Cracker or Cigar Box Amp as seen on Makezine.com

This is a quick demonstration of how to mod a 1k ohm potentiometer to be able to work in a region between 0 and 25-30 ohms. The Make schematic for the cigar box amp calls for a 25 ohm rheostat, which is either hard to find or much more costly than it needs to be.

Cigar Box Guitar Amp [LM386; as seen at Makezine] (Currently up for bid until July 12, 2010)

http://makezine.com/09/crackerboxamp/

The amplifier circuit from the pages of Make is a very simple one, but I had some issues finding the 25 ohm rheostat that the schematic calls for. This is the only modification to the circuit that I made. I used a 1k ohm pot for the volume, but I modified it with a 33 ohm resistor to get it as close to the schematic as I wanted to. The volume doesn’t completely cut out, but who wants to use an amp at zero volume anyways?

modified amp schematic

modified amp schematic

The trick to this mod was easy. I just used the parallel resistance formula where total resistance equals the inverse of the sum of the inverse resistances (or the inverse of the sum of conductances). Either way, I know that when the volume pot is at 0 ohms, most of the current will flow through the pot as if it were just a wire. If the pot were to go to infinite resistance, the current path would be through the shunt resistor.

I used my graphing calculator to graph y = 1\((1/1000)+(1/x)). Then I traced the line to find the Y-value (my shunt resistor). I chose the point where x = 25. Y was so close to 25 that I just rounded up. Well, I didn’t have a pile of 25 ohm resistors so I used a 33 ohm resistor.

The volume pot now works a little better. It’s not perfect, but it provides a better range than just a 1k pot.