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Old 31st August 2005, 04:29 AM   #1
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Default I need a little LM3886 help

I want to build a nice audio amp for a my car and it seems that the LM3886 is a good choice (once I get a damn DC-DC converter). However, I've been having some trouble building a test circuit. The datasheet here
shows a VERY simple application circuit on the first page. However, further down it shows a "Single Supply Application Circuit." Which one should I use? I tried the first once but I didn't get any results (i.e. sound). Does it need an opamp to boost the signal or can I directly input the source audio (CD player)? Any help is much appreciated...thanks!!

-Segasonicfan
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Old 31st August 2005, 05:13 AM   #2
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The circuit on page one is the right one. You want the dual supply variant. The single supply circuit is lower performance, and you do NOT want that output capacitor in the circuit. just search LM3886 or gainclone on google from some schematics. you can also build a home power supply with the usual transformer, bridge rectifier, and capacitors to test it.
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Old 31st August 2005, 06:46 PM   #3
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Thanks for the reply. I think my problem is the mute switch. Or, more accurately, the resistance between pin 8 and pin 4. It says in the datasheet:


RM Mute resistance set up to allow 0.5 mA to be drawn from pin 8 to turn the muting function off. RM is calculated using: RM is greater than or equal to (|VEE| − 2.6V)/I8 where I8 is greater than or equal to 0.5 mA.

This confused me as first because in the PDF the I8 looks like 18. So if I calculated that correctly it should be a maximum of 44.8 ohms for a 25v power supply, right?

Also, I'm using a 25v battery to test the circuit. However, the battery only outputs 2 amps, should this be any problem? Thanks for the help,

-Segasonicfan
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Old 1st September 2005, 03:44 AM   #4
mateo88 is offline mateo88  United States
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Just use a 10k resistor for mute. It has worked on every voltage I've tried so far, +/- 14v being the lowest.
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Old 1st September 2005, 05:16 AM   #5
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thanks. can anyone recommend a schematic they've used? I saw this one but i tried it (using 10k for mute) and I didn't get any results. The output was -20v which doesn't seem right. It's possible my amp was just busted though since I've already built the wrong circuits for it. So does anyone know a schem that's personally worked for them?

-Segasonicfan
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Old 1st September 2005, 07:48 AM   #6
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hrmmm i tried another 3886 amp and still no sound. I have a -20v readout on almost every pin too, which seems odd. What voltage caps should I use? I'm using 16v 22uf and 100uf...could that be the problem? The 0.1ufs are 50v so there's def no problem there. Also, does the amp NEED more than 24v @1amp to run? I just want to build a working test circuit before I make a full power supply for it. Thanks for the help!

-Segasonicfan
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Old 4th September 2005, 11:13 AM   #7
sr2002 is offline sr2002  India
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hey,

have u tried the AN-1192 file it has pretty decent schematic for bridge, parallel or bridge-parallel mode.

for ur aplication i'd recommend the paralel mode.

this file is available on nationals site.

i have recently made a stereo amp in bridge mode. it sounds terrific
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Old 4th September 2005, 11:17 AM   #8
sr2002 is offline sr2002  India
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i think 1 amp is a bit toooooooo low. i think u'll definately need more than that atleast 3amps at 24v. 24v is ok enough.

and to be on the safer side use 50v caps. because i bet if u connect a 3 or 5 amp suplly the 16v caps will blow at 24v( because after rectification 24v will become roughly 30-35v)

so be carefull
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Old 4th September 2005, 08:56 PM   #9
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Thanks for the replies I checked the AN-1192 and that parallel app looks great. Could I wire it just like them schem on page 8? That would be perfect for powering my sub. Hopefully I don't need the full 35v it says though, the best DC-DC converter I think I'm going to get would be 24-25v.

As for the 1amp 24v I know that's way too low I just wondered if that would work for testing purposes (for weak output). Appearently I don't think it will though since I never got any output. Also, I think I blew both my LM3886s b/c when I just connect the power to it (and the mute off resistor) I only get 3v on the output. From my experience when just connectign the power supply you should get the proper voltage (or close to it) on the outputs. That's how the LM3875 and LM4752 worked. It's a nice way to test if your amp is broken without constructing a whole other test circuit.

So I guess my real problem is the damn power supply...I think it would be way more expensive to make my own so I'm gonna have to search for one on ebay.. Although they don't seem to be too common :/

-Segasonicfan
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Old 6th September 2005, 04:49 AM   #10
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sr2002--could you tell me the exact schematic you used for you amp? I'm trying to find someone that's actually built an LM3886 amp based on a schematic successfully. There are so many variants out there and so I want to try one that I KNOW is correct.

And I solved the power supply problem. I found 2 12v 5amp (60 watt) AC adapters around the house so I connected them in series and voila! 24v @ 5amps

I made (yet another) LM3886 test circuit according to the page 1 datasheet schem but still didn't get any sound. Also, I only got +3v out on the output. I'm starting to think my IC is fried. I'm gonna drive over to Jameco tomorrow and pick up another one with some proper 50v caps. Hopefully then I can make a working circuit. All I want is to get one working circuit, then I'll worry about parallel modes and such. If I can learn how to build LM3886 amps like I know how to build LM4752 amps I will be one very happy camper Thanks for all your help and please keep helping! Thanks guys,

-Segasonicfan
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