Direct Box (instant music recording instrument for Musicians)
At the input of the device, there are two parallel Jack connectors, J1 and j2. The first one will serve to connect the source (guitar, etc), and the second will allow for the possible connection of the amplifier. A 3-position attenuator allows the connection of different sources. In position 1 the gain is 0dB, in position 2 the gain is 15dB (R10 / R15) to connect electric pianos or synthesizers or line levels, while in position 3 the attenuation is 45dB (R10 / R15 and R16) for Connect the speaker outputs of the amplifiers.
R10 and R16 values can be revised if the levels do not match your own application. We recommend not to reduce the resistance value of R15 (47kΩ), which would result in a reduction in the input resistance of the device, which should be as large as possible to avoid disturbing the parallel connection of the amplifier.
The SW2 switch allows you to isolate the ground of the Jack connectors from the grounding of the mixing console to avoid a possible ground loop undesirable in the sound because it produces the very undesirable 50Hz buzz.
Amplifier and symmetry
Step IC1 is a differential amplifier with unit amplification. The selected integrated circuit (TL072) is a low-noise dual-power amplifier with high input resistance, low deformation rate and a reasonable cost.
The Vcc/2 polarization required for operation is generated by the R1/R2 voltage divider, which directly polarizes the IC1b stage, while polarization of IC1α is provided by the R9 resistor. Capacitors C1 and C2 ensure filtering and avoid disagreement between the stages. IC1b has been connected as a unit gain voltage inverter.
At the output, terminals 7 and 1 have two opposite phase signals and the same amplitude since it is a symmetric output stage. C7 and C8 capacitors (50V) allow the separation of the continuous component ("hidden" power supply) that is displayed on the XLR connector terminals 2 and 3 of the mixing console.
The resistors R13 and R14 allow this 48V supply to be recovered if the SW1 switch is in the 48V supply, filtered by capacitors C5 and C6. An electrolytic capacitor hardly disconnects the high frequencies and this function is left at C5.
Install the small parts first. If you can select low noise (metal coating) resistors for amplification and attenuation steps (R3 to R5, R10, R15 and R16), this advice applies to any audio circuit.
You do not need to make any adjustments, since the circuit must work from the start.
R10: 150kΩ 1%
J1,J2: Jack 6,35mm
XLR male for mount