Expert help needed to check PA150 design (LM3886)

domtw

Member
2008-03-24 3:10 am
Taiwan
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:
[IMGDEAD]http://img534.imageshack.us/img534/7500/pa150xd218.jpg[/IMGDEAD]

Thank you!
 
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.
 

domtw

Member
2008-03-24 3:10 am
Taiwan
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...:confused:

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 :)
 
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 !
 
My personal opinion:

The design is fine.
if your speaker is 4ohms ,(for 8ohm load its a waste)

Get 2 bare boards.
Get 6 x lm3886tf.
use 0.1% resistors for R1 , R4 , R10 , R3, R6 ,R12
use 10pF for c4 ,c6 , c11
use +-35V (25 0 25Vac 300VA toroidal for stereo channel)
use between 20000uF to 40000uF per voltage rail (40000 to 80000 in total)

If your speakers are going to be 8 ohms and have good efficiency I would suggest you try just using a single lm3886 per channel first. No gain in using that many chips in parallel for 8ohms you can't get more than 50W even if you have 4 in parallel.
 
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...:confused:

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 :)

(the mute pin (8) is not connected ? or what does the "B" represents ?) they are all connected to each other.

(- when you talk about the decoupling cap: are they C2 and C13 ?) solder extra ones onto the bottom onto the pins of the lm3886 chips.

(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 ?) wither by forgetting or just trying your luck...
 

domtw

Member
2008-03-24 3:10 am
Taiwan
Thanks for those constructive answers.
AndrewT: why did you say I could destroy my speaker ?
Because of not understanding the concept and physic behind what I build ?
Again, I agree that it would be much better but this is not my priority of the moment.
I want to make an amplifier with 6 channels controlled with minidsp and that could drive some 3 way speakers. That will be my initiation to active crossover. I will use it to experiment with speakers that I build and maybe also for my listening room if I like it.
So basically, I just need a reliable design.
I think I can follow a schematic correctly and soldering isn't a problem. And of course, I will test each amp with speakers that I don't care for.

Tangmonster: you are probably right that it might be a waste for 8 Ohms speaker but since I want this project to be quite flexible for my experimentation, an amp that can support relatively low load is a must.

I am still open for advise and suggestions:)
 
Hi,
we are agreed that the parallel implementation as shown might not be reliable.

A chipamp that goes "phut" is quite likely to send rail voltage down the speaker line.
That results in a damaged speaker.

If you go ahead with a parallel implementation without the knowledge to recognise faults and signs of misbehaving and protection methods then you are risking destroying your speakers.
 

domtw

Member
2008-03-24 3:10 am
Taiwan
That is exactly why I am here, to know if this design was reliable. If it needs modification I could use the naked board and implement the components suggested.
But, I haven't made my choice, I might use something different...
You are allowed to make suggestion for something more straight forward as you seem thinking I wont be able to make this work.
So far, your answer have been more disheartening than helpful...
 
It would probably be best to start with a single IC per channel. Use a transformer with 2 x 18 V or lower to provide low load stability. That is likely to be enough for mid-ranges and tweeters all the time. It will also suffice to test the woofers and tune the crossover. Once you have the entire speaker configuration and crossovers worked out, check the power requirements of the woofer and see, whether you need more than you have and how much more you need. Then decide which amplifier configuration suits your needs.