Direct boxes (also known as DIs, which stands for “Direct Inject”) began as a way to resolve a basic impedance mismatch between electro-dynamic guitar pickups (high impedance unbalanced) and sensitive studio electronics (quite low impedance balanced). Still today the primary function of DI boxes is to take an unbalanced, high-impedance signal and convert it to a balanced, low-impedance signal. This allows to send audio signal over extended cable runs without losing volume, high-frequency information and without picking too noise easily. DI boxes are essential pieces of stage equipment.
Mixing-console mic inputs in a live-sound context typically expect to handle signal levels from maybe as low as -55dBu, up to about +5dBu. Microphones generally have a very low source impedance of about 150Ω, from a balanced source, and expect to see a load impedance of about 1.5kΩ or higher. The output of an electric guitar is unbalanced, with a typical level range of between about -30 and 0dBu, perhaps going up to +10dBu for active basses. The source impedance is relatively high, at between 5kΩ and 20kΩ for normal electric guitar pickups (can be higher for special pickups and piezo). The load impedance to which electrical guitar should see should be much higher than it’s output impedance, anything up to 1MΩ is common. The load impedance can affect electrical guitar sound. Usually, the instrument to be DI’d will be connected to a local amplifier/speaker combo on stage, so some kind of signal splitting and feed-through provision is generally required.
Modern passive DI boxes typically use a transformer to convert high-impedance signal to low-impedance balanced signal. This style of transformer features electrically separate windings in the input and output stages, which can isolate ground-level voltages and eliminate ground loops. Typically a switchable ground lift lets you disconnect Pin 1 on the XLR jack of the DI box. Some DI boxes feature a switchable attenuator (called a pad) and/or “phase” switch to polarity reverse audio signal. Usually the input impedance of passive DI boxes can be up to few tens of kilo-ohms (up to about 150kΩ). Passive DIs typically are stepping down the signal voltage by about 20dB in unbalanced-to-balanced conversion.
An active DI includes a preamplifier which can provide a very high input impedance and also can usually provide an extra gain to boost the weak signals. Active DIs requires power, which can come via batteries, dedicated power supplies, or 48V phantom power, depending on the model. A good active DI can presents a high-impedance load (>250kΩ) to an unbalanced instrument source. Active DI boxes employ an active electronic buffer circuit at the instrument input to present a very high input impedance to the source instrument — typically 1MΩ or so. This allows the guitar pickups to work as intended in delivering the expected tonality and sustain.
The differences between DI boxes basically come down to build quality and ruggedness, the design of the electronic circuitry (especially the power supply arrangements), the features and facilities included, the headroom margin, and the quality of the transformer.
As direct boxes have evolved and many similar tools have entered the market, a number of devices similar to direct boxes have become available. This group of devices include in-line preamps, attenuation pads, isolation transformers and re-amping boxes (opposite function of DI).
Tutorial: DI Boxes & You
DI boxes part 1: What does a DI box do, and when do I need one?
DI boxes part 2: Active vs. Passive
DI boxes part 3: Amplifier DIs – using, advantages and disadvantages
DI Boxes Part 4: Stompbox/Pedal DIs
DI Boxes Part 5: Active vs. Passive revisited
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Using a DI Box for Acoustic Guitars – AudioTech
How to hook up and use a D.I. Box with acoustic guitar setups.
Radial-JDX-explained.mp4 – Peter Janis talks about how the JDX works and why.
Peter Janis talks about the Radial JDX amplifier direct box with speaker emulation.
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How to Choose a DI: Active vs. Passive
Guitars and Gear Vol. 42 – Radial PZ-DI Piezo-optimized Direct Box Demo
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I have several active DI boxes, and the Behringer ULTRA-DI DI20 is the worst one. Too noise, to much hiss, even using batteries (easily audible in a simple DI comparison). So it results in a very very poor signal / noise ratio.
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Radial Engineering’s USB-Pro makes it easy to connect to your computer, iPad or iPod to a PA system. You simply connect via the USB port and the USB-Pro delivers stereo balanced outputs via the dual XLR connectors.
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