# I need a little LM3886 help

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#### segasonicfan

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

#### Dr. Photon

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.

#### segasonicfan

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

#### mateo88

Just use a 10k resistor for mute. It has worked on every voltage I've tried so far, +/- 14v being the lowest.

#### segasonicfan

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

#### segasonicfan

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

#### sr2002

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

#### sr2002

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

#### segasonicfan

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

#### segasonicfan

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

#### segasonicfan

could someone please help me out?? I'm getting to the point where I'll just pay someone to help me get an LM3886 working...lol. I'd buy one of the chipamp kits but they're \$60 a pop which is a little rediculous I think (considering they sell LM3886's for a measily \$4.50 ish). I've already spent a good \$50 on parts though and I have 3 LM3886 amps lying around but I can't get any of them to work I obviously don't know what the problem is but these are my questions:

-am I using the right caps? I'm using standard polarized electrolyics. I know they aren't the best for audio application but like I said I just want to get the damn thing working. Do I NEED to use ceramics though? Also, is 35v enough tolerance or should I get 50v ones?

-I connected to AC adapters in series to make the power supply for this (2 12v @ 5amps=24v 5amps). Anything wrong with this?

-I'm using all 1/2watt regular resistors, should I use ones that handle more?

-does the LM3886 require any sort of preamp? I'm just trying to amplify a simple CD player.

It seems
this schematic is probably the best to go with. Finally, can someone PLEASE give me the output voltage of a working amp in refference to -vin? Or a list of the voltages on the functioning pins would be great too.

#### Craig405

Hi,

I dont know anything about gainclones but here are some ideas.

Does this chip require + and - 24V, if so your wall adapters will probably only provide 24V and ground, this might account for the 20V offset at o/p?

Check your circuit by starting at the beginning with your multimeter and checking every connection and resistor value and rebuild the circuit diagram from scratch. Check it matches what it should be

Regards
Craig

#### wes-ninja250

Group buys *ARE* a good deal!

Code:
``````[FONT=courier new][b]Index  	Quantity  	Part Number  	Description  	Customer Reference  	Backorder Quantity  	Unit Price
USD 	Extended Price
USD[/b]
1 	2 	LM3886TF-ND 	IC AMP AUDIO PWR HP 68W TO220-11 	  	0 	5.58000 	\$11.16
2 	8 	MUR860OS-ND 	DIODE ULTRA FAST 8A 600V TO220AB 	  	0 	1.14000 	\$9.12
3 	5 	PPC22KW-1CT-ND 	RES 22K OHM METAL FILM 1W 5% 	  	0 	0.34800 	\$1.74
4 	5 	PPC10.0KXCT-ND 	RES 10.0K OHM METAL FILM .50W 1% 	  	0 	0.19400 	\$0.97
5 	5 	PPC1.0W-1CT-ND 	RES 1.0 OHM METAL FILM 1W 5% 	  	0 	0.34800 	\$1.74
6 	5 	PPC680LCT-ND 	RES 680 OHM .50W METAL FILM 10% 	  	0 	0.25600 	\$1.28
7 	5 	PPC2.7W-2CT-ND 	RES 2.7 OHM METAL FILM 2W 5% 	  	0 	0.60400 	\$3.02
8 	6 	P10323-ND 	CAP 100UF 50V ELECT FC RADIAL 	  	0 	0.44000 	\$2.64
9 	2 	P10321-ND 	CAP 47UF 50V ELECT FC RADIAL 	  	0 	0.28000 	\$0.56
10 	10 	BC2054-ND 	CAP FILM MKP .1UF 63VDC 2% 	  	0 	0.42500 	\$4.25
11 	2 	P6473-ND 	10000UF 50V ALUM ELECT CAP 	  	0 	6.61000 	\$13.22
12 	5 	PPC2.2W-2CT-ND 	RES 2.2 OHM METAL FILM 2W 5% 	  	0 	0.60400 	\$3.02
13 	5 	PPC1.0W-2CT-ND 	RES 1.0 OHM METAL FILM 2W 5% 	  	0 	0.60400 	\$3.02
14 	1 	1N4004DICT-ND 	RECTIFIER GPP 400V 1A DO-41 	  	0 	0.26000 	\$0.26
15 	1 	P11250-ND 	CAP 10UF 50V ELECT FC RADIAL 	  	0 	0.28000 	\$0.28
[b]	Subtotal	\$56.28[/b]
[/FONT]``````

Can you really make a PCB for less than \$3.72?

I suppose you might be able to save a little bit by not shopping at Digikey -- but not much, especially if you value your time.

Brian's done the work to basically supply good-quality, well-matched components for about the same price as Digikey, making the PCBs effectively free.

When I was deciding how to build my gainclone, this struck me as a sure-win formula -- pretty much guaranteed good results, and a SIGNIFICANT reducton in manual labour. I *can* make a PCB for around \$4, but it will be single sided, through holes will not be plated, and it will take me an afternoon to clean/resist/etch/drill the board.

Never mind the fact that the traces will not be as high-quality as those from a commercial board maker, or that the layout is already debugged...

Wes

#### segasonicfan

Well I live 2 miles away from Jameco, so I get all those parts cheaper than Digikey and free of shipping charges I also already have all of the passives and I'm not interested in having a PCB...I hardwire everything. It makes for a much smaller amp (that's what I did with my LM4752 GC).

Is not having -24v my problem then??

#### wes-ninja250

> I'd buy one of the chipamp kits but they're \$60 a pop which is a little rediculous I think (considering they sell
> LM3886's for a measily \$4.50 ish)

> I also already have all of the passives and I'm not interested in having a PCB...

Then why are you making snide comments about the cost-effectiveness of the LM3886 kit, when clearly you don't want/need one?

If you're interested in the circuit, just duplicate it out of the schematic in the PDF on chipamp.com. It's available for all DIYers, not just those who want to build their amps on Brian's PCBs.

Wes

#### segasonicfan

whoooa...Wes you need to chill out. I wasn't making 'snide remarks' about it. I have nothing against the guy, there's no need for you to get all defensive. I'm just saying I can get the materials cheaper. However, if I continue to have no success with building my own LM3886 amp I'll probably buy his kit.

So do I need a seperate power supply to output -24v? I'd like to make my own but I don't know anything about transformers or where to find one for this project :/

-Segasonicfan

#### wes-ninja250

> So do I need a seperate power supply to output -24v?

That depends, are you using a normal circuit, or a single-supply circuit? "Normal" circuits depend on having voltage rails equally distant from ground (0V), which is where the amp bias is.

AFAIK, all the circuits on here are dual-supply "normal". The only time when single-supply should even be considered is for applications where AC power isn't readily available.

You need to evaluate your circumstances and design to determine if you're using/need a single-supply configuration. If you do, you're looking at needing double the "usual" voltage.

Wes

#### wes-ninja250

PS: Look at page 6 of the LM3886 datasheet.

#### segasonicfan

ahh! Now it all makes sense! I DO want to do the Single Supply operation. Someone told me earlier that the sound quality wasn't as good or something and to stick with dual supply. But I'm going to be powering this in my car so I def want the single supply operation

I'll pick up the couple extra parts I need for it and give it a try

-Segasonicfan

#### wes-ninja250

You're running this in your car? How are you going to get a decent rail-to-rail voltage?

Most of the amps on here run on at least 18VAC transformers.. yieldng ~25VDC rails. My amp's rails will be at ~ +/- 34VDC. You would need 50VDC (if I'm correct - big if!) to achieve the same power as 25V rails from your amp.

Wes

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