With the rise of “amp-in-a-box” modeling processors that include line-level outputs, line isolators have become increasingly popular with guitarists — with boxes like the Walrus Audio Canvas and Radial Highline series replacing the old standby DI box in some rigs. So, what’s the difference between a DI vs. line isolator?
The DI box — also known as a direct injection box, direct input box, or simply DI — is an essential piece of gear that lets you plug the 1/4″ output of an instrument “directly” into a mic preamp or mixer. You’d be hard-pressed to find a studio or venue that doesn’t have at least a couple DI boxes kicking around their gear collection. Using an electronic component called a step-down transformer, passive DI boxes convert the high-impedance, instrument-level, unbalanced signal from a guitar or bass into the low-impedance, mic-level, balanced signal that a preamp channel is designed to receive.
A line isolator also converts a signal from unbalanced to balanced — but unlike a DI box, a line isolator is strictly a line-level device. When you plug into a line isolator, your signal goes in line-level and comes out line-level, without any gain changes or impedance changes — getting it into the mixing board’s line input with a minimum of electronics in its path.
DI Box vs. Line Isolator Rule of Thumb:
- Use a DI box if you’re sending the instrument-level high-impedance output of a guitar or bass to a preamp channel of a mixer or audio interface.
- Use a line isolator to connect your device’s line-level low-impedance outputs to the mixer, avoiding the need to add any gain at the preamp stage.
In this article, we’ll take a closer look at why you might want to use a DI box vs. a line isolator.
Line-Level, Instrument-Level, Mic-Level — What Do They Mean, Exactly?
These terms are used to describe the different standards for signal strengths (voltages) and impedance ratings that you might encounter in audio gear:
- Microphone-level is the weakest audio signal level. With an approximate voltage of -60 to -40 dBV, a mic-level signal requires a mic preamp circuit to boost it up to line-level. Mic-level signals are low-impedance (Lo-Z) — generally around 150 to 600 Ohms.
- Instrument-level signal output by passive instruments is significantly stronger than mic-level, in the range of -30 to -10 dBV. These instrument-level signals are high-impedance (Hi-Z), often 10 kOhms and up. (Instruments with active pickups, however, generally output a hotter signal with lower impedance, often approaching or overlapping with line-level territory.)
- Line-level is stronger than instrument-level, with voltage of -10 dBV for consumer line-level or +4 dBu (equivalent to +1.79 dBV) for professional line-level. Line-level outputs generally have a low output impedance of 600 Ohms and under, and they’re designed to feed line-level inputs with a high input impedance of 10 kOhms and up. “Low output impedance drives high input impedance” is the rule of thumb for signal integrity.
- Finally, speaker-level is the very-high-current “post-power-amplifier” signal level that’s required to drive passive loudspeakers. You’re likely to encounter this signal level in just one use case: connecting a power amp’s speaker output to passive speakers or cabinets.
These distinctions matter because mismatched signal levels can cause noisy signals, too-weak signals, or distortion (the unwanted kind!).
DI Box vs. Line Isolator: What’s the Difference?
When you plug into a line isolator, your signal stays at line-level all the way down the chain.
In contrast, when you plug into a DI box, the DI steps your signal down from instrument-level (or line-level) to a very low-voltage microphone-level signal. When this signal gets to the mixer, it’s going to need quite a lot of preamp gain (think at least 40 dB) to get up to the line-level signal needed within the mixer.
When you plug into a DI, you’re essentially turning the level down just to turn it right back up again on the other end.
Now, adding a whole bunch of preamp gain isn’t necessarily a bad thing: adding gain to mic-level signals is exactly what the preamp channels on a recording console are designed to do, and do well.
But if your goal is to take the sound you get out of your modeler and capture it exactly as it is, then a line isolator lets you send your line-level signal into the mixer as is, bypassing the mixer’s preamp circuit and any coloration, character, or (heaven forbid) noise that it may add to your signal. In that sense, a line isolator is actually more “direct” than a direct box.
DI Box vs. Line Isolator: What’s the Same?
Both DI boxes and line isolators use an electronic component called a balancing transformer to convert an unbalanced signal (such as from a 1/4″ TS instrument cable) into a balanced signal (on a 3-pin XLR connector). This conversion allows the signal to travel over long cable runs with greatly reduced susceptibility to noise and electromagnetic interference.
