Why Would I Need a DI box for my Acoustic?

The Radial SB-4 Piezo, a DI box designed specifically to deal with the challenges that come from recording piezos and using them live on stage.

You’ve got a really nice acoustic guitar that has a built in pickup. You want to record with it, or play it through a PA. Heck it might even be a piezo pickup (turns head and spits on ground) and you want it to sound good.

All things are possible of course.

If we examine the pickups in acoustic guitars, they tend to come in a couple of broad based flavours. The most common is the piezo which requires a power source to work. Piezos come in a couple of sub flavours, those that have a preamp, such as what we find in a Martin SC-16 or in the still piezo but better sounding Taylor Expression 2 pickup system, or those that have no preamp such as the popular and inexpensive under saddle systems from folks like Fishman and others. The less common is the contact pickup / internal microphone as one can find in the K&K Pure Mini family. Pure Minis are by default contact transducers that sit inside the guitar under the bridge plate (similar to the positioning of the piezo in Taylor guitars), but you can extend that to include an in body microphone as you would find in a Maton guitar from Australia such as those played by Tommy Emmanuel.

In any of these contexts, the connection is a TS guitar cable into a jack somewhere on the body. This means a high impedance output unless there is a preamp built with a low impedance output involved in the configuration. Since that is much less probable, we have to deal with the limitations of high impedance.

The High Impedance Challenge

The longer the cable from the guitar to whatever it is plugged into, the more loss you get, starting with the high end. For longer runs, and for better fidelity, you want low impedance. Higher end microphones are all low impedance. Some interfaces limit you to one or no high impedance inputs, a bit of a pain when recording. Some PA / mixers limit the number of inputs as well and have the further challenge of distance from the player. Hence as noted, you want low impedance.

Low Impedance Connections

Low impedance runs do not use traditional guitar cables. They use what are known as balanced lines. These can be the often found TRS cables that look like TS cables unless you check, or the more durable and more reliable XLR type connectors found on microphone cables. What we really want is a High Impedance to Low Impedance conversion very close to the guitar. Impedance conversion is done by a transformer and some impedance converting direct boxes are passive while others are active giving better performance and using the interface or mixer input’s +48v phantom power. This is delivered over the XLR cable, so it is simple.

The DI Box

I wanted to be able to record acoustic guitars not just by using a microphone, but also to use the pickup if the guitar has one. I prefer the K&K Pure Mini family over any piezo, although Taylor’s Expression System 2 goes a way to eliminate that nasty nasal tone that most piezos have. I use an Active DI from Radial Engineering called the StageBug SB-1. They also have one called the StageBug SB-4 built specifically for piezo pickups (image above).

In my case, I support Canadian builders when I can, and over decades, I have always been impressed with the build quality and audio characteristics of the products from Radial Engineering. I use their Bigshot ABY boxes to build wet/dry and wet/dry/stereo rigs and have a few of their tube based preamps as well as their old super clean FET boost. Radial has never let me down, and when I determined I needed to get some DI stuff I looked first to Radial, found what I wanted and stopped there. For acoustic use, I like the active DI units, as I do for microphone activators. For electric guitars, I have been proven successful with their passive DI boxes (D1 Pro, D2 Pro).

In either case, you connect the StageBug via XLR cable to a microphone input on your interface or mixer and activate the phantom power. This power allows the transformer to cleanly change the impedance. Connect your guitar using a standard instrument cable to the instrument input on the StageBug. Both units offer a high impedance output as well to run to a tuner or an acoustic amplifier that is close to the instrument.

Audio Samples

I wanted to give a sense of what a DI signal to a high quality preamp would sound like as opposed to running the signal direct to a PA or even a really good acoustic amplifier. I used a couple of different guitars for these short samples. Recording was through the DI to an Apollo Solo with a UA-610B preamp set to the factory guitar preset in the UNISON slot. The recordings were made in Presonus Studio One. There was a Tuner on Presonus insert and a Multiband Dynamics Compressor / Expander in the Post slot on the output channel.

First up is a special Taylor build of a Grand Concert called the E14ce. The sides and back are solid Ebony and the top is Sitka Spruce. When I heard this specific guitar played live at a Taylor Roadshow at The Arts Music Store, my ears stuck up because there was a tone from it like nothing that I heard before. It has Taylor’s Expression System 2 pickup system. I ran cable from it to a Radial StageBug SB-4 Active that is designed specifically for piezo systems. The DI kept the wide frequency response while warming things slightly to get rid of that nasal noise common to piezos, even really good implementations such as those from Taylor.

Next I used my Boucher BG-52 Bluegrass Goose through a Radial SB-1 DI. This DI is built for more traditional pickups. The BG-52 is a solid rosewood sides and back guitar with an Adirondack Spruce top. The pickup in it is a K&K Pure Mini. This is a transducer style pickup, not a piezo and is naturally warmer without being brittle.

This is the Radial SB-1, suitable for non-piezo acoustic instruments such as guitar, and string bass

My Thoughts on DI for Acoustic

The first question, is can you get by without one. The answer is as usual it depends. The big dependency here is the cable run length from the guitar to where it is going if you are using a built in pickup. If you have a cable run longer than about 3m. I would say yes, you want a DI box. They were both under $100 CAD each, added no noise and allowed me to run the guitars over low impedance balanced lines into the balanced inputs on the Apollo. That made for a nicer signal path and better tone in my opinion. Yes, it is another piece of gear, and one more cable, but as the devices are very small and go in your guitar case or gig bag, that’s not a real consideration. They use phantom power from the mixer, the PA or your interface so no wall warts or batteries to die. So the second question is should you have one if you record or play live with an amplified acoustic and to this my answer is a resounding yes.

Thanks for reading. Should you have any questions on this or other similar topics, click this link to send your question in. Until next time, peace.