Active DI Box

With floating balanced input

This circuit adapts an unbalanced audio signal source (such as a guitar pickup) to an amplifier or mixing desk with a balanced input. It also has a rotary switch that allows the input signal attenuation or gain to be set to several different levels.

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Component list

Component List
Resistors
R1 = 470Ohm
R2,R16 = 1MOhm
R3,R6,R7,R8,R13,R14,R17,R25,R26,R30,R31,R32 = 33kOhm
R4 = 15kOhm
R5 = 8.2kOhm
R9,R20 = 680Ohm
R10,R18 = 56Ohm
R11,R12,R21,R22 = 100kOhm
R15 = 12Ohm
R19 = 47Ohm
R23,R27,R28 = 10kOhm
R24 = 100Ohm
R29 = 330kOhm
R33 = 0Ohm (wire link)
R34 = 2.2kOhm
Capacitors
C1 = 1µF MKT or MKS
C2,C8 = 47pF
C3,C5 = 2.2µF MKT or MKS
C4 = 100pF
C6 = 470nF
C7 = 47pF
C9,C10 = 220µF 25V
C11 = 470µF 50V
Semiconductors
D1 = 1N4148
D2 = LED, red, 3mm
U1 = LM833
U2 = TL072
U3 = TL071
Miscellaneous
S1 = 6-position 2-pole rotary switch, PCB mount
K1,K2 = 6.3mm stereo jack connector, PCB mount
J1 = 3-pin pinheader
J2 = 2-pin pinheader
Supply connector, panel mount, 2.1mm center pin
3-pin XLR socket, panel mount
Case, Hammond type 1455K1202