DI box
The DI box (abbreviation for D irect I njection ) is an in -tone technique used apparatus, comprising an unbalanced signal into a balanced signal is converted.
How it works and uses
DI boxes are required when a signal source with an unbalanced output (usually coming from a jack or cinch plug) is to be connected to a device with a balanced input (usually via an XLR plug). Typical examples of unbalanced signal sources are guitars with built-in pickups ( electric guitar , electric bass , acoustic guitar ), keyboards and other electric instruments or CD players ; Common symmetrical inputs are found on mixing consoles or audio interfaces . The symmetrical signal routing enables an effective reduction of interfering signals, especially with long cable lengths . The ground connection can also be separated (ground lift), for example to interrupt hum loops . (This does not always work satisfactorily, because DI boxes can be found that separate the signal ground but not the housing ground; in connection with XLR cables in which pin 1 is bridged with the connector housing, this leads to despite the DI box Ground loops.) In addition, DI boxes can match the impedances of signal sources to those of the subsequent inputs .
Building types
DI boxes are available both as individual devices in small, box-shaped housings (see picture) and in the form of plug-in units for 19 "racks , which then contain several channels.
A distinction is made between passive and active DI boxes. The former achieve symmetry solely through a broadband low-frequency transformer , the latter contain active amplifier electronics. The power supply then required is provided by battery or phantom power .
Advantages and disadvantages of the two variants
With active DI boxes, any high input impedances and any small output impedances can be realized, while the input impedances of passive DI boxes are at most in the order of 500 kΩ. Another advantage of active DI boxes is that active balancing avoids the disadvantages of transformers. High quality transformers are quite expensive, depending on the design, they are sensitive to magnetic interference and can have a harmonic distortion factor that cannot be neglected (especially at low frequencies and higher levels) . They also offer a limited bandwidth compared to active balancing. However, transformers enable real potential separation , which is why there are high-quality, active DI boxes with transformer balancing.
The advantages of passive DI boxes are that they do not require a supply voltage and, due to the transformer, electrically isolate the signal. In principle, they can also be used the other way round, i.e. to convert a symmetrical into an asymmetrical signal. An application in which a DI box is used in reverse is called re-amping . A guitar or bass track that has already been recorded is subsequently played back via a guitar or bass amplifier and recorded again.
In practice, active DI boxes are used for signal sources with high output impedance and low level ( e.g. piezo pickups ) because of their high input impedance. On the other hand, a high-quality passive DI box is sufficient behind an instrument with a line output level.
Special variants
Normally, a DI box should not cause any discoloration of the sound, but rather transmit the signal as accurately as possible. However, special DI boxes with sound filters for electric guitars have a special position . These contain a sound filter that is supposed to mimic the characteristic frequency response of guitar speakers. The background is that guitar loudspeakers make a significant contribution to the sound of a guitar, but this contribution is missing if the signal is not picked up with a microphone in front of the loudspeaker. As a rule, such DI boxes are looped either between the amplifier and loudspeaker, or in the effects loop.
This solution has advantages and disadvantages and is therefore controversial. The sound of the electric guitar can be influenced by the selection and position of the microphone. The reproduction of the loudspeaker sound is also not always satisfactory, especially at very high playback volumes. On the other hand, if microphones are not used, the problems with crosstalk and feedback on stage can be reduced.
With the electric bass, however, an active DI box is usually used, because the sound coloring by the loudspeaker plays a rather minor role here.
literature
- Hubert Henle: The recording studio manual. 5th edition, GC Carstensen Verlag, Munich, 2001, ISBN 3-910098-19-3
- Roland Enders: The home recording manual. 3rd edition, Carstensen Verlag, Munich, 2003, ISBN 3-910098-25-8