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)
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?
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.