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Help me reverse engineer this 9volt mini opamp clean boost/preamp.

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  • Help me reverse engineer this 9volt mini opamp clean boost/preamp.

    I bought a used guitar acouple years ago that someone made a boost circuit and put in. It was connected to a stereo output jack, a 9volt battery and a 250k pot all inside the guitar cavity. Its on a super cheap/simple perf board. The components on the board seem to be a texas instruments TL882CP opamp, 3 tantalum caps, 1 ceramic disc cap and 3 resistors. It functioned great before I removed but I removed it because I was sick of having to replace 9v's and didn't have a use for it. Like a dumbass I didn't write down what wire went were. Now that I want to use it again I am stuck with no clue how to hook it up. There are 5 wires wired onto the circuit but don't remember if there were other wires involved going to pots or grounds or whatever. Anyone have any clue of how this functions just by the list of components? I don't really understand schematics yet so I can't draw the circuit out.

  • #2


    • #3
      It would be easier to help you if the photos were better focused. If you want to learn how to figure this out, we can help you to do that.

      If you just want to be told where to hook up the wires to, we need better photos.


      • #4
        From your description (I can't see any pictures at work), it doesn't seem to be particularly exemplary or special in any way. In the absence of any tone controls, safe to assume that the TL082 dual op-amp is being used with one op-amp dong the amplification, and the other half being used to provide a stable reference voltage for virtual ground. One tantalum cap will be on the output pin (either 1 or 7), and the others likely used to provide smooth battery power. One resistor will be in the feedback path (between pins 2-3 or 5-6) and the other from the inverting pin (pin 2 or 6) to ground. The third resistor likely goes from the noninverting pin (5 or 3) to Vref, and the ceramic cap likely goes from the other onboard circuitry to the inverting pin on the gain stage (again, 3 or 5). The output would go to the 250k pot.

        Am I close?


        • #5
          I would first like to be told what to hook up where and then it will be easier for me to understand it. Let me know if you need more pics or if you want me to tell you what is soldered to what. Thanks abunch guys.


          • #6
            This is what I can see from the pictures. This circuit uses one section of a TL082 dual op amp. There is no feedback resistor on the board, so I have to assume that the 250k pot is used for the feedback loop.

            So hook up the wires as follows and see what happens.

            The single white wire on the right side of the board is the circuit ground. Connect this to the guitar's ground.

            The blue wire is the circuit output. Connect this to either the output jack or a bypass switch.

            The white wire that is twisted together with the blue wire is the circuit input. Connect this to either the guitar's volume pot or to a bypass switch.

            The two remaining white wires go to the 250k pot. This is a gain control, so one wire should go to the center wiper and the other should go to the CCW terminal.

            You need to add the battery clip wires to finish off the installation. The red wire connects to the circuit board at the bottom center at pin #8 of the IC. The black should connect to the ring terminal of the output jack.


            • #7
              Awesome. I'll give this a try.

              On the pot (looking down on the bootom of it or shaft pointing away) the left and middle tab are soldered together awaiting a wire and the right is by itself awaiting a wire as they all still have solder on them. I just found something. If you look at the tantalum that is connected to the blue wire -the hole between that and the resistor next to the opamp had a wire going in from the top and soldered to that resistor. Is this where the battery positve wire was?


              • #8
                So wire one white wire to the separate terminal and the other to the side with two terminals already connected.

                Yes, the red battery wire should go to the corner pin of the chip (pin #8) which is also connected to one end of the 100K resistor.


                • #9
                  I'll bet it's the circuit from Ralph Denyer's "Guitar Handbook".
                  Steve Conner
                  Noodle of Reality
                  Last edited by Steve Conner; 04-13-2011, 10:52 AM. Reason: added link - hope it works!
                  "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"


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