DIY A/B switch Box

Discussion in 'Effects, Pedals, Strings & Things' started by salvatruco, Sep 22, 2008.

  1. salvatruco

    salvatruco Member

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    I am building this A/B switch box [​IMG] basically, i am going to run one active instrument and one passive instrument to one output... can some one share info on how to add a level pot for each input??? thanks:YinYang
     
  2. ylo

    ylo Member

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    Add two good quality opamps with pots to adjust the gain, or just one for the passive guitar side. I believe Craig Anderton's book (Electronics for Guitarists?) has a simple circuit.
     
  3. salvatruco

    salvatruco Member

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    thanks, I'll try that book:AOK
     
  4. ylo

    ylo Member

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    Salvatruco

    This should work: non-inverting unity gain opamp circuit shown at http://www.beavisaudio.com/techpages/Buffers/

    For gain, replace the feedback path from the output to input (-) with a 1 k pot wired as a variable resistor in to out, then wire the (-) input of the opamp to ground with a 1 k pot. This will give you a gain from unity to 2. For more gain, use a higher value pot, 5 k or 10 k. The gain is (R1 + R2) / R2, where R1 is the pot setting and R2 is the fixed resistor to ground.

    I've built this. The noise is low and it provides enough gain to match the volume of my Gibson with passive pickups to my ESP with active EMGs. It also does a great job driving long cables to the amp.

    You could also cascade two inverting opamps, one with unity gain and the other with adjustable gain.
     
  5. salvatruco

    salvatruco Member

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    you answer some of the questions i had about this circuit!!! thanks alot.. I think, i have to do some parts shopping now...:YinYang:YinYang
     
  6. ylo

    ylo Member

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    :jo
    You're welcome . . . but . . .

    I just realized that adding the gain pot probably will not work with a single 9 volt supply. I'm using a bipolar supply on mine (+9 V and -9 V to the opamp).

    I can send you the slightly different circuit I am using relative to the Beavis one, which I believe adds a capacitor to float the feedback path above ground.

    I have e-mailed this to you.
     
    Last edited: Sep 24, 2008
  7. spookyelectric

    spookyelectric Supporting Member

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    I thought I'd revive this thread as this looks like a fairly simple box to build. How would one go about adding a couple LED's to each side to be able to tell which side is selected?
     
  8. stinkfoot

    stinkfoot Member

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    The drawing in the original post has LED's included... One thing that struck me when I read the thread was that the Boss LS-2 does exactly what the OP wanted. It can be set to switch between loop A and B (plug one guitar into each return) with individual volume controls. That pedal is a really useful tool - possibly one of Boss' best compacts, if you ask me.

    /Andreas
     
  9. walterw

    walterw Gold Supporting Member

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    yeah, since you're talking about throwing true-bypass out the window, might as well get something with a decent buffer, switching and options.
     
  10. GTechNick

    GTechNick Member

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    I hate to dig this thread up from the grave but I need some help. I just tried to wire an a/b box using the above diagram but with a DPDT instead 3PDT but it is not working properly. I assumed I could just bypass the led portion that has to be powered, but it doesn't seem to be working. It is only passing a signal through when I hold the switch down, and only 1 of the outputs is working. Any help would be greatly appreciated. :bow
     
  11. stinkfoot

    stinkfoot Member

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    The LED portion is the third row (far right, top to bottom). So for a DPDT sans LED's, use rows 1 (left top, center and bottom) and 2 (middle top, center, bottom).

    By your description, I suspect you don't have the right switch. You need a latching (on-on) switch for this - a momentary switch would act like yours (change state when you press it, and go back to the previous state as soon as you let it go).

    As to why only one of the outputs is working, are you sure you're using the correct jack as the input? You can use the "Y" (labeled "input" in the drawing) as the input, which will then be sent to either A or B. Or you can use A and B as two inputs (for instance with two guitars), and the switch will select which one to send to "Y".
     
  12. GTechNick

    GTechNick Member

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    I think this is my issue. Thanks.
     

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