Magazine Archive

Home -> Magazines -> Issues -> Articles in this issue -> View

DIY Direct Inject Box

A high quality DI box circuit


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.

Finished DI Box.


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.

Printed circuit layout.

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.

Performance

Input:Unbalanced 470Kohm. Max level -10dBu.
Output:Balanced 75ohm. Max level +15dBu.
Gain:28dB approx with values shown. (To reduce gain, increase R4).
Frequency Response:+1 -2dB 20Hz to 18kHz.
Distortion:Within 0.1 %.
Noise:104dB below input.
Output noise:-81dB. RMS 20Hz to 20kHz.


Circuit for DI Box.
(Click image for higher resolution version)



Previous Article in this issue

Realistic PZM Modification

Next article in this issue

The Units - On Record


Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.

 

Home & Studio Recording - Mar 1984

Previous article in this issue:

> Realistic PZM Modification

Next article in this issue:

> The Units - On Record


Help Support The Things You Love

mu:zines is the result of thousands of hours of effort, and will require many thousands more going forward to reach our goals of getting all this content online.

If you value this resource, you can support this project - it really helps!

Donations for August 2020
Issues donated this month: 0

New issues that have been donated or scanned for us this month.

Funds donated this month: £34.00

All donations and support are gratefully appreciated - thank you.

Please Contribute to mu:zines by supplying magazines, scanning or donating funds. Thanks!

Monetary donations go towards site running costs, and the occasional coffee for me if there's anything left over!
muzines_logo_02

Small Print

Terms of usePrivacy