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Aguilar OBP-3 to DIY Preamp Pedal

Discussion in 'Pickups & Electronics [BG]' started by theTinkerer, Nov 29, 2017.

  1. theTinkerer


    Mar 7, 2017
    Long-time reader, first-time poster.

    I have a bass-less preamp lying around that I want to convert to a pedal with 3PDT switch and LED. I have VERY LITTLE experience with this type of stuff, but I did some research and altered the schematic with my best uneducated guess.

    Did I plan this correctly? Any advice would be super appreciated. Thanks!

    Original Schematic


    Altered Schematic (grayed-out parts didn't change)


    Any fatal flaws?
    Last edited: Nov 29, 2017
  2. The attachments are not working, for me.
  3. theTinkerer


    Mar 7, 2017
    My bad. Should be fixed now.
  4. I see a few issues.

    Firstly, you have the volume pot wired backwards. The wiper terminal should be the output, not the input. You only wire volume pots the way you have shown when there are multiple volume pots in the circuit, because that is the only way to make them work independently of each other. That kind of wiring compromises the taper of the pot, and should be avoided whenever possible.

    Secondly, it is traditional to have stompboxes switched on/off by the insertion of a cable in the input jack, rather than the output jack. You don't have to follow this tradition if you don't want to, but it adds confusion to the process of powering up/down, and if your rig is ever used by someone else, then they probably won't know about your nonstandard choices, and leave the pedal powered up.

    Thirdly, you have the volume pot running before the input of the preamp. This is necessary when you want a volume pot to work in both active and passive modes, but since you are building a stompbox, it might serve you better to have either two volume pots, or a dual ganged pot with one 500k wafer and one 25k wafer. This way, your active mode can place a pot after the preamp, rather than before it. This will give you a better functioning volume pot, when the preamp is being used.

    Another thing that I noticed is that you do not have an external power input. Running a stompbox on battery power can be annoying. You might consider adding a standard 9V or 18V DC input.
    theTinkerer and tubestuff like this.
  5. theTinkerer


    Mar 7, 2017
    Wow. Thanks for the feedback. I'm sure this stuff is fairly common-sense for most, but this is incredibly helpful for me.

    RE: Volume pot placement: If I don't want a volume knob for the clean signal, but just to regulate the active mode, could I simply move the volume knob to between the switch and the preamp? Or perhaps pre/vol/switch?
  6. You can put the volume pot after the preamp. Make sure you have the right value pot. The impedance will be lower at the output of the preamp, so use a 25k pot.
    theTinkerer likes this.
  7. theTinkerer


    Mar 7, 2017
    Seriously, thanks for the feedback. I took your advice to heart and made lots of edits. I think the only thing I have yet to do is move the volume pot after the pre.
    • Rewired volume pot
    • Added DC input
    • Moved battery ground to input
    • Had way too much fun playing around in Illustrator

  8. The last step is moving the volume pot to after the preamp.
    theTinkerer likes this.

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