Three Channel Audio Mixer Circuit

Three high/low-sensitivity switchable inputs

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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.

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Parts:

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

Capacitors:

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

Transistors:

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

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Notes:

• 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.