built a 5w mini amp with LM384, need help

[MetalGod]

hi all, I've built a small practice amp into a stompbox enclosure and figured it would be of interest here.  I used an LM384N - its works and sounds great, but when the volume is increased above 1/4, it becomes unstable.  I've made a quick youtube video so you can see how it sounds with it working and also when it becomes unstable... http://www.youtube.com/watch?v=DPGAMxI9dJM

The circuit is right out of the data sheet application - exactly the same with the exception of the smoothing cap on the supply rail.  I used 100uF whereas the datasheet application schematic showed 0.1uF.  Here's the datasheet, the schematic is on page 5... http://www.national.com/ds/LM/LM384.pdf

Any ideas why it's acting up?

[Mark Hammer]

Conceivably, the source impedance it expects to see is different from what you are providing.  remember that as long as the volume is turned down, one leg of that volume pot is providing a series resistance into the input pin.  As you turn the volume up, that series resistance is decreased until you are likely reaching a point where the resistance is too low.

Just for the hell of it, stick a 3k3 resistor between the wiper of that pot and the input pin, and see if you can get the volume up higher.

P.S.: Thanks for having this problem.  I have a 384-based amp sitting on a board I got from something else (or maybe surplus for a buck or two) and a couple more of them in the parts drawer.  It'd be nice to know more about what they don't like before boxing one up.

[phector2004]

that's a neat built-in tremolo section 

[MetalGod]

Conceivably, the source impedance it expects to see is different from what you are providing.  remember that as long as the volume is turned down, one leg of that volume pot is providing a series resistance into the input pin.  As you turn the volume up, that series resistance is decreased until you are likely reaching a point where the resistance is too low.

Just for the hell of it, stick a 3k3 resistor between the wiper of that pot and the input pin, and see if you can get the volume up higher.

P.S.: Thanks for having this problem.  I have a 384-based amp sitting on a board I got from something else (or maybe surplus for a buck or two) and a couple more of them in the parts drawer.  It'd be nice to know more about what they don't like before boxing one up.

Thanks for that Mark, will let you know what I find and post back here.

I had another thought too - the voltage supply is currently obtained via three PP3's in series for 23v (they're not quite measuring 9v, hence I'm not getting 27v from them). Would this cause any problems? - might it be better just to get a 24vdc wall wart?

[MetalGod]

just an update - the pedal is current RIP 

I wired up a 24vdc wall wart and the smoothing cap on the power supply blew (twice), rated for 40v.  Gonna try this again when I get more LM384s later in the week.

[petemoore]

  Gotta measure those WW's, they'll put out more than what they're rated for at Max current, put a load on it, the voltage'll drop.
  24/40 is a near doubling, I've had 12vdc WW's say 18.+Vdc, and had damaged ones put out...something ugly I couldn't really measure.

[MetalGod]

  Gotta measure those WW's, they'll put out more than what they're rated for at Max current, put a load on it, the voltage'll drop.
  24/40 is a near doubling, I've had 12vdc WW's say 18.+Vdc, and had damaged ones put out...something ugly I couldn't really measure.

hi Pete,

The wall wart is switchable to 15v, 16v, 18v, 19v, 20v, 22v, 24v - I had it set to 18v when the 40v cap blew (it measures only slightly higher with no load at 18.5v).

The weird thing is the polarity - the connector has the tip marked as negative, but when I measure the voltage with my DMM with the black probe to the tip and the red probe to the barrel, I get a negative voltage.  If I swap the leads around, obviously it becomes positive.  Weird!