@ m-tech: I found your schematic so intriguing that i went out this morning and bought all the parts. I'm building it now. 
For now i only have a 25-0-25 400W transformer, but if all goes well i'll be doing a SMPS to feed it.
What kind of power output can be expected from this? My final goal is 250W/8ohm. But with 28v rails that my transformer can offer, maximum power on 8ohm is 100W.
For now i only have a 25-0-25 400W transformer, but if all goes well i'll be doing a SMPS to feed it.What kind of power output can be expected from this? My final goal is 250W/8ohm. But with 28v rails that my transformer can offer, maximum power on 8ohm is 100W.
Okay... I built it, blew my first pair of FETs due to a stupid wiring mistake, then did the series lightbulb trick before i installed the second. It worked for a while... sort of. The bulb would always glow medium brightness, the regs would get fairly hot, the drivers got hot, the MOSFETs were just barely warm (i did mount them onto a heatsink). It would only work well at low volume.
Anyway, now it doesn't work at all. Sticks to the negative rail every time. I get -11.8v at the output of the LM311. I swapped the 311, swapped the drivers, still the same. Then i noticed that the top mosfet got disconnected from its positive rail. I wired it back and stupor - still the same. I also cannot make sense of the voltage readings on it - gate voltage reads the same as the positive rail (i triple checked, it ISN'T shorted), source reads positive rail (since it's connected to it), drain reads negative rail! I think it might have failed open.
I'm using +/-12v regs as they had run out of 9v ones.
Anyway, now it doesn't work at all. Sticks to the negative rail every time. I get -11.8v at the output of the LM311. I swapped the 311, swapped the drivers, still the same. Then i noticed that the top mosfet got disconnected from its positive rail. I wired it back and stupor - still the same. I also cannot make sense of the voltage readings on it - gate voltage reads the same as the positive rail (i triple checked, it ISN'T shorted), source reads positive rail (since it's connected to it), drain reads negative rail! I think it might have failed open.
I'm using +/-12v regs as they had run out of 9v ones.
The feedback line is DC coupled so it was obvious that a fault producing DC at the output would cause the whole thing to stick to one of the rails. I disconnected the feedback and now the LM311 and drivers work as normal. The problem is in the output stage, but i can't figure out what it is for the life of me.
Well, it turns out that my problem is shoddy parts. I have a 7812 that will only start when its output is shorted to ground momentarily, and a LM311 that will only start oscillating when it feels like it. When it does start, the amp works fine. I feel that it clips a little too early, but probably i just need higher voltage rails.
Did it before. And indeed i noticed some funky things. Anyway, the hardware version is in need of new p-channel FETs - the two i had left are now blown. Reason unknown, but both have failed after the same action - attempting to see the gate waveform. One is open, the other acts like it's shorted when put in-circuit even though it doesn't test shorted. They kinda look fake so i guess it's just low quality parts. I'm using IRF540/9540 instead of 530/9530 btw. And the 8v2 zeners are 12v in my amp, which reduces efficiency a bit judging by the simulation, but shouldn't be anything critical.
I also simmed what happens when the top FET goes open. In simulation, the output sticks to the negative rail for a bit then only half of the waveform is reproduced (but the amp still works, just hella distorted). On the real thing, the output sticks to the negative rail... and stays there. It comes up briefly when the transformer is unplugged till the caps discharge.
Anyway, while the amp was working, the positive side of the waveform clipped very very early compared to the negative. There's definitely something fishy going on with the top FET.
Edit: I would like to understand the circuit more. How does this scheme avoid cross conduction, since the output transistors are basically driven off the same waveform? I know that the zeners and the resistors do it somehow, but i don't understand how.
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I think i understood it now. It's the 1N4148s with the resistors in parallel doing all the magic. Now i see - with the arrangement shown the top diode only conducts while the PWM signal is positive, and the bottom when it's negative, so the gates of the power MOSFETs are driven accordingly.
Figured everything out. For one, the hissing was coming from my laptop's soundcard. I knew it was noisy but on a class AB amp it outputs a different kind of noise. Perhaps all the switching noise from the laptop power supply influences the functioning of this amp. Any other signal source sounds CLEAN.
Second, my low output was mostly due to the output coil. Right now, my ugly stripboard has reached 80W/4ohm, and except the 7812/7912 all parts are just a little warm. Power supply is +/-30v.
I see on the oscilloscope that there's a bit of shoot-through. I won't handle that until i get it done on a PCB - it could very well go away by itself. But first, i'll be making a +/-50v SMPS to power the final version.
Second, my low output was mostly due to the output coil. Right now, my ugly stripboard has reached 80W/4ohm, and except the 7812/7912 all parts are just a little warm.
Power supply is +/-30v.I see on the oscilloscope that there's a bit of shoot-through. I won't handle that until i get it done on a PCB - it could very well go away by itself. But first, i'll be making a +/-50v SMPS to power the final version.
Thanks. Unfortunately they don't have 7910 in stock here, only 7810. I think i still have LM317/337 somewhere, i'll rig them up and see what happens. In simulation i get a slight increase in final transistors dissipation with 9v supplies so it might not be the best idea after all. Edit: I see about 400mV of switching frequency ripple at the driver transistors' collectors, i'll put some bypass caps next to them and see what happens.
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Hi all i need help, i`m following m-tech schematic using tl071 op-am and LM311P comparator, when power on there is no sound at all, only hear thumb at speaker, check speaker dc output almost 0v, i`ll post my picture here if required and now i`m blur after checking many times +-9 supply and +- 35v supply all is connected.
regards,
azmi
regards,
azmi
hi Aucosticraft, yes im using zener as regulator, LM311 pin 8 tl071 pin 7 and Q1 have +8.5 vcc, LM311 pin 4, tl071 pin 4 and Q2 have -8.5 vee, im using T106-2 for the lc filter and make it 28UH, still checking my board, actually i`m trying helping my younger brother for the electronic project, hes study electronic course.
I hope can help him with class D amp.
Regards
Azmi
I hope can help him with class D amp.
Regards
Azmi
I use +/-30v and it's working fine here.
Check the following voltages: Pin 6 of TL071, pin 3 of LM311, pin 7 of LM311, emitter junction of Q1 and Q2.
If you get DC close to one of the rails on pin 6 of TL071 there's something wrong with TL071. If you get DC on pin 3 of LM311 then LM311 is bad. If you don't get DC on the above two but get DC on pin 7 of LM311 and emitter junction then the parts are good but the amp isn't oscillating. Try touching the comparator pins with your finger, see if you get any reaction from the speaker.
Check the following voltages: Pin 6 of TL071, pin 3 of LM311, pin 7 of LM311, emitter junction of Q1 and Q2.
If you get DC close to one of the rails on pin 6 of TL071 there's something wrong with TL071. If you get DC on pin 3 of LM311 then LM311 is bad. If you don't get DC on the above two but get DC on pin 7 of LM311 and emitter junction then the parts are good but the amp isn't oscillating. Try touching the comparator pins with your finger, see if you get any reaction from the speaker.
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