crossed safely at only +90deg!   
      
   Killing many birds with one stone:   
   - perfect transient with no peaking;   
   - natural low cut off in the whole amp (sort of built-in anti-rumble   
   filtering);   
   - reduced error signal at VLF as more feedback is applied;   
   - no need to use two identical shelving circuits in push-pull amps -- just   
   one extra electrolytic.   
      
   A drawback - a despised electrolytic as a shaping component.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)