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Complete beginner. Need help with LM386 amp project

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Tommyduke

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I have recently purchased a **broken link removed**. I was very impressed with what this little thing could do and have looked into the design. I found this forum searching for Little Gem projects in google.

I am very interested in trying to build one of these for myself. I don't know how simple something like this is. Can anyone suggest a good guide for beginners or recommend a better 'first project'?
 
Yes, a tiny one - it's just something like an LM386, perhaps a 741 (or other opamp) and a 9V battery.

First of all, let's not get the LM386 confused with an opamp. It is not. It is an audio power amplifier... it's a very good first project, because all the hard stuff of audio amplification is taken care of right inside the chip with very few external parts. It uses some fairly large capacitors for coupling of the audio signals, but pound for pound it is one of the best audio amp chips I've come across, although my last project that used one I picked the AN7586 (which uses even bigger coupling capacitors) probably for the power output. When you look up the LM386 data sheet they also suggest the LM4877 or LM4951 for more power and less THD.

One area you need to take care in would be the input side. You must make sure you have a shielded cable coming in, and that shield needs to be carried right up to the input of the power amp. Because you are amplifying, any noise on this input will show up very big on your output. If that noise comes from the output side, you're in for some big feedback oscillations.

And don't 'ground' your shield at both ends, this causes ground loops, intorducing noise. The shield needs to be tied to ground at one end only, usually the source end is best, but in the case of a guitar you will probably end up grounding it at your amp box.

And yes, if you are a beginner, take a soldering lesson. This will save you tons of time in the long run, not to mention all the troubleshooting you might do due to a loose or cold solder joint. It freaks people out when they see me 'pull' on a wire or other solder joint, but if you can't physically pull them apart, then it's a valid solder connection. If it pulls off, it's probably cold soldered. Positive mechanical support, don't rely on the solder to hold the parts together (known as tacking). One time I fixed a unit in less than 5 minutes w/o even powering it up after 2 techs looked at it but couldn't find the problem. It was an assy used to 'shake' these big coal feeders for power plants to get rid of the dust. It had some monster triacs on it, and the first thing I always do is, yes you guessed it, pull (or push) on the wires. One gate solder joint looked good, but when the wire was pushed it went through the pin eye with the solder attached to the wire, but not the pin. On just inspection, it looked just like any of the other connections. But this thing caused large vibrations, and when vibrating one channel didn't work. On the bench, however, everything worked to spec (no shaking) electrically so identifying the problem was not easy... unless you knew what to look for...

Mike
 
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I doubt it uses the 741 as a power stage, it'd have to be boosted by transistors and it would be

The bridged LM386 amp looks like it'll cook, the speaker should be capacitively coupled otherwise there'll be some DC as the two ICs won't be matched. Bridging also divides the impedance by two and the LM386 can't drive impedances below 4Ω.
 
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