XPost: sci.electronics.design   
   From: nbill@bellsouth.net   
      
   On Wed, 02 Mar 2011 08:40:42 -0800, John Larkin wrote:   
      
   > I've always sort of liked the classic "GE" tape head/mic preamp   
   > circuit:   
   >   
   > ftp://jjlarkin.lmi.net/GEcircuit.jpg   
   >   
   > but it occurred to me that it might also make a nice headphone amp...   
   >   
   > ftp://jjlarkin.lmi.net/GE_headphone_amp.JPG   
   >   
      
   I plugged this into LTspice and with a bit of twiddling appears   
   to work well, at least under simulation. I used a supply V of 15V,   
   transistor input resistor (R1) = 100K, input capacitor (C1) = 1uF,   
   transistor emitter resistor (R2) = 3.3K, feedback resistor (R3) = 22K,   
   cap in series with feedback resistor (C3) = 10uF,   
   bypass capacitor (C2) = 2200uF (needs to be big or lose LF gain/damping)   
   output capacitor (C4) = 470uF (let it charge before connecting)   
   inductor size of 10 henries (for sim assuming perfect, 0 ohms)   
   transistor collector resistor (R4) = 3.6K but can vary,   
   mosfet source resistor (R5, parallel with C2) = 47 but can vary.   
   Transistor = whatever (2N5550 in sim), mosfet = IRL530.   
      
   These values optimize for medium output power (570mW) into 50 ohms   
   and reasonable power (around 300mW) into 32 ohms and 100 ohms but   
   with off-center clipping. R4 and R5 can be varied to deliver the   
   desired power into the desired load... some of the values I tried...   
      
   100 ohm load.. R4=22K  R5=22 PDQ=1.5W PDR5=0.3W   
     Pout=730mW into 100, 370mW into 50, 220mW into 32   
   50 ohm load... R4=9.1K R5=22 PDQ=2.2W PDR5=0.9W   
     Pout=1000mW into 50, 660mW into 32, 530mW into 100   
   50 ohm load... R4=5.6K R5=33 PDQ=1.6W PDR5=1.1W   
     Pout=788mW into 50, 490mW into 32, 400mW into 100   
   50 ohm load... R4=3.6K R5=47 PDQ=1.2W PDR5=1.1W   
     Pout=570mW into 50, 330mW into 32, 290mW into 100   
   32 ohm load... R4=5.1K R5=22 PDQ=2.5W PDR5=1.7W   
     Pout=1180mW into 32, 760mW into 50, 380mW into 100   
   32 ohm load... R4=3.3K R5=33 PDQ=1.7W PDR5=1.7W   
     Pout=790mW into 32, 560mW into 50, 280mW into 100   
   32 ohm load... R4=2.2K R5=47 PDQ=1.2W PDR5=1.6W   
     Pout=500mW into 32, 400mW into 50, 200mW into 100   
      
   (PDQ is mosfet dissipation, PDR5 is R5 dissipation)   
      
   R5 sets the overall power level, then adjust R4 to achieve   
   balanced clipping.   
      
   Output impedance is fairly low, not much variance as load changes.   
   Gain is approximately R3/R2 plus a bit. Distortion increases as   
   R4 (and open loop gain) decreases but it appears rather "tuby".   
      
   A 10 henry inductor is probably overkill, anything 1H or more will   
   probably be fine, for a HP amp there's plently of overhead and the   
   negative feedback will mostly correct for deficiencies, smaller   
   inductors just have less output at 20hz. Could probably use the   
   secondary of an output transformer with the primary insulated..   
   but watch out for core saturation. Should have fairly low resistance,   
   preferably less than a few ohms (can tweak values to compensate).   
   Someone in the thread said large inductors are "unobtainium" but   
   that's BS, transformer windings ARE huge inductors, for this level   
   of power a winding of a power transformer will probably work.   
   For class A amps using an inductor or transformer output doubles   
   efficiency and halves the supply voltage needed for a given output.   
   It also presents a high impedance at audio frequencies so that only   
   the load determines the impedance (thus the gain) of the output stage.   
   It's possible to use a resistor load but won't perform as well.   
      
   >   
   > Audio tends to be nonsense, but at least the audio guys have fun   
   > playing with circuits, whether they make a lot of sense or not.   
   >   
   > John   
      
   But this one does make sense. There's a reason this basic circuit   
   has been around about as long as transistors...   
      
   Terry   
      
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