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Old 3rd March 2007, 08:33 PM   #1
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Default Help my LM3886

Hi there everybody. Altough i'm a long time diy reader, this is my first thread.

I'm assembling a gainclone using a LM3886 chip based in this project:

http://www.chipamp.com/docs/lm3886-manual.pdf

BUT, today I assembled everyting using this power supply:
Click the image to open in full size.

...and I had some problems.

fist of all, I had a humm noise witha 2 amps current, the I unplugged the input pin (number 10) and the hum stopped, the current also dropped to a few milliamps.

When I plugged the speaker, it simply "popped" and no sound was heard. So I checked the speaker and it was burned.

measuring the LM output, it has 34Vdc, witch is more than sufficient to burn a speaker.

So, I plugged only the V+ , V- and GND to the LM and still I had the same output voltage.

Click the image to open in full size.

what is the problem? Maybe my power supply is with some problem?

Ok, I understand a bit of electronic, but it is my first chip amp.
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Old 3rd March 2007, 08:42 PM   #2
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A schematic of the amp circuit would be helpfull
Sounds like a problem with the chip or circuit around the chip, which probably has gone by by you most definetly had rail voltage at the outputs I would check for shorts in th NFB loop ect.......
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Old 3rd March 2007, 08:59 PM   #3
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IF your power supply is actually like it shows on the diagram. then you have positive voltage on the negative rail. you could blow not only the IC but also electrolytic capacitors as well.

the bridge rectifier is backwards on the negative side.
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Old 3rd March 2007, 09:44 PM   #4
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my bad, I uploaded the wrong schematic for power supply. this is the one that I'm using:

Click the image to open in full size.
(paintbrush edition)

the power supply output looks good, black tip on ground makes +34 and -34
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Old 3rd March 2007, 09:48 PM   #5
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Halo: the diagram of the circuit you can find at
http://www.chipamp.com/docs/lm3886-manual.pdf
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Old 3rd March 2007, 10:34 PM   #6
sek is offline sek  Germany
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Hi,

have you tried referring to Brian Bell? He's the one you bought from and there are a couple of known problems with the chipamp.com boards regarding their non-ruggedness with different power supply wirings.

The problem is not the board. The problem is Brian's bad (as in: incomplete) manual.

As you have already built everything, your best bet probably would be to take some high-res pictures, upload them somewhere and link them here. Try to do some snapshots of both sides of the amp board and a detail of your power supply wiring.

If your PSU really delivers +/-34V, the supply side looks good, though.

Hope this helps,
Sebastian.
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Old 4th March 2007, 12:10 AM   #7
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Well, I'll try to describe step by step.

This is my transformer, removed from an old receiver...
Click the image to open in full size.

...plugged into a rectifier bridge. The black (-) and red (+) goes to the..

Click the image to open in full size.

...filter board. there is a 12v bus, but this one is ok. It will be only for lighting.

Click the image to open in full size.

Is the output ok? I think so...

Click the image to open in full size.Click the image to open in full size.

Wiring the power output to the LM3886

Click the image to open in full size.

and soldering into the LM, but only the power supply directly into pins 1 (V+), 4 (V-) and 7 (0V)

Click the image to open in full size.

And now measuring the speaker output (pin 3) withr pin 7 (GND) and we got 32volts DC and 71 vac!!!

Click the image to open in full size.

General view from the mess. the voltimeter and the amperimeter are online!
Click the image to open in full size.

before you can ask, I did try this procedure with another chip and I got the same result. Monday I'll try to put this in the osciloscope.
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Old 4th March 2007, 10:00 AM   #8
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I dont know if it would have anything to do with your problem, but pins 1 AND 5 are supposed to be +ve (aka VCC+?) for the LM3886.

You should also have power supply decoupling at the chips pins, 100nF followed by an electrolytic (I used 100uF) between the rails and ground seems to be recommended.
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Old 4th March 2007, 10:16 AM   #9
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Hi,
if you just connect the power supply without any other circuit components, the ic will latch up or oscillate or whatever. Any measurement will be useless.
Regards
Jürgen
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Old 4th March 2007, 10:10 PM   #10
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I would like to thank all of you for your tips, it was really really helpful.

I just made this baby works. I had to change a few resistors and capacitors, but still there is one odd thing: if I unplug it from the sound source (i.e. cd player) you can hear a noise, something like when you plug a guitar in the amp, that bzzzz, you know? but I'm still working on that...
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