LM3876 chip amp very quiet and distorted... - diyAudio
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Old 26th November 2009, 07:33 PM   #1
jdsnape is offline jdsnape  United Kingdom
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Join Date: Nov 2009
Default LM3876 chip amp very quiet and distorted...

Hi,

I've just built my first chip-amp, based on the example on the first page of the datasheet, except with pin 8 tied low with a 100uF capacitor and the variable resistor omitted on the input.

I've been inputting signal from the headphone socket on my laptop and the output of the amplifier is very very quiet (I've checked the volume's not just down on my laptop :P)

Is this due to the signal I'm inputting to the amp? or is it likely to be another sort of mistake? I have noticed that the amp draws ~35 volts when on, up from the 24 the transformer provides under no-load, is this normal?

I will have a poke around, but I thought I'd ask to see if I'd made a common mistake that causes this problem!

many thanks,

Joel
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Old 26th November 2009, 11:57 PM   #2
jaycee is offline jaycee  United Kingdom
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Join Date: Apr 2004
Location: Norwich, UK
When you say "pin 7 tied low" do you mean tied to GND, or directly to -V ?

If you have tied it to ground, then the chip is in Mute mode. You connect it to -V with a resistor that allows a minimum of 0.5mA to flow. Try a 22K resistor (allows about 1.4mA to flow and makes sure it's out of mute mode). The capacitor can be omitted, but it gives a pop-free startup.

I would recommend you put a 22K resistor to ground at the input, and couple the input with a 2.2uF capacitor (preferably nonpolar). Rf1 can also be 22K. I have a similar setup with an LM3886 (a virtually identical chip) which works well.

http://www.darkmatter.myby.co.uk/lm3...weramp-sch.png


If you have tied it directly to -V, you have probably fried the chip's mute circuit.
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Old 27th November 2009, 08:11 PM   #3
jdsnape is offline jdsnape  United Kingdom
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Join Date: Nov 2009
I had tied it to GND - I see why it didn't work now! I thought it couldn't be that because I could still hear some sound when the i/p signal was turned right up - guess its not a perfect mute.

Unfortunately while I was testing for shorts with my multimeter I'd forgotton to discharge the rail capacitors so when I accidentally touched two pins together there was a big blue spark....then when I turned it on it gave off the magic smoke (I feel very stupid, but then it is the first time I've ever done that tho, always wondered what it looked like and I've learnt to discharge the capacitors first the hard way!)

I've re-built the circuit sans chip but with your suggestions, and am just waiting for another one comes in the post Monday to see if it works this time!

many thanks for your help

Joel
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