Sometime (few weeks) ago I finished my first P3A amp. I have designed the pcb myself. I did the testing as described at 60-80W Power Amplifier and all went fine, also adjusting the bias. And the amp also did (and still does) sound good and wasn't noisy.
Now I have finished balanced input on seperate board and mounted all stuff into chassis. I measured voltage drop across resistors and it was almost 100mV. I tried to adjust it back to 50mV, but even with the trim pot at maximum resistance, the voltage was still almost 70mV .
As I said, the pcb is my own design. I have mounted driver transistors (including Q4) onto same heatsink as power transistors, of course they are electrically isolated.
Now I have finished balanced input on seperate board and mounted all stuff into chassis. I measured voltage drop across resistors and it was almost 100mV. I tried to adjust it back to 50mV, but even with the trim pot at maximum resistance, the voltage was still almost 70mV .
As I said, the pcb is my own design. I have mounted driver transistors (including Q4) onto same heatsink as power transistors, of course they are electrically isolated.
Where did you locate Q9? Is it near one of the driver transistors Q5 or Q6?
Q4 should not be on the main heatsink - thermal feedback will cause drift. Put it on it's own heatsink.
I would also suggest putting 100uF capacitors on the power supply rails, in parallel with C+ and C-, near the output transistors to help prevent oscillation.
When adjusting bias, do it with no speaker load and with the amp's input shorted to ground.
Q4 should not be on the main heatsink - thermal feedback will cause drift. Put it on it's own heatsink.
I would also suggest putting 100uF capacitors on the power supply rails, in parallel with C+ and C-, near the output transistors to help prevent oscillation.
When adjusting bias, do it with no speaker load and with the amp's input shorted to ground.
why is 0mVdc and 60mVdc different from 13mVdc?
What voltages are you measuring?
Q9 (the Vbe multiplier) must be connected to Q5 & Q6 (the drivers). Not to the main heatsink.
All three should be adjacent to each other and connected with a short thick strip of aluminium.
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I understand you now.
Vre cold = 0mV
Vre hot = 60mV
This indicates that the temperature compensation is NOT working.
Q9 is To92. Glue it between the two To220/To126 drivers that are joined by the aluminium strip.
I don't know the PCB (don't post it here) but Q5 must monitor the temperature of the drivers, not the outputs.
Vre cold = 0mV
Vre hot = 60mV
This indicates that the temperature compensation is NOT working.
Q9 is To92. Glue it between the two To220/To126 drivers that are joined by the aluminium strip.
I don't know the PCB (don't post it here) but Q5 must monitor the temperature of the drivers, not the outputs.
Okay, I removed all To126 transitors from heatsink and installed those suggested 100uF caps. Now I was able to set the bias and stabilize that voltage to 50mV.
However, driver transistors are still in close proximity of main heatsink. And I'm afraid that Q9 is too far from drivers (3 cm from closest driver, maybe). Do you think that connecting drivers and Q9 via aluminium strip would help, even if drivers are very close to main hs. Or do I have to make a new pcb?
However, driver transistors are still in close proximity of main heatsink. And I'm afraid that Q9 is too far from drivers (3 cm from closest driver, maybe). Do you think that connecting drivers and Q9 via aluminium strip would help, even if drivers are very close to main hs. Or do I have to make a new pcb?
I finally had time to make those modifications to amp. Now things seem to work correctly. Thanks a lot for help!
It seems that I don't know enough about those temperature related things in PCB design. Could somebody suggest some good reading about the subject? (And maybe about audio PCB design in general?
It seems that I don't know enough about those temperature related things in PCB design. Could somebody suggest some good reading about the subject? (And maybe about audio PCB design in general?
there is extensive threads on each and every amplifier regarding pcb issues ....the diference actually is bearly audible .... but yes a proper pcb will produce very very low distortion .....
always the point is very simple : you can design a pcb that has all the goodies collected from audio threads but this can only be done in a 2 layer pcb ....then again 2leyer pcb cost alot of monye if done diy and also present their own problems regarding capacitance and inductunce between traces ....
ideal amplifier pcb should be :
--- devided in 3 areas
a) power transistor area
b) psu on board area
c) small signal area
--- now this has to be designed under specific rules
* star ground philosophy
* power traces as small is possible
* decoupling and bypass as close to the power transistor possible
* then ground trace of the above as small is possible
* ltp in a way that it can have thermal junction
* and all the above with symmetry and order just for the looks ....
well i can tell you that i have excersized drawing a pcb ( single leyer) in order to fullfill the above specs for the simplest amplifier ever the P3A and never managed to cover all of them in one pcb and i am doing this from time to time for 3 years now
it has to be done in two leyers and then the P3A is not an amplifier for a thing like that ...if you invest in a 2 leyer pcb you may as well built something else , even though the P3A is not an amlifier to be underestimated ...if done properly is one of the best i listen so far
always the point is very simple : you can design a pcb that has all the goodies collected from audio threads but this can only be done in a 2 layer pcb ....then again 2leyer pcb cost alot of monye if done diy and also present their own problems regarding capacitance and inductunce between traces ....
ideal amplifier pcb should be :
--- devided in 3 areas
a) power transistor area
b) psu on board area
c) small signal area
--- now this has to be designed under specific rules
* star ground philosophy
* power traces as small is possible
* decoupling and bypass as close to the power transistor possible
* then ground trace of the above as small is possible
* ltp in a way that it can have thermal junction
* and all the above with symmetry and order just for the looks ....
well i can tell you that i have excersized drawing a pcb ( single leyer) in order to fullfill the above specs for the simplest amplifier ever the P3A and never managed to cover all of them in one pcb and i am doing this from time to time for 3 years now
it has to be done in two leyers and then the P3A is not an amplifier for a thing like that ...if you invest in a 2 leyer pcb you may as well built something else , even though the P3A is not an amlifier to be underestimated ...if done properly is one of the best i listen so far
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