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Old 9th March 2008, 12:45 PM   #1
Derfla is offline Derfla  Australia
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Default My first LM3886 project (please help urgently!!)

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!)
Click the image to open in full size.
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
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Old 9th March 2008, 01:08 PM   #2
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This is my approach for point-to-point wiring GC:

Gainclone HUM, THUMP problems - The solution

Perhaps bit different if you're using PCB as the PCB would have a grounding point already.
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Old 9th March 2008, 01:26 PM   #3
Electrons are yellow and more is better!
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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
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Old 9th March 2008, 02:37 PM   #4
AndrewT is offline AndrewT  Scotland
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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.
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Old 10th March 2008, 11:11 AM   #5
Derfla is offline Derfla  Australia
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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?
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Old 10th March 2008, 12:17 PM   #6
Electrons are yellow and more is better!
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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.
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Old 10th March 2008, 01:11 PM   #7
Derfla is offline Derfla  Australia
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ahh so would these be better instructions to follow
http://dogbreath.de/Chipamps/GainCar...nCardCopy.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
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Old 10th March 2008, 02:28 PM   #8
AndrewT is offline AndrewT  Scotland
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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.
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Old 14th March 2008, 03:06 PM   #9
Derfla is offline Derfla  Australia
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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/
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Old 14th March 2008, 08:44 PM   #10
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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.
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