The Brooke Siren Systems 'High Performance Direct Inject Box' is one of the most versatile and reliable units currently available, effectively accommodating the entire range of DI techniques in one compact and rugged unit.

Direct injection at its simplest just involves taking the output from an instrument directly into the microphone input of a mixer; by cutting out the combined effects of an amplifier, loudspeaker and microphone, a clean, accurate, low-noise signal is ensured, with perfect separation as obviously no crosstalk or sound spillage is possible.

A DI box is needed in order to correctly interface the normally high impedance output of an instrument with the low impedance microphone inputs found on most mixers. In most early designs this matching was accomplished simply by a transformer, with a high impedance primary winding matched to the instrument, and a low impedance secondary to feed the mixer.

However, this system has its limitations, for a typical transformer with a sufficiently low output impedance will usually have an input impedance that is too low to completely avoid 'loading' effects, which can cause serious signal deterioration. Transformers also tend to be susceptible to external magnetic field pick-up and are said to lack transient response and suffer from other distortions, and whilst the highest quality transformers undoubtedly overcome some of these problems, there is a general trend towards electronic, 'active' DI boxes, which have none of these performance limitations.

Facilities

The BSS active system uses a low-noise electronic input amplifier to achieve a very high input impedance, with a transformer output stage for a balanced, fully floating output that will operate into 600 ohms or more. The front panel features two ¼" jack sockets and the XLR output connector, in addition to a number of small recessed switches. The jack socket marked Input acts as the unit's On/Off switch, thus avoiding the possibility of unnecessary battery drain during transit, while the second jack provides a convenient parallel-wired link output for an amplifier to be used simultaneously. Although the outer casing is made from tough ABS material, all the circuitry is housed within an inner steel box, which provides excellent screening against mains induced hum and RF pick-up. In combination with the heavy gauge steel front panel, this makes for a virtually indestructible unit that is equally suited to use on stage or in the studio.

A three position input attenuation switch offers comprehensive signal matching facilities. The 0dB setting is for use with very low level sources such as guitar pick-ups; the very high input impedance offered by this switch position ensuring complete freedom from 'loading' problems. Magnetic pick-ups suffer both a loss of level and a severe curtailment of frequency response when 'loaded' with an input impedance that is too low. However, the BSS unit's electronic input amplifier is able to present an input impedance of 1 Megohm (one million ohms), which is significantly higher than simple transformer systems can offer, and which maintains optimum signal quality.

For higher output instruments which include onboard preamps, such as 'active' guitars or keyboards, the —20dB attenuation setting can be used, offering an input impedance of 47Kohms. This is 'line level' sensitivity and interfaces well with the 'preamp output' found on many instrument amplifiers - a preamp DI signal, of course, includes any equalisation used on the amplifier, which can be an advantage when this is contributing significantly to the player's sound.

The -40dB setting is for use with the very highest level sources, offering the useful facility for taking a DI signal from a power amplifier output.

A two position low pass filter is provided, which can introduce high frequency roll-off centred on either 8 or 4kHz. The filter, which is operative only with the —20 or -40dB attenuation settings selected, has a very gentle 6dB per octave slope. The 8kHz setting can be used quite effectively to make hiss less evident with little detrimental effect on the signal, although the 4kHz corner frequency is perhaps best reserved for bass instrument use, where the curtailment of HF response may be less noticeable.

Earth-Lift

An 'earth lift' facility is essential for many direct injection techniques, providing a means of avoiding the problems caused by multiple earth paths; the characteristic 50Hz hum of an 'earth loop' can arise whenever more than one route to earth is present in the recording system, such as when an instrument is connected to its own, earthed, amplifier, but then also connected to the recording system's earth through a DI lead. The BSS unit's 'Earth Lift' switch can be used in these circumstances, to separate its input earth from its output earth, eliminating the problem by leaving each system seeing just a single path to earth.

The Phase Reversal facility provided is most useful when a signal is picked up both by a microphone and through direct injection; the two signals need to be 'in phase' to prevent cancellation occurring when they are used in combination, and the switch can be used to reverse the phase of the DI output to match a 'rogue' microphone or compensate for an incorrectly wired lead. In 'live' recording and PA work DI signals from instruments using loud backline amplification can sometimes usefully be employed with the opposite phasing to the frontline mics, as this can help to reduce sound spill to some extent, by means of phase cancellation.

Powering

Battery or 'phantom' power can be used, with automatic switching governing the choice of supply. Main and Standby battery compartments are provided for two 9V PP3 type batteries, with a nominal life of around 200 hours. When the voltage of the main battery falls to a level that would degrade the performance of the system, the unit switches to the standby battery to ensure continued operation at full specification. The 'Battery Low' LED will flash to give warning of this condition, but although the standby unit will last almost as long as the main battery, it is advisable to replace an inoperative cell at the first opportunity in order to preserve the obvious benefits of the back-up facility.

The convenience of phantom powering is available, by means of a small adapter unit which can be fitted into the main battery compartment. If a battery is still kept in the standby position, the automatic switching is able to ensure continued operation even if the phantom supply is interrupted, as well as facilitating use where no phantom is available. By using a DC to DC converter, the phantom powering adapter is still able to maintain full isolation between the input and output earths when the Earth Lift facility is in use, and therefore does not degrade any aspect of the system's operation.

Conclusion

The Brooke Siren Systems DI Box is a high performance, professional unit, with many features and operational qualities to justify its reputation as the 'state of the art' in its field.

The rugged construction and standby powering system certainly enable this unit to be used with absolute confidence in the most demanding applications, whilst the low-noise 'active' system is able to ensure optimum signal transfer quality with all types of input. The additional facilities of phase reversal and low pass filter are of genuine practical value, and the system as a whole performs excellently in all respects.

Although a DI box is only required to perform a fundamentally simple task, this unit's performance goes beyond the basic requirements to offer perhaps the most refined and complete system currently available.

The BSS DI Box lists at £66 ex. VAT.

Further details from Brooke Siren Systems, (Contact Details)