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 Circuit

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

  • When SW1, SW2 or SW3 are open the input sensitivity is suited to high-output devices like CD players, tuners, tape recorders, iPods, miniDisc players, computer audio outputs etc.
  • When SW1, SW2 or SW3 are closed the input sensitivity is suited to low-output, low-impedance moving coil or electret microphones.
  • Sometimes, the 750 Ohm value for R3, R13 and R17 resistors could be not easy to find. In this case, two 1K5 resistors wired in parallel can be used to replace each item.
  • To make a stereo mixer, all the parts must be doubled excepting R6, C3, C5, SW4 and B1.
Source: Red Free Circuit Design

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