Introduction: Active Pickup 9 Volt Adapter

This project started out of frustration dealing with constantly dyeing batteries used in the active pickups in the bass played by my friend in the band No Response. He wanted a 9 volt adapter that he could use in place of batteries while practicing or recording, where mobility is not a necessity. He would run through a battery a day during practice and recording, and even after tons of searching, we could not find a commercially available adapter like this. So as far as we know, this is the first of its kind documented.
The design of this unit is to allow ultimate flexibility and ease of use. A 12 volt 1000 mA power supply provide the power, which is then filtered and regulated by a circuit built into a 9 volt battery shell, which is inserted into the guitar as a normal battery is. The circuit consists of a RF choke and a 4300 uf capacitor to filter out all noise and hum, and a 9 volt regulator to make sure the pickup is being fed a constant 9 volts. An in-line connector is used to hook the adapter to the power supply, so if the cable is stepped on during playing it will disconnect to avoid damage. A nice paint job completes the package.

Step 1: Parts and Pieces

This project requires very easy to obtain parts. You will need:
9 volt battery - Dead is preferred as we will just take it apart
lm350 or lm317 - This is the heart of the voltage regulator. Either transistor can be used interchangeably in this project. 
240 ohm resistor - I used two 120 ohm resistors because I could not find a 240 ohm to save me.
1500 ohm resistor
3300 uf capacitor - at least 16 volts
1000 uf capacitor - at least 16 volts
Ferrite bead - to make the RF choke
Power connector - I used a 1/8 inch mono audio jack for this project
12 volt power supply - I used a 1000 mA supply to be on the safe side and make sure I had enough power. 
You will also need miscellaneous wire, solder, soldering iron, hot glue and hot glue gun, as well as other simple hand tools.

Step 2: Dissect the Battery

First we need the case from the battery so we can later put the circuit inside of it. Start by finding a dead battery that is not leaking and pry up the flaps on the back side of the battery. Push on the end where the clips are to push the cells out of the battery. Cut the clips off from the cells, and throw out the cells. We need just the two end pieces and the battery case. All batteries differ, so the internals will look different form brand to brand. I find Energizer or Duracell work the best for this project, as the clips are easy to solder to. 

Step 3: Assemble the Circuit

Assembly is easy enough as this is a simple circuit. But since everything is being fitted into a relatively confined space, how the circuit is assembled matters. I found that the circuit fits well with the regulator circuit placed on the end piece where the clips are located, and the capacitors in the space above. The RF choke fits nicely above the smaller capacitor. Remember as you are assembling, that the circuit cannot be wider or longer than the end pieces, or taller than the battery case. The parts I used just barely fit, so every fraction of a millimeter has to be considered as the circuit is assembled. The vertical size (the length of the battery) is the dimension I found to be the most difficult to work around. As you are assembling, make sure to test fit all the components into the battery case and plan ahead. If you use similar components to what I used and follow the assembly steps given in the pictures, the components should fit in the battery case snugly but easily. It is a good idea to aim for the battery being a few millimeters shorter than a regular battery to ensure the power cable does not prevent the battery from fitting in the guitar. 

About the RF choke: The choke is made by winding a single wire around the ferrite bead. the wire is threaded through the hole in the center, and then back around the bead and through the hole again. The more turns used the better, as it will be able to effectively filter out more and more frequencies.

Once the circuit is assembled, it is a good idea to insulate everything. I did this by covering the regulator  circuit with electrical tape on all sides and then filling inside the electrical tape with hot glue, leaving the transistor  out the hot glue so it could more effectively dissipate heat. I also shrink-wrapped all connections, and applied electrical tape to the terminals on the capacitors. It doesn't have to look pretty, because nobody will see it. As long as there is no chance of short circuits that is your goal.

Step 4: Finishing Touches and Using It

Once the circuit has been placed in the battery case, drill a hole in the back end and run the power wires out of it. Put the back end on to the battery and fold over the ends of the case. Hook it up to power and check the output to make sure it is putting out 9 volts. I found that due to the cramped quarters in the battery case that short circuits could spring up easily, so check for them now. If everything is all set, then it is time to solder the connectors onto the power input on the adapter and the power output on the end of the cable of the power supply. Make sure that you double check the polarity of the connections so that it is not accidentally reversed. That will be bad for the circuit. 
Now that everything is assembled, you can give it a paint job like I did or just cal it quits here. To use the adapter, plug it into the guitar as you would a normal battery. If the guitar has a door on the battery compartment like my friend's did, you can snip the corner of the door off with wire cutters to allow the power cable to come out of the compartment. Now all you do is plug the power supply into the wall and plug the adapter into the end of the power cable and enjoy battery-free playing. 

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Bio: Why fix it if it ain't broken? Because it's fun.
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