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Direct Inject Box

Article from Electronics & Music Maker, April 1981


The Direct Inject Box (D.I. Box) allows the signal from an amplified instrument to be fed directly into a balanced line mixing desk, and as such is invaluable on stage, and in the home or professional recording studio avoiding many of the disadvantages of using a microphone. It is much cheaper to build the D.I. Box than to buy a good microphone, and it eliminates acoustic feedback and 'spill-over' of other sounds into the instrument channel.

The Circuit



The D.I. Box takes its input from the instrument amplifier and converts it to a balanced line output at microphone level. Figure 1 shows the circuit employed. The input signal is fed via a switchable attenuator to the 47k potentiometer. This means that the box has an input impedance of about 47k when used as a low level (line) input and over 700k in the speaker level mode. As shown the input is dc coupled and if a dc offset appears on the input the potentiometer will be noisy as it is moved. If this occurs a 470nF polyster capacitor should be connected in series with the input.

Two J-FET op-amps, chosen mainly for their very low power consumption, form the phase and antiphase generator. The first op-amp inverts the signal and divides its level by 4, whereas the second op-amp merely re-inverts the output from the first. The two outputs are thus of the same level but exactly out of phase and can be used directly. A 100R resistor is included in each output as a protection against short circuits, and a capacitor is obviously required to block the dc level. The op-amps are biased to half rail by R10 and R11 [Errata: Should read "R9 and R10"] which hold the non inverting inputs at 4.5 volts and R3 which provides a dc offset for the input signal. Diodes D1 and D2 protect the op-amp in the event of severe overload, and play no part in the normal operation of the circuit.

A single 9 volt battery is used to power the circuit. This is switched in the usual way by using a stereo jack socket on the input.

Construction



A printed circuit board holds all the resistors, capacitors and semiconductors except R1 and RV1. Mount and solder the components and Veropins on the PCB, with IC1 left to last. Bolt the PCB to the lid of the box and the connectors, pot, and switch to the base. If a box other than the one recommended is used, check that it is deep enough to take the chassis Cannon plug. Solder R1 and the battery connector in position, and wire up the connections to the PCB using screened cable.

The prototype used a small piece of polystyrene foam glued to the lid above the battery position to hold the battery in place. Finishing consisted of lettering and varnishing the front, and sticking four rubber feet on the bottom.

Operation



Use a jack-to-jack to connect the D.I. Box to the extension speaker socket on the amplifier or speaker cabinet, or the amplifier slave out jack, remembering to set the high/low switch accordingly. If an additional speaker output jack is not available, a jack-to-two-jack splitter lead can be used to connect the D.I. Box input in parallel with the speaker cabinet. RV1 should be adjusted for a convenient signal level to the desk.

Unlike direct injection of the instrument output or preamplified signal, the D.I. Box passes the full sound of the amplified instrument, including the effects of tone controls, signal processors, and amplifier distortion (the latter is often an important part of guitar sound) from the speaker outputs to the mixing desk for recording or amplification by the group P.A. Alternatively, the unit can be connected to the 'Slave Out' or 'Link' jack socket of the amplifier avoiding the distortion of the output stage. This is particularly useful for amplifiers which are also used as sub-mixers e.g. with keyboards, since output stage distortion is especially noticeable on a mix of different signals.

It is important to note that the D.I. Box design that follows is intended for mixers with balanced line inputs. If the mixer in question does not have such inputs it is debatable if the D.I. Box is worth using; a simple wire connection being the easiest. If hum problems do occur, or particularly long connections are required, better results will be obtained with the E&MM Line Driver/Receiver presented in last month's issue. The Balanced Line System, although designed for microphone use, will handle signal levels of up to 400mV without any trouble.

Obviously some instruments cannot be D.I.'ed — the most notable example being an organ with a Leslie cabinet. The D.I. is also a matter for personal opinion, indeed many claim that the sound produced is too dry. Additionally, problems will occur if tonal adjustments are made before the D.I. connection to compensate for a poor speaker cabinet. It is, however, a much under-rated technique, offering several advantages — cheaply.

Figure 1. Circuit Diagram of the D.I. Box.
(Click image for higher resolution version)


Figure 2. Internal Wiring of the D.I. Box.
(Click image for higher resolution version)


Figure 3. The D.I. Box PCB.
(Click image for higher resolution version)


Figure 4. XLR Cannon-to-jack lead connections.


Internal view of the D.I. box.


PARTS LIST

Resistors — all 5% ⅓W carbon unless specified
R1 680k (M680K)
R2 1M0 (M1M0)
R3 220k (M220K)
R4,5,6 56k 3 off (M56K)
R7,8 100R 2 off (M100R)
R9,10 100k 2 off (M100K)
RV1 47k log. pot. (FW24B)

Capacitors
C1 100n carbonate (WW41U)
C2,3,4,5 22u 16V tantalum 4 off (WW72P)

Semiconductors
IC1 LF353 (WQ31J)
D1,2 1N4148 2 off (QL80B)

Miscellaneous
PL1 XLR chassis plug, 3-pin (BW92A)
S1 Sub-min toggle 'A' (FHOOA)
JK1 Jack socket, stereo (HF92A)
B1 PP3 battery
Printed circuit board (GAOOA)
Case PB1 (or alternative) (IF01B)
Veropins, 1mm (FL24B)
PP3 connector (HF28F)
Knob R52 (or alternative) (HB29G)
Twin screened cable 1m (XR21X)


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Car Ignition Timing Strobe

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Power Controller


Publisher: Electronics & Music Maker - Music Maker Publications (UK), Future Publishing.

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Electronics & Music Maker - Apr 1981

Feature by Chris Lare

Previous article in this issue:

> Car Ignition Timing Strobe

Next article in this issue:

> Power Controller


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