Three Channel Audio Mixer Circuit

Description

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:

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

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.