Publication : 04 oct 2018
A DI-Box is a device usually used between a musical instrument and a mixing desk/console.
Its main role is to transform an asymmetric input signal of high impedance (e.g. from a guitar pickup) to a symmetric signal of low (or medium) impedance.
The conversion between an asymmetric/symmetric signal aids in blocking unwanted parasitic signals/frequencies (50 Hz mains, light dimmers etc.), buzzing sounds originating from grounding imbalances.
Musical instruments with passive pickups/microphones, like acoustic/electric guitars, have a high output impedance (order of 10 kOhm) and a low signal amplitude (order of 100 mV). Guitar amplifiers are adapted to this specifications with a high impedance input, so in this case, the DI-Box is not needed. If we want to connect a guitar to a mixing desk, the inputs have a low impedance. If we connect a guitar to this input, we experience a loss of signal quality (low and high frequencies) and amplitude. Using a DI-Box in the later case greatly improves the signal quality.
A DI-Box can be designed for passive or active operation. Passive design does not use a power supply. Active design uses a battery (usually 9V) or Phantom power supply (48V) originating from a mixing desk.
The active stage comprises of an operational amplifier (op amp) set-up as a voltage follower without gain (or attenuation). That means that the voltage on the output (pin 6 of IC1 on the schematic) will be exactly the same as the voltage on the input (pin 3). The same will happen when an audio signal is used. Additional adjustable gain could be easily added at this stage with minimal components.
The benefit of the active stage is that the input of the op amp is of high impedance and the output is of low impedance.
The downside of the active stage is that it requires a power supply for it to work. Another downside is that the op amp does not provide a total galvanic separation between the input and the output, which can lead to sound issues due to ground loops or ground imbalances (buzzing sounds). More precisely, the op amp does not eliminate those issues, it does however limit them.
The transformer TR1 is used to transform the input asymmetrical signal into a symmetric signal on the output. This stage is important as it has a big effect on the quality of the output. A cheap transformer (3€) can be used as well as a professional one (70€). The quality of the transformer will affect the frequency response of the output as well.
For this project, an existing design was used and can be found on the following link:http://sonelec-musique.com/electronique_realisations_di_003.html
The schematic from the above mentioned website (Sonelec-Musique) was replicated inside Autodesk Eagle software. The component dimensions were adapted to the components used for the project (see the components list below). For the transformer, we chose to use a Monacor LTR-110 (https://www.monacor.com/media/FLE/LTR110.pdf). A custom footprint was created inside Eagle to adapt for the component dimensions, which are not built-in. For the self-inductance, we used a VK200 self-inductance, found at our supplier (https://www.gotronic.fr/art-self-de-choc-vk200-3545.htm) and adapted the schematic to appropriate dimensions. Classic jumper pins were added for external connections; input, output, battery and the ground lift switch.
DI-Box schematic, made in Autodesk Eagle software.
The schematic was transformed into a PCB design using the same Autodesk Eagle software. Careful consideration was made for the routing of the ground connections to avoid a any loops. Three additional holes were added for attaching the PCB to the box in the assembly part of the project.
The PCB files were exported to Gerberfiles for the fabrication of the PCB. The Gerberformat is an open 2D binary vector image file format. It is the standard file used by PCB industry software to describe the printed circuit board images: copper layers, solder mask, legend, etc.
The PCB was fabricated using the LPKF ProtoMat S103 circuit board plotter for producing PCB prototypes.
PCB board design (top view), made in Autodesk Eagle software.
Fabricated PCB board (bottom view).
The PCB was assembled using the components stated above.
The box was designed using an online box generator (https://www.festi.info/boxes.py/). The generator is parametric so the parameters were changed to fit our needs. Plywood of 5 mm thickness was used as the box material. The generated vector file was modified using Adobe Illustrator vector software. Two holes were added for the input and link 6.3 mm jack connector, and two holes were added for the XLR output connector and the ground lift switch.
The result of the box plans can be seen here below:
Box design, prepared for laser cutting.
Additionally, the box cover was designed separately. A general purpose schematic was added to provide an overview of the box function and illustrate the purpose of each connector/switch.
Box cover design, prepared for laser cutting/engraving.
The box was fabricated using a laser cutting machine and 5 mm thick plywood. It was then glued together. Before assembling the connectors, the box was sanded with a fine (#180 and #240) sand paper and protected using wood protection oil in transparent matte colour.
Salle D133 Bâtiment D
Fait avec par la Team UOF