Audio: Push Pull PA

April/20 2016

transformer push pull Audio PA is not HIFI, not small, not cheap, but I'm attracted by it's simple and classic, low voltage high power output( compared to OTL).  for a given supply voltage, if it's output transformer given n:1 voltage ratio, it's max output power is:

Pmax = V2/(2*n2*RL)

n is normally 1.5 ~ 3. 

OTL  : Pmax = V2/(8*RL)

build a simple push pull Audio PA

this one looks simple. i'm super lazy, omit R11(100 ) seems no harm, cause change R12 will give the correct bias current. but, practically build that teach me a lot about push pull PA.

the R11 actually is essential part for AC path. what if i connect R12 with a bypass capacitor?  yngz@crystralradio.cn told me that won't work, yeah, i suddly get that, cause for push pull, the transistor responsible amplify half of waveform, that's actually DC current.  this implicitly mean any capacitor bypass won't work in the push pull ! even connect bypass from B5's mid to Emit of transistor.

thing won't that perfect as i expected, i reallize the R11 is essential, but still, i can't get it work. 

i made some other thing bad, this time is B6. I use a B6 contain too much coil, but i ignore one thing: the too much coil turns mean too much DC resistance, this DC resistance in the output coil of B6 exceed 50 ohm, this mean most of energy is consume by the transformer, instead of 8 ohm speaker.  much bad impact is, the B6 input will see the impedance at least the DC resistance, that's exceed 200 ohm.  in such a scenario, impendance ratio is totally mess,  you can't  8*16=100 ohm impedance seen by the transistor's collector. too much impendance present to the transistor leading to oscillation.  

reach to final success? no, my PA audio quality is terrible. this time things simple, one of my Push pull transistor is broken. ok, i got the good audio this time. 

but wait, several minutes later,  DC bias current exceed 1A, cause a Power protection,  thermal drift, too much. the cure is connect a degenerate resistor to common E joint, i use 15 ohm get a good result.