Many musical instruments have electrical audio outputs that are not ideally suited to direct connection to a mixing desk. In order to get the best possible quality and 'match' to the mixer input, an interface circuit is required.

Direct inject boxes normally contain either a matching transformer which converts the medium impedance of the pick-up down to microphone level, low impedance, or a simple solid state amplifier which effectively does the same job.

Advantages

The circuit here has some significant advantages:

1) The output from a pick-up, while not being particularly low, likes to work into a very high impedance input. Any shunting of the pick-up tends to 'muddy' the sound, losing clarity and depth. This circuit gives an input impedance of 470K ohms which is a significant improvement over transformer DI units.

2) The ICs used are ultra low current devices which consume practically nothing when no signal is present. This allows the use of a low capacity battery.

3) The output stage gives a real 6dB advantage in output level giving peak output voltages of nearly twice the battery voltage.

4) A transformer output gives total freedom from hum problems and is designed to operate into either a low gain mic input of any type, or any line input (that has a gain of about 20dB).

5) Component values around the circuit give it an excellent noise performance.

Circuit Description

Input from the musical instrument is via a standard 3 pole jack which serves as the input termination and the on/off switch.

IC1 is connected as a conventional voltage amplifier with a gain of (R4 + R5)/R4 at audio frequencies. Its output impedance is controlled by R6 and forms one half of the audio drive to the output transformer, the other half being supplied by a unity gain inverter amplifier, IC2. High frequency stability of the transformer is controlled by R9 and C7 and the audio output meets the outside world via an XLR connector.

Power is supplied by the PP3 battery: the mid rail reference voltage is set by R1 and R3. The purpose of the diode is to avoid any possible damage to the input devices within the TL061s by the mid rail straying above the +ve voltage when the circuit is being switched off.