If you’ve ever rocked the stage or studio, chances are the sound technician kindly requested a clean signal, directly from your guitar or bass. There’s a plethora of reasons why running a 50 meters long instrument cable just won’t work. Solution, a D.I. box.
In it’s most basic form, a D.I. or Direct Input box will accept the jack from your instrument and spit out a balanced signal through it’s XLR connector. This output signal is suited for transport over long cables, directly into a mixing console or recording equipment. But why exactly?
The first reason is the (bass)guitar’s ‘output impedance‘. Everything within the realm of electronics with an ‘output’, comes with a property called output impedance. Whether it’s an amplifier, battery, microphone or (bass)guitar. The higher the output impedance, the more opposition a current flow (e.g. audio signal) experiences to ‘get out’ of the device. A high output impedance can be problematic because;
- It’s more difficult to interface with a receiver. Connecting to a low input impedance, like a microphone input, results in severe signal loss. The reason being that the out- and input impedance together form a voltage divider (explained here). Rule of thumb: low output- into high input impedance = good, high output- into low input impedance = bad. Added distortion and high frequency degradation are other possible detrimental effects.
- It’s prone to pickup noise and outside interference along the way. Input impedances are usually high as well. Noise entering the cable is basically trapped between the in- and output device and will find it’s way onto the signal.
Passive (bass)guitars have a very high output impedance, making them horrible at driving long cables. Instruments with piezo pickups are even worse, This is also the reason most instrument amplifiers have a high input impedance of 500 kOhm or more, with 1 Mohm being an industry standard. Instruments with a build-in preamp, also called active, tend to have a low output impedance, overcoming some of the issues associated with passive, but you’re still left with an unbalanced signal.
Unbalanced v.s balanced
A perfect example of an unbalanced signal connection is the well known instrument cable. The signal passes through the single core conductor. The outer mantel connects the grounds (or return). The mantle also doubles as a shield to stop outside interference from reaching the inner core. A properly shielded cable is always a good idea, but it will not stop all interference. Some will get through.
Another disadvantage is that any interference picked up by the shielding ends up in the ground of the amplifying electronics. It’s sometimes falsely believed this doesn’t affect the signal itself, since ground always retains a 0 volt potential, no matter what you throw at it. But even ‘ground’ has some impedance. Depending on the amount of interference and design of the equipment, a voltage can develop. Result; noise.
Balanced signals and cables take a different approach. They use a second signal wire, dedicated to completing the electrical circuit. The mantle is now solely used for shielding. The signal going through the second conductor is the exact opposite of the first. The receiver (mixer, recorder, etc.) will amplify the difference between these two signals. Hence the synonym; differential signal. This will yield an extra amplitude of 6dB, increasing the signal to noise ratio. It also makes it immune to interference picked up by the cable. Interference will be the same on both conductors. Since only the difference is amplified, the interference is ignored.
Galvanic isolation is the last on the list of reasons for using a D.I. box. Imagine a stage with hundreds of meters of audio cabling, power lines, lighting control, you name it. Your better of isolating yourself from this jungle for obvious safety reasons. Galvanic isolation, through a transformer in most cases, will prevent any serious current flow between you and the rest of the stage. DC current (technician accidentally turned on phantom power) is completely blocked altogether. A huge benefit of transformer D.I. boxes (like my upcoming Sidekick) compared to solid state counterparts.
The grounds of the transmitting and receiving side are possibly still directly connected. Let’s assume the XLR cable shielding is connected to both devices through pin 1. It is quite common to have a relative voltage potential on different ground points. Different electrical groups, faulty equipment or just bad grounding schemes are to blame. We now have a closed loop through power-earth, with a voltage potential and an inevitable impedance which is growing with cable length. This is what we all know as a ground loop, generating horrible noise and humm. Breaking the circuit is your only option. The ground lift switch on a D.I. box will do just that. No closed circuit means no current can flow so no noise is generated. This is only possible with a balanced circuit though. An unbalanced circuit, again, uses shielding and ground through the same conductor.
D.I. with benefits
I believe a D.I. should be kept simple. It’s a tool used in cluttered, busy situations. Establishing rock-solid, isolated connections from your (bass)guitar to the rest of the world. That doesn’t mean it has to be a boring, featureless box though.
Tubes and (isolating) transformers go together like peanut butter and jelly. It made a lot of sense to use our favorite amplifying element, the tube, to create the Sidekick as my ultimate, active D.I.
An active D.I. will amplify the signal somewhat to achieve an even better signal to noise ratio. Plus, it will make the performance of the signal chain less dependent on the preamps found in mixing consoles. An active D.I. can also achieve a higher input- to output impedance ratio while maintaining the same, or higher signal levels. This results in an even better signal transfer.
Loads of artists use an amplifier to shape their distinctive sound. Connecting your (bass)guitar directly to a D.I. however will lose that part of the signal chain. A microphone in front of your speaker is your only option, but can be problematic. Picking up surrounding audio, stage rumble, and considered a ‘hassle’ by many for small to medium gigs. The Sidekick allows you to connect the speaker output of your amplifier and use it the exact same way as you would with your (bass)guitar. This includes every bit of character between your instrument and speaker.
Compacted into one
To sum up the above, you start with an unbalanced, high impedance source and end up with a balanced low impedance output. Capable of running over hundreds of meters of cable and isolating itself from any outside influence. That’s a D.I. box in a nutshell. Add a tube in the mix plus some special features and unprecedented low noise levels and you get a deBont Amps Sidekick.