As a rule, balanced signal is what you want for any long cable run. With an unbalanced cable (like a typical 1/4″ TS guitar cable), you’re limited to about 20 feet before you run the risk of picking up noise. But once your signal is running through a balanced XLR cable, it’s much less susceptible to noise and interference. That’s why every long cable run in a pro studio or venue uses balanced cable — whether you’re plugging into a stagebox and running through a snake across an auditorium to the front-of-house mixer, or plugging into the patch points in a studio’s live room and sending your signal who-knows-where through the walls, floors, ceilings of the building, into the control room down the hall.
Both DI boxes and line isolators serve to maintain the integrity of your signal by converting it to a balanced signal.
DI Deep Dive
Want to know more about DI boxes and why you might want to use one in the first place? Glad you asked!
Why Record with a DI Box in the Studio?
In the studio, when you plug into a DI box and record your guitar or bass “direct,” you capture the instrument’s “dry,” unprocessed signal, giving you the flexibility to do re-amping, or add effects/processing later, without having to re-record the guitar performance. Your typical mono DI box has a single input for your 1/4″ TS cable, and two outputs: an XLR out that sends the dry/direct signal to be recorded, and a 1/4″ “Thru” output that feeds your pedalboard or amp. This “Thru” output lets you play through your effects, amp, and cabinet. You can mic up that cab to capture the “wet” signal, while simultaneously capturing the “dry” signal — giving you more options later when it’s time to mix.
Why Play Through a DI Box Live?
When you’re playing live, the sound engineer may prefer to grab the clean direct signal from your keyboard rig and route that to your monitor, rather than futz around with miking up an keyboard amp, adding more gain on stage, and increasing the chances for feedback. Or maybe your live sound tech wants to get your bass direct and mic up your bass amp, to have more options for reinforcing your rig. It’s all about getting a clean signal to front-of-house so the person behind the board has greater control over the mix, and can make you sound as good as possible in the house while giving you what you need in your monitors.
Active vs. Passive DIs
DIs come in two flavors: active and passive. As a general rule of thumb, you’ll want to use an active DI if your guitar/bass has passive pickups. You may use a passive DI if your guitar/bass has active pickups or a built-in preamp with a battery, or if you’re using an active device, like a preamp pedal, before plugging into the DI. You can also use a passive DI to connect a device with line-level outputs, like many keyboards or drum machines.
What Do Those Switches Do?
DI boxes sometimes come with extra features like a ground lift switch (to kill off unwanted ground loop hum), polarity flip (to help reduce feedback if you’re playing an acoustic-electric instrument live), filters for reducing interference, pad switches to attenuate too-hot signals, and more bells and whistles.
My Mixer/Interface Has a Hi-Z Input — Do I Still Need to Use a DI?
Many mixing boards, preamps, and audio interfaces have “Hi-Z” inputs on board with a DI circuit built in, giving you the option to plug in a guitar and “go direct” with no need to plug in a separate DI box. However, not all DI circuits are equal, and a high-end DI box can just sound really darn good. The DI box isn’t merely an adapter — it can be part of the signal chain you use to sound your best.
DI Box vs. Line Isolator: Which One Works for You?
DIs and line isolators look pretty similar on the outside: they’re both unassuming little boxes that typically have 1/4″ inputs and XLR outputs. Both boxes serve to maintain your signal integrity, helping keep noise to a minimum over long cable runs. Some line isolator units — like the Walrus Canvas Mono and Canvas Stereo — even have a switch that allows them to function as either a DI or an LI.
So which one should you use?
Are Your Outputs Instrument-Level or Line-Level?
Most electric guitars and basses, along with most single-effect stompbox pedals, have instrument-level outputs. Some guitars with very-high-output active pickups, or acoustic-electric guitars with a built-in preamp, may start to approach a line-level signal. Keyboards, synths, drum machines, and DJ rigs are likely to have line-level outputs.
Many modern guitar modelers, like the Line 6 HX Stomp or HeadRush Prime, have outputs that can be set to line-level OR instrument-level (sometimes called “amp level”), depending on where you want to connect their outputs. Navigate to the global settings menu on your modeler, and set the output to whatever you want to use.
To find out if your specific device’s outputs are line-level or instrument-level…
See if there’s a label on your device’s output jacks:
- If an output is labeled “Line Out”, it’s typically line-level.
- If it’s labeled “Guitar Out or “Instrument Out,” it’s usually instrument-level.
Check for an Output Impedance spec in your device’s manual:
- Output impedance under 1 kOhm probably signifies a line-level output. You might try plugging this output into a line isolator and see if it makes a difference for your tone!
- Output impedance over 1 kOhm most likely means it’s an instrument-level output. Stick with a DI box for getting this Hi-Z signal to the mixer!
Leave a Reply