Why do you need a DI box

Why do you need a DI box

It would be great if every type of audio device from microphones to instruments to compressors all spoke a universal language but thanks to history, physics and our endless pursuit of better sound, we’ve wound up in a world where to make one device play nicely with another we often need some kind of translator in the middle.


The DI Box


One such device is the DI box (Direct Inject). Its job? To take instrument type signals such as those at the output of an electric guitar or bass and condition them to interface properly with the input of a microphone preamplifier. To put it more crudely, if you want to plug your instrument straight into the desk, you need a DI in between! As musicians and engineers, most of us intuitively know when to use a DI, but perhaps have never taken the time to understand why.


Microphone Signals


To truly understand why the DI is required we need to first discuss the type of signal the microphone preamplifier is expecting, versus the type of signal a typical instrument produces. Microphones typically produce very low level signals. The nature of their heavy diaphragm turning tiny fluctuations of air pressure into electricity means the output signal yielded might be as little as 0.001-0.01 volts. Sorry to bring up voltage but I promise it will help us to understand later! This tiny signal is what the microphone preamplifier is designed to accept.


The microphone preamplifier also expects that the signal connected to it is balanced. I’ll save the minutiae of balancing for another post but in a nutshell a balanced signal is one that is carried by three wires on three-pin connectors like XLR and TRS. The three signals are ground, the audio signal and a polarity inverted copy of the audio signal. Colloquially: ground, hot and cold.


The role of the microphone preamplifier is to take that very low level, balanced signal and amplify it significantly so it can interface with the other equipment in the studio such as compressors and mixing desks. The amplified signal level is known as “line level” which is nominally around 0.7 volts. To get from 0.001V to 0.7V is a heck of a lot of amplification and that’s why you will see preamps with 60-80dB of gain available!


Instrument Signals


Somewhat annoyingly, the average level produced by a typical electric guitar or bass is neither here nor there. It’s significantly higher than that produced by a microphone but it's still significantly lower than line level. If you plug your bass directly into a microphone preamplifier you will overload it but if you connect it directly to a compressor that is expecting line level your signal will be really weak. Instrument signals are also almost always single ended or unbalanced.


Before we discuss how the DI box addresses the above conundrum, we need to have a quick chat about impedance. I’m going to try to keep this digestible because understandably the mere mention of impedance can cause the eyes of musicians and producers to glaze over. If you’re already falling asleep feel free to skip the next two paragraphs and you will still understand most of the reasons a DI box is required!


All audio signals are alternating current, and impedance is a circuit's ability to oppose alternating current, measured in Ohms (Ω). To get a signal from A to B we actually don’t need to move much current, what we need to transfer is the voltage. So, many moons ago very clever people discovered that if you make the transmitting device (output) exhibit a very low impedance, and the receiving device (input) have a higher impedance we get a very good voltage transfer with no loss of high frequencies. Typical modern output impedance is around 600Ω and typical input impedance is around 10kΩ.


This isn’t completely standardized, and of course the microphone preamplifier and the electric guitar are once again exceptions. The construction of a guitar pickup results in an unavoidably high output impedance output; 15kΩ or more. The input of the mic preamp, however, expects a relatively low impedance signal.


To recap, we now have three problems to solve when it comes to plugging a guitar into a preamp.


  1. The instrument produces a signal that is much louder than the microphone preamplifier is expecting.
  2. The instrument produces an unbalanced signal and the microphone preamplifier is expecting a balanced one.
  3. The output impedance of the instrument is way too high to sound good when loaded by the low input impedance of the microphone preamplifier.

The humble DI box is the solution to all three problems. Using a special type of high quality transformer, or some active circuitry, it creates a really high impedance input (100kΩ-1MΩ) so that the instrument is loaded properly and high frequencies are retained. It then attenuates that signal significantly, so that the level doesn’t overload the microphone preamplifier. Finally it takes that single ended or unbalanced signal and generates a balanced one. This means that even if the DI is very far away from the microphone preamp, it won’t be susceptible to picking up noise along the way. Just like that all of the issues your mic preamp had with your instrument are resolved and everyone is getting along.


Through (Link) Connectors


In many instances, such as live performance the instrumentalist may wish for their signal to be routed simultaneously though a DI and an instrument amplifier. For this reason many DI box manufacturers add an extra jack labeled “through” or “link”. The instrument should theoretically be connected to the DI input, and the amplifier to the link jack, however in practice it doesn’t matter which way around you use them, as the link is nothing more than a passive split of the input. The two jacks are literally just joined together internally.


Closing Remarks


If you’ll indulge me in one last tangent before signing off, I’d like to gush a little about my favorite electronic component. There is just one component that can perform all three of the above functions. It attenuates, it balances and it impedance matches, and that component is the audio transformer. It still amazes me to this day what an elegant solution a transformer is in a passive DI box and I take great pride in having scoured the earth and exhaustively tested to find the transformers that make our DI boxes sound so great.

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