Expert help needed to check PA150 design (LM3886)

[domtw]

Hello,
I found a kit using 3 LM3886 in parallel but I have no idea on the reliability of the design.
That would be really nice if some expert could have a look and comment its schematic:


Thank you!

[tangmonster]

Exactly the one I bought on ebay

except I only bought the bare pcb.

my non expert input:


1)
c4 ,c6 , c11 is too high , you can lower the value to 10 or 8pf as in this schematic:

DIY BPA300 6x LM3886 300W audio Amplifier

2)
do NOT mess around with R1 , R4 , R10 , R3, R6 ,R12. 1% is not good enough ,rather populate 0.1% wither by buying 0.1% resistors or hand matching with a good multimeter.
I did blow up a lm3886 chip and nearly a speaker after more than a months use of using 1% resistors.
a prebuilt one bought on ebay will most probably have 1% resistors.

Another thing:
this design is overkill for 4ohm loads you won't get much more than 100W at +-35V
heat will be better dissipated though. and I suppose if you have tricky speakers with slightly lower impedance this circuit will be great.

[tangmonster]

Another thing.

I wasn't happy with the placement of the 100nF decoupling caps so i soldered 6 extras to the bottom pins of the lm3886 chips.

Another thing

If you try and test and leave off c4 ,c6 , c11 my board immediately started oscillating and R7 goes up in flames!

[domtw]

Thanks for your answer.
I might buy the naked board and choose the components myself.
I think 100W @ 4 Ohms should be OK but I have no idea how you can see that in the design...

There are a few thing I don't really understand in the schematic I posted.
For example:
- the mute pin (8) is not connected ? or what does the "B" represents ?
- when you talk about the decoupling cap: are they C2 and C13 ?

I also don't get what you mean by "If you try and test and leave off c4 ,c6 , c11".
You mean if I forget them ?

Excuse my ignorance

[AndrewT]

Hi,
build a single 3886 first and learn how it works, why it works, what you can do to help it work and what you must not do to prevent you damaging your equipment.

When you have learned all these lessons, re-evaluate that parallel implementation. I think you will be surprised at how bad that parallel schematic is.

[domtw]

Hi Andrew,
You are right, that would be the best, to understand how it works but I don't really have that much time...
I would like to buy something that works fine already as I want to use my too little free time concentrating on speaker building...

[AndrewT]

go ahead and destroy your new speakers.

[domtw]

Thanks Andrew!
I wont get this kit if the design seems that bad to you.
Any suggestion for something reliable and cheap?

[balaboo]

That was not at all a helpful reply.

domtw:
I would recommend that you check the National application note and follow the suggestions to use 0.1% resistors.

Also, the C4, C6, C11 capacitors are part of the Zobel network that helps provide a high-frequency damping load on the IC output - otherwise it will oscillate and blow up.

Yes, the decoupling capacitors are C2, C13 - make sure they are as close to the IC pins as possible.

Go ahead and get the nekkid board !

[domtw]

I am getting confused...

Balaboo:
Thanks for your answer.
Have you tried this board yourself?