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Old 2nd November 2006, 02:46 AM   #1
Minion is offline Minion  Canada
diyAudio Member
Join Date: May 2006
Location: Cowican Bay , vancouver island
Default Problem with LM3886 guitar amp???

Hi Folks, I have been building a Guitar amp for the last week or so and Finally got it finnished today but I can not seem to get it to work at all.....

It uses this as the Preamp:

Click the image to open in full size.

And the Standard Implementation of the LM3886 from the data sheet for the Power amp stage.....

I have gone over both schematics many times and compared it to my PCB and traced the steps and all seems ok accept that it doesn"t work.....

I tested the Preamp stage seperate from the LM3886 stage and got nothing , so I then tested the LM3886 Stage (before the 10k Volume pot) and got nothing accept for a faint Heart beat like sound... I then tested the LM3886 after the Volume pot and I did get a very quiet and intermitant distorted signal from my guitar so I though that maybe the Volume pot was bad because I got a signal after the pot but no signal before so I switched the Pot but still didn"t get a signal...I have also tried different Speakers with no success......

I also noticed that the heatsink got very hot very quick and I am useing a HUGE heatsink (about 6in wide and 3 in high and 3 in thick) and thermal compound but the chips I am useing are isolated (all Plastic).......

I tested the PSU also and it seems fine as I am getting all the Correct voltages from it(+/- 15v for preamp and +/-24v for power amp) and it is fairly well filtered with 8000uf per rail useing 16 1000uF 50v Nichicon VR caps......

So I am thinking the problem is probably somewere in the preamp stage and possibly in the LM3886 stage but I can not see how I have have meticulously gone through the circuit many times and I can"t find the Problem....

Maybe someone here with more experience can look at my PCB design and compare it to the schematic and see any problems that would cause it to completely fail??

Here is the pcb design I did:

Click the image to open in full size.

I know the grounding scheme isn"t the best but it shouldn"t cause total failure....The only other place I think that I might have made a Mistake is in wireing the Pots but I don"t think that would nescessarilly cause total failure....Also checked the PCB for burn out components and such and didn"t find anything, I did blow a few 0.5amp fuses but that is because they are just too small as switching to a 1amp solved that problem , well for a little while

If I can"t find the problem , is there anyone reading this that knows of a simple but reliable and good sounding Guitar Preamp design??? (I think next time I will put preamp and poweramps on seperate boards)

Thanx a Lot guys....

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Old 2nd November 2006, 04:18 AM   #2
Minion is offline Minion  Canada
diyAudio Member
Join Date: May 2006
Location: Cowican Bay , vancouver island
Don"t worry about it guys I figured out the problem and Got it working...I have to change some scratchy pots and maybe try some different speakers but at least it works now.....

The problem was my own fault and my bad implementation of supressing RF Interferance.....
I thought I would put a ferrite bead to ground off of the output to supress interferance but what happened was the signal was just leaking to the ground plane and not powering the speaker....

So I simply cut the ferite bead out of the circuit and it all of a sudden worked...Yippee!!!

The sound isn"t as good as I want and some of the Pots are scratchy (reused ones) but after replaceing the Pots and trying out some different speakers I should find a good sound....

here is a Pic of my new amp and my first amp build....

Click the image to open in full size.

Not bad looking ehh??

I"m going to put some decals on the front showing the functions and have the 1 to 10 dial for the knobs.....

The amp also uses a high quality adjustable JBL crossover that I scrounged up.....

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Old 2nd November 2006, 04:21 AM   #3
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Join Date: Oct 2004
Location: Raleigh/Atlanta
Very nice, but make the dial go to 11

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Old 2nd November 2006, 05:35 AM   #4
jerishi is offline jerishi  United States
diyAudio Member
Join Date: Jan 2005
Location: wa. state
Default guitar? amp

I agree- great work!... but I'm wondering if you need the 2 speakers. Are you playing electric guitar, or is this an amp for an acoustic guitar (w/pickup) that you simply wish to make louder. I think there's a difference in the way it'll work. Anyway, I'm impressed with the P.C.B. layout .
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Old 2nd November 2006, 04:39 PM   #5
Minion is offline Minion  Canada
diyAudio Member
Join Date: May 2006
Location: Cowican Bay , vancouver island
Hi, Thanx...I have the 2 speakers hooked up to the adjustable crossover so I get a wider frequency responce ,all the low Frequencies go through the 12in woofer and the mid and High frequencies go through the 6in midrange speaker and there is a Dial on the front that adjusts the crossover frequencies.....

I had never seen a Guitar amp with a Crossover before so I thought I would try it.....

And anyone is welcome to use the PCB design if they want just remove the ferrite bead comeing off of the speaker out to ground as that was the reason why I couldn"t get it to work but becides that it is a Nice sounding amp and will sound better after I get some better pots and speakers....


PS: I will be useing it for Bass and electric guitar, I haven"t tried it on my Bass yet but will do so later today...
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