High Power P3A design questions.

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#61
Jean said:
ttt.

I got my MJL4203 and 4281a devices, still waiting on mje15035/15034s to arrive .
Click to expand...

:cannotbe: :cannotbe: :cannotbe:
🙁 🙁 🙁

Just like me!!!

I think they are out of stock for the MJE1503X for more than 3 months... (yes, I but directly from OnSemi)

I've checked OnSemi Web Site (myOn) and...

The order is there... But not shipped yet :bawling: :bawling:
 
#62
drivers thermal runway

Plz Pay attention to Drivers of output transistors
They must be thermally coupled very well unless the thermal runaway might occur due to multiplication of positive thermal conduction coefficient feedback due to SIZIKLAI PAIR CONFIGURATION. As a care there temperature must not exceed safe limits of about 64C'
 
#63
I just replaced bd139/bd140 and mjl193/194s with mje15034/35 and mjl4302/4281 in 2 channels of my 4ch p3a amplifier. My bias current went up from 95 to 240 after changing transistors. Should I run higher bias with different transistors or turn it down ?

This weekend I plan on comparing my p3a channels to see if I can hear any difference between different transistors . Also, I am adding heatsinks to the Q5 and Q6 pair just in case.
 
#64
Jean said:
I just replaced bd139/bd140 and mjl193/194s with mje15034/35 and mjl4302/4281 in 2 channels of my 4ch p3a amplifier. My bias current went up from 95 to 240 after changing transistors. Should I run higher bias with different transistors or turn it down ?

This weekend I plan on comparing my p3a channels to see if I can hear any difference between different transistors . Also, I am adding heatsinks to the Q5 and Q6 pair just in case.
Click to expand...

Do as you like, I run my P3A at 200mA bias current. I think it sounds better at 200mA than at 100mA.

/Freddie
 
#65
As long as you have the output devices on large enough heatsinks you can quite safely increase the bias curent as high as you want (up to the safe limits of the design), all that will ahppen is that your transistor life may be shortened a bit (I doubt that this should be much of a real concern) from the higher opperating temp.

But, as far as this project is going, I'm looking to get some prototype boards made up in the next week or two and hopefuly building up a couple of test channels before sending off for some 'proper' PCB's and the rest of the stuff I'll need to build them properly.
 
#66
Comparing transistors

I just finished a second P3A built with the the new OnSemi parts.
My initial impression is that I dont like it, the higher frequencies are "harsh or edgy" and the vocals sound "nasal" if that makes any sense. My wifes ears also hear it as different " the singer has a cold".
I only played it about an hour so Im hoping for some "run in" either of the parts or my head or better yet an assembly problem.
My recolection of the 1st P3A was that it sounded great imediately and much better than the LM3886 that I assembled on an ESP board. Unfortunately this P3A sounds like the LM3886, and not at all layed back on the upper end or vocals.

The amp was built with some higher quality parts polystyrene caps at strategic positions as described in the assembly link at ESP. Power supply is identical for both amps. Ive got parts for another 8 channels so Im really hoping there is a distinct problem I can solve.

Any ideas would be greatly appreciated
 
#67
Re: Comparing transistors

Fritz said:
I just finished a second P3A built with the the new OnSemi parts.
My initial impression is that I dont like it, the higher frequencies are "harsh or edgy" and the vocals sound "nasal" if that makes any sense. My wifes ears also hear it as different " the singer has a cold".
I only played it about an hour so Im hoping for some "run in" either of the parts or my head or better yet an assembly problem.
My recolection of the 1st P3A was that it sounded great imediately and much better than the LM3886 that I assembled on an ESP board. Unfortunately this P3A sounds like the LM3886, and not at all layed back on the upper end or vocals.

The amp was built with some higher quality parts polystyrene caps at strategic positions as described in the assembly link at ESP. Power supply is identical for both amps. Ive got parts for another 8 channels so Im really hoping there is a distinct problem I can solve.

Any ideas would be greatly appreciated
Click to expand...
Try higher bias. It sounds like you are hearing some switching distortion.
 
#69
Fritz said:
OK Ill try more bias, it now occurs to me that I assumed the bias was the same as for the old transistors, about 100mA.

Any suggestions or should I just increment up and see if the sound improves?

Thanks

Fritz
Click to expand...
Use an o'scope and use a sine wave at a low power level and look for 'bumps' in the curve that indicate transistor turn on/off. Adjust to get rid of these. Be careful to not be fooled by the problem being before the outputs and measure the signal at several points along the path from input to output to see if it is clean.
 
#73
Yes I do, but I wouldn't recomend using the free version of eagle to try and produce a board with eight output devices, as the one I have posted here is as physicaly large a board that the free version will allow. So I'm not too sure you would be able to get all of the output devices on there easily without using two or more boards (although, perhaps if you ran them down both top and bottom sides of the board...).

Anyway, if you want the schematic, I have atached it below so you should be able to work ok from there (Just be careful with the power supply rails I have a feeling I may have acidently joined them at one point when I was doing my PCB layout.

Andrew.
 

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#74
Jean said:
I just replaced bd139/bd140 and mjl193/194s with mje15034/35 and mjl4302/4281 in 2 channels of my 4ch p3a amplifier. My bias current went up from 95 to 240 after changing transistors.
Click to expand...

I also experienced increasing bias current when mje15030/31 transistors were used. maybe they have lower Vbe?
 
#76
I abandoned it when I found out that adding more output transistors into a CFP creates all kinds of issues. One of them was ringing on square waves at high frequencies. As Rod once stated, the P3A is a simple amplifier for about 70W of power, going higher requires a more advanced design.

I'm currently working on a scaled down version of the Leach amp with 2 pairs of output transistors to run at about 100W into 8 ohms from 45V rails. For those wanting more power, use the Leach as is. It's an excellent amplifier.
 
#77
need sugestions for my pcb project

hello.

some days ago i found this upgrade for ESP3a and i want to build one. I made pcb project and i need opinions before i start with pcb-making.

note about heatsinks:

- all power transistors on big heatsink;
- drivers(T5, T6) on small 2 hestsinks.
- T7 on big heatsink

any comments are welcome.

regards

An externally hosted image should be here but it was not working when we last tested it.
 
#79
For best thermal stability the drivers should be mounted on a separate heatsink. The design of the output stage of the P3a is such that the driver temperature is what should be sensed by the Vbe multiplier, not the output stage. Forcing the driver transistors to be as hot as the output stage can result in less than optimum quiescent current.
 
#80
that's sounds not so good for me. previous version was with 2 heatsinks - separately for power transistors and drivers. so back to the roots 😀

ok. tommorow(now is midnight in my city 😀 ) i'll try to make new version. with separate heatsinks.

i have no idea about value of R25 - emmiter resistor of T7. any suggestions? 😕

here is schematic: http://www.diyaudio.com/forums/attachment.php?s=&postid=283772&stamp=1071610659

BTW: thanks for reply.
 
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