Most guitar effect pedals can be supplied with 9V DC. The main feature for such a power supply is low noise. A battery will give no noise, but power from the AC grid can cause 50/100Hz hum, plus higher frequency noise from other peripherals connected to the same grid. This design provides  500mA of 9V DC with very low noise.

The basis of this project was a transformer with two secondary windings that provides 2x12v AC. I decided to build a dual power supply, with two identical rectifier and regulator circuits in parallel. This enables me to draw a total of 1000mA, but more importantly, if an effect is prone to transfer noise to other effects, this can be put on an isolated power supply.

The AC current from the secondary coils on the transformer is rectified by four 1N4007 diodes, and then fed into the LM7809 voltage regulator IC. The LM7809 is the heart of the power supply, and provides a steady 9V output. Noise, however, will be an issue after the LM7809. This is why the four capacitors are added. A 470$$mu$$F capacitor helps smooth the rough rectified current from the diodes. The 10$$mu$$F capacitor on the output will do somewhat the same on the output. The two 100nF capacitors will remove high frequency noise.

The LED’s I added just because I wanted to (and because I like thinks that light up when they are turned on).

The PCB was drawn in Eagle CAD, and milled with files created by pcb2gcode. Eagle files here: sch brd. After soldering, both the power supplies was tested, and they both gave steady voltages. Testing with an oscilloscope showed a very steady voltage, but with very small amounts of ripples.

Testing with effect pedals, by switching between battery and the power supplies gave no audible difference.

Personally I was very pleased with the result. I will, however, consider to add heat sinks to the LM7809’s, because those babies can get hot!

PS.
This design can be used to provide other voltages by changing the LM7809 with an other regulator IC, as long as the voltage from the rectifier is 2V higher than the output voltage. I.e. LM7812 could be used to provide 12V.