Three Channel Audio Mixer Circuit

Small portable device - 9V battery operation
Three high/low-sensitivity switchable inputs

Although the modular Portable Mixer design available on these web pages has become a hit for many amateurs, some correspondents required a much simpler device, mainly for mixing mono signals. This design should fulfil their needs, featuring three inputs with switchable high/low sensitivity and unusual level-control circuits, providing high overload margins and low-noise figures, proportional to gain-level settings. Low current consumption due to a simple, five-transistor circuitry, allows the Mini Mixer to be powered by a common 9V PP3 battery for many hours.

Circuit diagram:

Three Channel Audio Mixer


P1 = 5K
P2 = 5K
P3 = 5K
R1 = 180K
R2 = 2M2
R3 = 750R
R4 = 1K
R5 = 15K
R6 = 220R
R7 = 1.5K
R8 = 820R
R9 = 150R
R10 = 100K
R11 = 180K
R12 = 2.2M
R13 = 750R
R14 = 1K
R15 = 180K
R16 = 2M2
R17 = 750R
R18 = 1K


C1 = 1µF-63V
C2 = 100µF-25V
C3 = 220µF-25V
C4 = 100µF-25V
C5 = 220µF-25V
C6 = 1µF-63V
C7 = 100µF-25V
C8 = 1µF-63V
C9 = 100µF-25V


Q1 = BC550C
Q2 = BC547
Q3 = BC557
Q4 = BC550C
Q5 = BC550C

Misc. Components

B1 = 9V PP3 Battery
J1,J2,J3 = 3mm Mono Jack sockets
SW1,2,3,4 = SPST Toggle or Slider Switches

Source: Red Free Circuit Design