Potentiometer ARCING inside :(
Anybody know what's going on  ?
I'm using a 100k to control a 50k ohm load .
i smell lots of smoke .
It's a LM386 with pots at the output section to a 8W power amp .

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gmoon6 years ago
Something's a little off here...

First of all, most POTs are rated for only about 1/4 of a watt--if you tried to use a POT as an attenuator on the output, it would probably burn out, 'cause some 386's can supply up to one watt....IF the load was a speaker.

But it isn't.  You're using the 386 as a preamp for a separate power amp. It's very likely you are correct, that the input impedance for that power amp is much higher (50K?) than a speaker. So the current draw should be very much smaller.

And it sounds like the POT is correctly connected as a voltage divider (one side to output of the 386, other side to ground, and the center wiper to the power amp).

You have a coupling capacitor between the 386 output and the POT/power amp? If not, you're passing DC through, and that could be the problem.

Otherwise, check your wiring...

NOTE: that HUGE 470 uF coupling cap in the "Portable Guitar Amp" instructable? You only need a large capacitor with a low-impedance load like a speaker (4 or 8 ohm), not with a 50K load. Plug the numbers into this RC calculator--compare the cutoff frequency for a 8 ohm load vs. a 50K load...
Thereyouhaveit (author)  gmoon6 years ago
Anyway , i now think it's a bad idea to have a LM386 preamp .
Thanks anyway , i guess its the cap .
gmoon6 years ago
I don't blame you for dropping the LM386 as a preamp. 386's usually have a pretty harsh and abrupt "decay" when used as such; the more distorted, the more abrupt the cutoff. There are a few 386 stompboxes out there, but it takes some additional circuitry to prevent the unpleasant artifacts.

For a simple replacement, maybe look at the Fetzer valve ?

Good luck!

Oh yeah--the REPLY buttons are missing from our two comments below, so I couldn't reply to your msg. Another day, another bug...
Thereyouhaveit (author)  gmoon6 years ago
You didn't tell me it was harsh , I LOVE IT .
Well , the input impedance is TOO LOW to be useful with my strat .
Why not add a simple buffer, like a JFET before the LM386, to increase the input impedance?

Any input Z over ~250K should be sufficient, and the JFET buffer on AMZ is about 1M ohms...
Thereyouhaveit (author)  gmoon6 years ago
Oh , never thought of it .
gmoon gmoon6 years ago
And now those two comments are "sticky" and above everything else? Weird.
Its a bug in the comment featuring system.
Oh, OK-- thanks.
Thereyouhaveit (author) 6 years ago
By the way , does anyone know the parts of the Piledriver pedal from modampkits ?
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