Hi Ian,
The problem was that the original P3A design was alternated in a wrong way - a second pair of the output transistors was added and the emitter resistors removed from the drivers (Re2, referring to post #35) - that was the main mistake. Without those resistors, even if the Vbe spreader transistor is placed correctly - the drivers are tracked - Sekhar ends up with runaway in any case.
Cheers,
Valery
The problem was that the original P3A design was alternated in a wrong way - a second pair of the output transistors was added and the emitter resistors removed from the drivers (Re2, referring to post #35) - that was the main mistake. Without those resistors, even if the Vbe spreader transistor is placed correctly - the drivers are tracked - Sekhar ends up with runaway in any case.
Cheers,
Valery
Sekhar, by the way - why do you need the second pair? More power? But then - lower load or higher rails?
you are posting things that are not right ....do some more reading before advising others
I found some interesting reading for you. From ESP site: link here.
"Parallel Transistors
When power transistors are used in parallel, some amplifier designs rely on close matching of all devices. Using an emitter resistor for each device forces some degree of current sharing, but with very low values (less than 0.1 Ohm) the devices should be matched because the resistance is barely sufficient to ensure equal sharing under normal operation.
Matching paralleled transistors usually requires that Vbe and gain are both matched. Gain matching should be done at a range of collector currents to ensure that the transistors share the load equally. It is the nature of bipolar transistors that the one that takes the most load (because of higher gain or lower Vbe) will get hotter, and this will increase gain and lower Vbe even further, causing it to take even more of the load. Use of a common heatsink ensures that die temperatures are held reasonably close to each other.
Using emitter resistors will always help, but in some cases may not be enough to ensure long term reliability - especially if the devices are used close to their maximum ratings. Some designers include series resistors in the base circuits - these might help, but may do more harm than good, and are not generally recommended. With MOSFETs, gate resistors are always needed to prevent parasitic oscillation, but these do not affect current sharing.
Because the temperature of all paralleled transistors should be the same (for the reasons described above), it is essential that all power transistors (bipolar or MOSFET) share the same physical heatsink. By doing this, the average temperature will be very close to the same for all devices. Always use current sharing emitter or source resistors if possible, and feel free to match the Vbe and gain of paralleled transistors (or Vgs and gain for MOSFETs).
"
EFP and CFP is not the same , farther more, CFP characteristics and pros cannot be repeated with additional transistors in the output while preserving the same audio qualities like its done in all EFP OP schemes ...
Task like that is not investigated deep enough .... Many used configuration with many CFP outputs mostly in pro amps to achieve a bit better efficiency or work with ground collectors but then the amp is far more limited in the quality and of course properly compensated...
Clearly not the case here
As for the P3a as was it will work and produce music while being solid rock stable at most conditions is the one side The other is how to achieve a bit more from it in terms of speed , sound quality and a better contend of harmonics ....
Task like that is not investigated deep enough .... Many used configuration with many CFP outputs mostly in pro amps to achieve a bit better efficiency or work with ground collectors but then the amp is far more limited in the quality and of course properly compensated...
Clearly not the case here
As for the P3a as was it will work and produce music while being solid rock stable at most conditions is the one side The other is how to achieve a bit more from it in terms of speed , sound quality and a better contend of harmonics ....
hi vzaichenko, i wanted to use the P3A as a subwoofer amplifier , the subwoofer is rated at 250W RMS at 4E and I have tried the sub with 60-80W of power from a TDA7293 as well as a LM3886 didnot get good results that's why tried the P3A with 42V and 4E load that should theoretically fetch me some where in the range of 150-180W of power, So i used to 2 pairs to get the stated power ...
OK, I see - 4 ohm load. Makes sense. Thank you for the answer.
Let us know how it goes after corrections 😉
yep sure will ... thanks for pointing out my error .. does the emitter resistor need to be 0.1E i current have it as 0.25E and is it imperative that the collector resistor be 0.33E ?
Use your .25R emitter resistors. The higher the emitter resistors, the better the output devices will share output current. And use the recommended .33R collector resistors also. Thermal tracking relies on their value.
Best regards!
Kay was quicker.OK will try and report back by the earliest ... I had a couple of more questions can I use bcp53 and bcp56 as driver and mmbt5551 as the input LTP pair as getting good through hole parts is difficult...The bcp53,56 are smd transistor with VCE of 80V@1A and a ft of 140mhz in a SOT-223 package??
Emmm... should be ok, but BC546 are lower nose / higher gain ones - replacement with mmbt5551 may result in slightly higher noise and higher distortion.
bcp53 and bcp56 - looking good, however in CFP arrangement every detail is important, so just be careful at power-on - have your oscilloscope attached at the output to notice any possible oscillation.
bcp53 and bcp56 - looking good, however in CFP arrangement every detail is important, so just be careful at power-on - have your oscilloscope attached at the output to notice any possible oscillation.
Yes sure i will try P68 as well , actually I have heard so many good reviews about P3A so i was very tempted to do so ...
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