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Old 10th December 2012, 12:50 PM   #1
NWE is offline NWE
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Join Date: Dec 2012
Default LM3886 white noise and bumps

Hallo everybody.
I am at my second adventure with chip amps. First one was a mono TDA7294 based machine that died for a mistake of mine. Now I took the same power section (+/- 40V and +/- 20V DC) and tried to assemble a mono bridge LM3886. It worked for 5 seconds and then started giving a white noise through.
So I shut down for a while, noted that the chips were pretty warm, and turned on again. Same story.
All of this was accompanied by bumps at switch on and off.
I suspect something on the mute pin 8, but oscillations on the pre-amplification stage could be the cause as well.
Any hint anout this?
Thank to anybody who may help.

Hallo again.
I found that the cause of the white noise was the LM318 I was using as a signal amplifier. I had it connected with 10kohm on the inverting input and 75kohm on as feedback (gain should be -7,5) and powered with +20V 0V leads. Bypassing the LM318 no noise went through. Using 220uF caps on the pins 7-8 of the 3886's I had the bumps deleted.
Now the points are:
1- I am using a 0-50 kohm potentiometer for volume (took it from a broken down Sherwood ampli). I cannot get the volume to 0 neither shortcircuiting the signal to its GND. Any ideas why?
2- I cannot reach a very powerful output: the toroidal transformer is rated 110VA and the LM3886 in bridge configuration should yield at least 50W on 8ohm. My Proton AM2 is much louder, and it's a 50W on 8ohm. Any ideas?
Thank you.

Last edited by NWE; 15th January 2013 at 11:45 AM. Reason: More info
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