Step 1: Parts
LM339 quad comparator
3 x 1k smd resistor (no bigger than 1206)
3 x 470 ohm smd resistor (no bigger than 1206)
2 x 1uf capacitor (no bigger than 1206). I suggest yo buy this somewhere other than Jameco, since the minimum order quantity is huge for this component. I just used through-hole capacitors for this one instead
2 x 0.22uf capacitor (no bigger than 1206)
2 x 0.1uf capacitor (no bigger than 1206)
1 x smd prototyping board for SMD 16-pin ICs
1 x audio connector, your choice
1 x 9V battery connector
3 x 500K potentiometers, or fixed resistors of many different values (CANNOT just use 470K resistors and get a "balanced" output)
3 x wires
3 x LEDs size and power are your choice. The maximum current from the LM339 is just about 16mA, so you cant use high power LEDs. I suggest 3mm LEDs, blue for low, red for mid, and yellow for high, but that is yor decision. The one in the pictures used one red 3mm led, and SMD blue and yellow LEDs. The SMD LEDs were 5050, so I couldn't fit the red one in.
*optional* you can add headers for an external display. The output of the LM339 will drive PNP type transistors if connected directly to the base.
Step 2: The schematic
The potentiometers can either be soldered in a "bridge" over other components, or you can drill through the board and mount them on the other side.
If using resistors, you will attach them in step 5.