A simple DC power filter circuit in a small aluminum box that has proven to be quite useful in the studio.
A pair of inductors and a large value capacitor form a filter that will remove all of the radio frequency and hum from a DC power supply for a pedal. Since the circuit is so simple, I just made a ground buss from one jack to the other with a piece of solid copper wire. The inductors were soldered to the capacitor and then that assembly was positioned in the box and soldered to the ground wire and the jacks.
The schematic illustrates the design of the simple circuit. Component values are not critical and the ones that I used were based on the items I had on hand when I decided to make the unit.
Here are the Mouser parts numbers for the components that I used:
330uH inductors: 434-22-331
470uF capacitor: 647-UVK1V471MPD
The aluminum box is quite small and a 9v battery will not fit into it – I think it is a Bud CU-5470-B. Since the circuit is symmetrical, the input and output are not designated and the box can be connected in either direction according to what works best for the position on the pedalboard.
The inductors that I used are limited to 100ma so the filter cannot be used to power a circuit that requires more than 100ma of current. Other small inductors are available that are rated for higher current capacity if that is required.
Will this eliminate hum from my cheap wall wart?
The inductors will help reduce radio frequencies and clock noise, but are not going to have any significant impact on hum at 60 or 120Hz, however, the 470uF is going to help reduce it. So, you will get some hum reduction but how much depends on a number of factors. Try it out and report your results.
So elegant, so simple yet very effective!
I was wondering if an opamp gyrator could be used to simulate the two inductance?
Sound like a case where theory doesn’t agree with practice but…
Why not add one/two diodes on the 9V side?(another rectification point and simple polarity protection)
How big does the cap have to be to power multiple pedals from a 1 amp source? Don’t the pedals feed power from the capacitor?