Improving the classic 18V mod for guitars
Your question gives no clue regarding the voltage available for the task and the maximum current required. Assuming that there's a few volts to spare and that the current is &lt; 100 mA a simple solution would be to use the ancient LM317 adjustable voltage regulator.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Adjustable voltage regulator circuit.

The LM317 adjusts its output so that it is 1.25 V higher than the ADJ pin. I have chosen the resistor values to make the maths easy (100 &Omega;/volt) so you can scale them up to use a 1k pot for R3, etc.

You'll need to calculate the power dissipation in U1 and check if you need a heatsink. \$ P = (V_{in+} - V_{OUT
min}) I \$.