My first LM3886 project (please help urgently!!)

[Derfla]

Hi, everyone, I'm new here and I have been reading lots about gainclones and decided to build some of these fantastic amps
I have all the parts ready but I just need some clarfication on some things
my power supply for each channel will be a 100VA square transformer with bridge rectifier and two 4700uf caps across the DC output (voltage is about 28.5v)
all resistors used are 0.5W and the electrolytics are 50VDC, 100nf's are 100V
the heatsink for the LM3886TF chips was taken from an old amp
now here's my circuit (dogbreath.de has been of much help thank you!)

An externally hosted image should be here but it was not working when we last tested it.

I am confused about where all the grounds go, do they all get connected together and to the mains earth? and also where in the circuit does the input signal's 'negative' go? what about the speaker output negative? it would of much help if it was indicated on my schematic (modified from dogbreath.de)
the schematic is correct isn't it? and am I missing anything?

Thanks! =]
Derfla

 

[gainphile]

This is my approach for point-to-point wiring GC:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=118735

Perhaps bit different if you're using PCB as the PCB would have a grounding point already.

 

[peranders]

Before you do anything I recommend you to read these two documents.

http://www.national.com/an/AN/AN-1192.pdf
http://www.national.com/ds.cgi/LM/LM3886.pdf

 

[AndrewT]

Hi Derfla,
that's a DC coupled amplifier with no RF filter and no output Thiele network and with wrongly placed decoupling caps and it is liable to burn out your speakers.

 

[Derfla]

could you please explain what you mean andrewT?
and for the mute line i've seen people include a switch there so does is the amp on when the switch is closed?

 

[peranders]

Have you compared the application note and the datasheet how to do a proper connection? Study the typical application and compare it with what you have come up with.

 

[Derfla]

ahh so would these be better instructions to follow
http://dogbreath.de/Chipamps/GainCardCopy/GainCardCopy.html
and can I still use the 100nf caps across the electrolytics?
I also have a dual gang 10k potentiometer so can I use this for volume control?

thanks for the help

 

[AndrewT]

Hi,
do not put the decoupling caps(100nF) across the electrolytics. Put them direct onto the pins of the chipamp.
I now see an output Zobel, but still no RF filter.
The NFB is DC coupled and the input is AC coupled. Make them both AC coupled.

 

[Derfla]

what do you mean by an RF filter?
and what does NFB mean? and how do I make it AC coupled??
or would carlosFM's schematic be my best bet?
http://www.vikash.info/audio/p2plm3886/

 

[Andy Bartha]

I would recommend CarlosFM's schematic and his power supply. Do you need all that gain? Remember you will be amplifying everything that the input sees. On your schematic you have a bit more than 30dBs of gain.

 

[Derfla]

okay! I shall use those two then
the snubbers are the 0.1R resistors and the 3.3nF caps right?
i'm having trouble finding those small value caps:
C1 2.2uF, Cs 300pF, C6 3.6uF, and C7,8 3.3nF
are they cube polyester ones, little round orange ones or electrolytics

An externally hosted image should be here but it was not working when we last tested it.

 

[gootee]

You can use the polyester 'box' (or any shape) caps for any of them for which the values are available in polyester, which would probably be all except the 300 pF. Polypropylene would be better, but would be larger in size. Wima makes some nice polypropylene box caps. For the smaller values (e.g. 300 pF), if you use ceramic caps, try to use C0G or NPO dielectric type. Or you could use polystyrene (best, probably) or silver mica caps.

 

[norazmi]

Anyone know where i can get PCB Design for LM3886? I believe it will be simple circuit, since i dont know how to use PCB layout software, any help would be appreciate

Regards.

 

[Puffin]

Norazmi. The cheapest place (£ to $) I know is DIYClub.biz

http://eshop.diyclub.biz/product_info.php?products_id=441