Phantom Power for Active Pickups

I am sick and tired of changing 9v batteries for my active pickups. These pups sound awesome, but they drain the battery pretty quickly when I forget to unplug after I play. I recently installed some Fishman modern Fluence pickups (which sound AWESOME!!!), but it is hard for me to remember to unplug since I am used to passive.

Female 1/4" TRS

Male XLR plate

DC power jack (5.5mm/2.1mm)

Stomp switch

Stomp Box

LED (not needed) and appropriate limiting resistor (necessary if using LED)

Most active pickups use a trick with your guitar's input jack to turn on your pickups when plugged in. To do so a female TRS (stereo) jack is used for the guitar's output, and the negative terminal of the 9v battery is attached to the ring (R of TRS) contact. When a 1/4 inch mono guitar cable this ring cable shorts to ground by way of the guitar cable's sleeve (S) contact. I will be using this contact to supply 9v to the guitar.

My first question was "will this cause undesirable interference?" To answer that I first looked up the high spec for microphone phantom power, it is 7mA @ 48v. Next I checked Fishman (the manufacturer of my pickups), and my pups draw 2.5mA @ 9v. The current I am expecting to draw s well under the spec for microphone phantom power, but the fact that voltage is so much lower means that electromagnetic interference (EMI) should not be a problem. The 9v power supply is of an isolated design for use with audio, plugged into a power conditioner further isolating the proposed circuit from EMI. In other words, it should be fine.

I decided to use an XLR to balanced TRS cable so that it could plug into a commercially available shielded audio quality cable that would be unlikely to be misused. The goal is for the tip to carry the signal, the ring to provide 9v, and the sleeve is the return path/ground.

After adding a power switch and indicating light to the circuit and matching the appropriate XLR pins the their TRS counterparts the above circuit design is complete.

Now that everything is designed schematically, we must plan the layout. This will vary depending on the box you use, just make sure that you have enough room to fit everything and still work inside.

After I decided where I wanted my parts, I prepared the stomp box by drilling out holes for my components. Now is a good time to test fit everything in case any changes need to be made to the physical design.

Now that the circuit is designed, the box is ready, and all of the parts fit, it is time to solder it all together!

Soldering is fun (opinion) and challenging at first (fact). If you are not great at soldering check out some how to articles and practice with spare parts.

Test your product to verify the voltage is only present on ring terminal, and perform continuity checks from tip to tip and sleeve to sleeve.

The last thing to do is pull out that annoying battery and short the battery terminals in your guitar. I just stuffed a piece of wire into the appropriate place, so that I could still use a battery in my guitar, in case I ever find myself away from the rest of my gear wanting to jam.

Now let's fire up the amp and cook some tubes!

Everything sounds great! No hum, no buzz! The only thing is that my cheap tuner began to behave oddly. This was easily solved by moving the tuner to my effects loop. Perhaps an audio buffer could be worked into later designs, or your signal chain to solve this problem. Maybe a better tuner would just work.

Either way this thing works, and now I am free from 9v batteries.