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Old 2nd March 2010, 12:53 PM   #1
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Default AC problem with LM3876

I recently built a stereo amp using this chip.
The problem is when I power it up i hear the 60hz in the speaker. There was a large pop through the speakers at power on im guessing this is from the caps.
At first there was no sound so i turned it off, That was a mistake i heard the awfullest whirring noise which probably ruined my tweeters. experienced this whole routine a second time then i decided to unplug everything and out of frustration i tried one more time. amazing i could barely hear the music as it played it got louder and louder but it was still heavy with ac distortion.

Now heres the interesting bit.
When i turn the power off the distortion goes awayand it plays the music perfectly for about 10 seconds (prolly the caps discharging)

Also upon initial testing there was -4.53 volts DC ON THE OUTPUT! just on the right channel so no testing was done to that channel.

Ive uploaded the power supply schematic and one channel schematic as they appear on headwize. Ive also uploaded a picture of one solderboard with its 10000uf cap for size comparison. I followed the general rule for gainclone keep everything as close as possible aside from the tranny
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Old 2nd March 2010, 01:06 PM   #2
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I just read on the site it says to be sure to take the dc from the caps and not the rectifier i had used three wired and tied them into a T. Im going to change this tonight and see if it has any effect on the working channel. still no clue about the dc output
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Old 2nd March 2010, 01:18 PM   #3
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you need a resistor from pin 10 to ground.
And btw. your PE connection network is dangerous, if you're not using a transformer prepared for safe insulated equipment.
regards

Last edited by Juergen Knoop; 2nd March 2010 at 01:20 PM.
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Old 2nd March 2010, 01:19 PM   #4
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What was source of this circuit diagram? No capatitors for HF compenzation and no resistor for ref GND on input pin.
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Old 2nd March 2010, 01:25 PM   #5
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The source was headwize as i stated before. here is a link to the project. From what i understand c4 and c5 in the power supply is for high frequency filtering. What size of resistor is needed on the input and whats its purpose?
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Old 2nd March 2010, 01:31 PM   #6
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Thake look on my thead LM3886 schematic + PCB
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Old 2nd March 2010, 03:08 PM   #7
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As a test, try using a capacitor on the input. Also measure your rails with the amp powered up, but connect it to an 8 ohm resistor instead of your speakers.
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Old 2nd March 2010, 04:41 PM   #8
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22k would be fine, but otherwise you can use what you have. Me would chose something between 10k-100k.
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Old 2nd March 2010, 05:45 PM   #9
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You need a 22K after the 1K input resistor to signal ground. Currently you have nothing setting input impedance.

A coupling cap is required unless you are 100% sure that your source has no DC offset - most of them do.
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Old 2nd March 2010, 09:43 PM   #10
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i have 5.1k 5k 220k 4.7k 3.2k 330k. I dont guess any of these will suffice.
my last post didnt show up so im going to retype it.

the original design had a coupling cap on the preamp which i had overlooked. I have some spare ones that are 150uf way over the 3.2uf specified.

will this fix the static because it really does sound like 60hz ac hum whether the source is connected or not. and theres still the dc issue on the right channel.

I figured out why the third power on was the charm. if the source isnt connected when the amp is turned on and you plug the source in nothing comes out but if the source is connected while the power is off and then turn it on the sound comes out along with the very very loud buzz thats present if its plugged in after power on.
also as i turn the volume on the pc up the static gets worse and a large popping starts occuring every second or so.
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