Home GuitarPCB Forum General DIY Pedal Discussion Power Filters for DC Power Supply

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  • #31286
    Big O
    Participant

    After powering up a few of my pedals with a 9V DC Power Supply (One Spot wall wart variety), some of the pedals without power filtering were rather noisy.  Powered by a 9V battery, they were quiet, which tells me my power supply is pretty noisy.  So instead of retrofitting the pedals with an internal power filtering circuit, I decided to build external power filter circuits that could be plugged into the pedals.  That is Power Supply Jack > Power Filter > DC Power Input Jack on Pedal.

    I researched standard power supply filtering on schematics of multiple pedals and came up with two ways to filter the supply – a simpler version with an in-line diode and 2 filtering caps to ground, and an in-line resistor with diode polarity protection and filter caps to ground as below.

    Below are the finished products.  First are the completed, boxed filters.  Green arrow is the power supply input.

    Below are the guts, which correspond to the circuit drawings above in order.  The bolt inside each enclosure serves as the ground.

    So what are the results?  Well both filters work, although the larger circuit works better.  But there is still some noise from the supply even with the filter compared to battery power which is quiet as a mouse.  Most pedals with built in power filtering are mostly quiet (including commercially available pedals) but there still is some minimal background power supply noise in a couple of pedals.  My first conclusion is that the One Spot is a noisy power supply as many have indicated on different forums.  A second conclusion is that some pedals handle power supply noise better than others (my Ramble FX Twinbender is dead quiet).  A third conclusion is that old 1-3 transistor circuits that tend to be noisy in general have their noise level exacerbated by external DC power supply noise.  They handle the power supply noise the least out of all the pedals I have.

    This also leads to a theory in my head – that maybe the power supply noise is not fully suppressed in the simple transistor circuit pedals since the power filtering circuit needs to be internal and tied to the pedal circuit ground.  That is, the filtering does not work as well in an external type filter that is not grounded internally to the pedal circuit.  I don’t know if this is true, but simple transistor pedals with DC power filtering built into their circuits are quieter than those with only the external filters.

    Looks like I may be purchasing a better power supply in the future.

    #31314
    TNblueshawk
    Participant

    Very interesting Steve. Thanks for sharing.

    #31619
    Big O
    Participant

    A follow-up to this topic.

    Purchased an isolated switch mode (not linear mode) power supply and tried all the pedals that had significant noise with the 1-Spot power supply.  There was no audible noise greater than that heard with a 9V battery as I switched back and forth between battery power and the new 9V power supply.  The 1-Spot is just plain noisy, especially with circuits that don’t have power filtering built-in.

    Lesson is – get an isolated, low noise power supply for your pedals if not using a battery.

     

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