The musician faced with a vast variety of microphones, mixer sockets, cables and instrument outputs often needs to be able to match and stabilise levels before he can use any of them. When working on stage with large numbers of microphones - for instance those on a large drum kit plus vocal mics, plus backline mics - it becomes a problem to achieve a reasonable PA mix if line level outputs from keyboards also need to be taken into account. In the studio it's often necessary to adjust levels, input impedances, or to split signals, before the mixing desk and associated effects can be used to their full. Unfortunately with this sort of wiring complexity problems with earth loops and line interference often occur.

Technical Projects aim to solve these problems with a range of four handy black boxes. Three of these are 'balancing boxes', designed to balance long cable runs, break earth loops and provide safety insulation, the fourth being an Active DI box. Three of the range are examined here: the GP (General Purpose); Hi Z (High Impedance) and the Active DI.

Construction of each of the boxes is similar, with the exception of the fact that the DI box has a battery compartment, being the only active design of the three. Each model measures 4½ X 3½ X 2" and weighs about one pound. The casings are extremely tough, formed from extruded aluminium, with aluminium front and back panels. XLR sockets are in chromed metal and ¼" jack sockets in black plastic. Two rubber strips along the base of each unit should also help keep the boxes in place on stage.

DI 100

This unit has 2 inputs; instrument and amplifier, which are both standard ¼" jack sockets. Power is connected to the internal amplifier when a jack is inserted into either socket. If the mixer to be connected is phantom powered the internal regulator will take a feed from this, otherwise a PP3 battery should be inserted in the battery compartment. Input signals are fed to a unity gain buffer which has an input impedance of 10 Megohms. If the amplifier input is used the signal is attenuated by 45dB. The output of the buffer is passed through a 'padding' attenuator, which can be switched in to provide a further 20dB cut, before being connected to the isolating transformer. The secondary of the transformer, coupled in a balanced configuration, is connected to an XLR socket for connection to the mixer desk.

A second output, via another ¼" jack socket, is provided for connection to another amplifier, speaker or second mixing desk. This can be connected to a second primary on the transformer to give a buffered, isolated version of the input or can be switched in parallel with the input, ie. direct.

The circuitry also contains a special ground compensation circuit which makes 'ground lift' switches obsolete.

Lo Z and Hi Z

These boxes are designed to be used as sending and receiving ends of a balanced line system. Two channels are provided in each which can be used separately, or as a stereo set-up.

Lo Z can be used for equipment outputs at the sending end as it takes low impedance inputs such as balanced mics or unbalanced instruments and provides a transformer coupled balanced output.

Hi-Z can be used for equipment inputs at the receiving end as it accepts a balanced line input and provides a transformer coupled output which can be used to drive high impedance inputs such as mixer line or microphone inputs.

Standard XLR connectors are used on each channel and a 'ground lift' switch is provided to break the path between the ground lines, thus isolating each side.

GP Box

The GP box is optimised for low impedance operation but can be used in most situations where isolation is required for safety, or to break hum loops. Again, XLR connectors are used for each channel and a 'ground lift' switch is provided.

Conclusions

Internally, these units are very well constructed with all of the components mounted on resist-coated PCBs. When the front and rear panels are screwed on the case is, as the manufacturers claim, virtually indestructible.

Technical Projects have provided the musician with professional, compact, high quality products which should cure most of the problems associated with equipment connection.

DI 100 £60; GP £55; LoZ £89; HiZ £79.

Further details from Theatre Projects/Court Acoustics Sales Ltd., (Contact Details).