No "puff" ??? With .22R emitter resistors on the same heatsink , while not ideal , one can run unmatched multiple pair BJT's. Just to "play" , I used two different batches of Mouser NJW's , some read 17mV Re ,some 14mv , average of 15.5mv (68mv per device). Ran 4R squarewave , real subwoofer ... no pooof! In multiple OP pairs with .1R Re , one very "weak" pair (much higher beta ) will "hog" the current and possibly fry.
OP devices closest to where the rails connect will be the first to blow regardless of matching (I did this ... twice !!
) A fast flyback diode at OP will also short out and save a "string" of outputs. Of course , I would not recommend anybody try these things to confirm ($$$ ) , just take my word for these things that the simulator will NOT tell you. My LT says ... same gain , absolute pre/driver cancellation (thermally) , more stable. Just one Vbe (OP's) , 3 pair T-traks would be the "ultimate BJT powerstage" sweet ... just too expensive (the T-Traks
) I have the PCB designed already , just "a leap of faith" (like PMA's CFP bias spreader). PS , that bias spreader will be the triples Vbe .os
Awesome post ostripper, that's good to know about parallels of multiple outputs in the real world. DIY AKA R&D
No software could ever replace that!
I like how you mention the sharing difference between 0.1 and 0.22 ohm emitter resistors with unmatched pairs!
Also how you mention the pairs closer to the rails blow first. It makes perfect sense being the wires or PCB paths to the rails are shorter to the closest pair. Now if you ran equal length power wires on both Emitters and Collectors for each transistor would it reduce this phenomenon?
Now with closely matched pairs, or pairs that have been checked for gain match, do 0.1 resistors do fine in your opinion? What about NO Emitter resistors, can you get away with it? I always assume even under matching once the transistors heat up they will vary in gain some with each other due to slight differences or temp differences along the heatsink.
I would always assume it's best in a BIG amp with multiple pairs it's best to add more pairs than needed to help better handle the total load because one pair or the other may carry just a heavier load than another pair at times. A similar example would be Semi Trucks, being one tire can handle more weight single than in dual, because sometimes one tire (or axle) carries more of the weight than the other, so they add more tires (or axles) and keep the total load below the max ratings of the individual tires. I look at Parallel Transistors in a similar light.
Now being about a SINGLE pair of MJL4281/4302, (since there's no parallels) could you get rid of the emitter resistors completely to help drive a low ohm load by not wasting power and volt drop. I know the IC CHIP AMPS NEVER use output resistors, and I'd like to do the same thing. A high current, low impedance DIY DISCRETE amp without using IC AMPS would be BLISS!
No software could ever replace that!
I like how you mention the sharing difference between 0.1 and 0.22 ohm emitter resistors with unmatched pairs!
Also how you mention the pairs closer to the rails blow first. It makes perfect sense being the wires or PCB paths to the rails are shorter to the closest pair. Now if you ran equal length power wires on both Emitters and Collectors for each transistor would it reduce this phenomenon?
Now with closely matched pairs, or pairs that have been checked for gain match, do 0.1 resistors do fine in your opinion? What about NO Emitter resistors, can you get away with it? I always assume even under matching once the transistors heat up they will vary in gain some with each other due to slight differences or temp differences along the heatsink.
I would always assume it's best in a BIG amp with multiple pairs it's best to add more pairs than needed to help better handle the total load because one pair or the other may carry just a heavier load than another pair at times. A similar example would be Semi Trucks, being one tire can handle more weight single than in dual, because sometimes one tire (or axle) carries more of the weight than the other, so they add more tires (or axles) and keep the total load below the max ratings of the individual tires. I look at Parallel Transistors in a similar light.
Now being about a SINGLE pair of MJL4281/4302, (since there's no parallels) could you get rid of the emitter resistors completely to help drive a low ohm load by not wasting power and volt drop. I know the IC CHIP AMPS NEVER use output resistors, and I'd like to do the same thing. A high current, low impedance DIY DISCRETE amp without using IC AMPS would be BLISS!
With absolutely equal inductance/resistance (sort of a starred power rail) with multiple pair OP's , the strongest device would blow first , most likely by just a few microsecond's.
Smaller heatsink - larger emitter resistor. Larger heatsink - a smaller emitter resistor will work. .1R and under your Vbe must have a variable coeffecient as the "window" of thermal feedback is smaller. Never tried designs without a resistor.
Driving low impedance loads has less to do with the emitter resistors and more to do with device SOA vs. supply voltage. IC OP's are nearly perfectly matched internally , do not need the resistors.
OS
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