Acoustic DI Boxes Part 1 – Introduction to Impedance


Ever see a musician who constantly wastes precious time during his set twiddling knobs? I used to be that guy. It’s not endearing. I had this brainstorm to build a DI box into an Ernie Ball volume pedal 1in theory it’s still a great idea so I could effortless control my volume with my foot without anyone even noticing. I had a friend build it for me and I used that pedal on tour and it worked flawlessly. Brilliant, right?

Then one day I plugged into a Fishman DI at a festival gig and my sound was so much bigger, with way more dynamics than what I was getting out of my pedal. I had to go back to the drawing board – what was I doing wrong?

Why do I need a DI box anyway?

Now some folks at this point may be asking “What’s a DI box?” “Why do I need one, isn’t that the venue’s responsibility?” Or saying; “I don’t like pedals.” or “I don’t like to think about these things.” If you are performing live it’s your responsibility to bring your own DI box – just as it’s your responsibility to bring your own mic if you’re singing. 2Please tell me you bring your own mic!  It should just be part of what you bring to a gig.

From Wikipedia: “A Direct Input (DI) box is an electronic device typically used in recording studios and in sound reinforcement systems to connect a high-impedance, line level, unbalanced output signal to a low-impedance, microphone level, balanced input, usually via an XLR connector and cable.”

Meaning: the cord that comes out of your acoustic plugs into a DI box and then the venue plugs an XLR cable into the DI box to connect it to the PA system. Do not get confused by “high-impedance” meaning more volume and “low-impedance” meaning less volume – impedance and  volume are not the same thing.

What the heck is impedance?

“Impedance? I just want to play guitar!”  Well if you play live acoustic guitar, understanding it just a little bit is EVERYTHING in making your piezo pickup acoustic guitar sound better.

Why is it no one told us?  Well, we’re going to need a plumbing metaphor to understand this.

Imagine a piezo pickup puts out a signal that equals a 5 inch pipe3This metaphor not to scale. When you plug it into a guitar pedal or a guitar amp4most guitar pedals and amps it squashes that 5 inch signal down into about a ¼ inch pipe and then we wonder where all the fidelity went.

A piezo pickup likes to see something like 2MΩ to 10MΩ input impedance. Where an electric guitar pickup is fine with 1MΩ or less. The piezo signal puts out a much wider frequency.

My custom made DI box was made with 1MΩ input impedance. My sound was getting squashed before it even reached the PA! So I set out to get the best live acoustic sound I could. How did I do that? Well, I went to a couple of music stores with decent return policies and I bought a bunch of DI boxes. I couldn’t get every one out there but I got quite a few 5Radial ProDI, Fishman Aura and ProEQ, LRBaggs ParaDI and Venue DI, Rolls, Behringer, Sansamp and I started plugging stuff in to see what worked for me.

Stay tuned in Part 2 where I put a number of different DI pedals through the paces.

Notes:   [ + ]

1. in theory it’s still a great idea
2. Please tell me you bring your own mic!
3. This metaphor not to scale
4. most guitar pedals and amps
5. Radial ProDI, Fishman Aura and ProEQ, LRBaggs ParaDI and Venue DI, Rolls, Behringer, Sansamp