How a Passive Direct Input Box Works December 15, 2019 00:00
If you’ve spent even a little bit of time in a recording studio or doing live sound, you’ve undoubtedly worked with a DI box. But what, exactly, do these little boxes do and why do we need them? In other words, why can’t you plug a guitar right into a mixing board or audio interface?
A direct input box is used when we want to connect an unbalanced, instrument output to a balanced, microphone preamp input. The most common scenarios for this are plugging bass and keyboards into the mixing board for live sound, or recording bass and guitar through mic preamps in the studio. DIs almost always have these features:
- Unbalanced, ¼” input
- Balanced, XLR output
There are two types of DI boxes: active and passive. Active DIs run on DC power, usually from the +48v phantom power from the preamp connected to the output, because they feature active components like transistors and integrated circuits. Passive DIs, the kind we’re talking about here, require no power and feature only a couple passive components.
DI boxes were created to solve a particular set of problems that arise when connecting instruments to pro-audio equipment.
One of the first things you learn (very often the hard way) is you can’t just plug anything anywhere. The most obvious and dangerous example of this is shown below.
But why not? Incompatibilities come down to three main things:
In the case above the answer is obvious: voltage. If you plug your guitar directly into the wall, the guitar will be hit with 120v mains power and something will explode or melt. In most cases the differences aren’t so obvious, but they’re just as real.
What makes passive DI boxes so elegant and simple is that they utilize a single component to solve all these problems: a transformer. A transformer is a large, primitive analog component that ought to be obsolete except for the fact that it can do several things at once all while sounding great.
A transformer’s construction is very simple: it’s a single magnet with two wires wrapped around a metal core. The wires themselves do not touch each other, yet signal transfers between them through the principle of electro-magnetism. That is, an alternating current (which in this case carries audio signal) in the first wire generates a magnetic field in the core, which in turn generates a corresponding current in the second wire. These wires are called the primary and secondary windings. In the following sections we’ll see how this simple component can do so many things by showing how it solves the three problems we identified above.
Instrument output levels span a very wide range from the millivolts up to ~10 volts. Preamp input stages, on the other hand, are optimized for the very low output levels of microphones, which rarely exceed the millivolt range. So the transformer must solve the problem of too much voltage.
The coils in a transformer have specified number of “turns” around the core, and the ratio between the turns of each coil is the most important feature of any transformer. This is because a transformer reduces or increases the voltage of a signal in direct proportion to the turns ratio. That is, if the primary coil has 200 turns and the secondary has 100, the turns ratio is 2:1, and the voltage induced in the secondary will be half of that in the primary. The standard turns ratio for a DI transformer is 12:1, so the output is always 12 times lower (-21.5dB) than the input. Now the mic preamp is happy and won’t be overdriven by the instrument’s output voltage.
Every device has its own inherent input and output impedance. Without going too deep into the wormhole of explaining the concept of impedance, it’s important to know one general rule of thumb: when connecting two audio devices, the input impedance should be at least 10x the output impedance. When this rule isn’t followed, things start to sound muddy, noisy, and distorted.
So for example, the output z of a dynamic microphone is around 250 Ohms. This is why the input impedance of mic preamps is usually at least 2.5k Ohms. Similarly, guitars usually have an output z of between 10k-100k Ohms, so guitar amp input z is usually 1M Ohms. Now look what happens if we connect a guitar directly to a mic preamp: the output z is significantly higher the input z.
In the same way that our transformer induces a different voltage across the secondary than the primary coil, it also transforms impedances. But whereas voltage is transformed in direct proportion to the turns ratio, impedance changes by the square of the turns ratio. Thus, the impedance ratio for our 12:1 transformer is 144:1.
Say we are hooking up a guitar with a 20k Ohm output impedance to a mixing board with a 2.5k Ohm input impedance for the mic preamps. The transformer divides the 20k output z by 144, giving us an effective output z of 138 Ohms. Now we are nicely in the range of our rule of thumb; the input z of 2.5k is at least 10x higher than the output z of 138. (You can also run the math the other way: the transformer multiplies the 2.5k input z by 144, giving us an effective input z of 360k, which is more than 10x greater than 20k.) Now we can be confident that the full signal will pass from the guitar to the console without distortion or other artifacts.
There’s a great saying that “all electrons want is to go to the ground and die.” This is a way of expressing that current always flows from high voltage to lower, and the ground is our 0 volts reference point. This is why the safety pin of the outlets in your house are wired to a pipe in the ground (check your basement!)—it gives electrons a quicker path to the ground than through your body. However not all “grounds” are a perfect 0 vots, and ground problems arise when two connected pieces of equipment have different ground voltages or when noise doesn’t have a quick path to ground without infecting the audio circuitry. These are the main causes of hum and buzz in the studio and live sound.
Pro-audio equipment addresses ground problems by using 3-pin, balanced connections where the ground and audio paths are separated. In a standard XLR cable, for example, pin 1 is ground (also known as “chass) and pins 2 and 3 are signal + and -, respectively. A well-wired studio with only balanced connections can be practically free of ground noise.
Instruments, however, use 2-pin, unbalanced connections which do not allow for isolating the ground and signal. In a standard ¼” guitar cable, for example, the tip carries the signal, while the sleeve carries the ground.
So what happens if we plug an unbalanced guitar directly into a balanced input? As you can see in the illustration below, the sleeve of the unbalanced guitar cable will connect to both the chassis and signal - parts of the balanced input. This provides a path for current to flow between the ground of the unbalanced system and the signal - of the balanced system, in other words noise, hum, and buzz.
Modified and reused under the Creative Commons Attribution-Share Alike 3.0 license.Original rendering by Søren Peo Pedersen
Transformers solve this problem by providing what's called galvanic isolation between the input and output stages. Recall that transformers transfer voltage from one winding to the next through electromagnetic induction, without providing a path for current to flow. Thus, they allow the balanced output stage to communicate the signal to the unbalanced input stage without allowing ground currents to form. That's it. An elegant solution to a complex problem.