This is the simple and low cost 3 channels mini audio mixer circuit diagram. This circuit supplied using 9V power supply. There are high/low-sensitivity switchable inputs for each input channel and each channel used low noise and high gain transistor of BC550C.

Components list:

P1,P2,P3_________5K Linear Potentiometers

R1,R11,R15_____180K 1/4W Resistors
R2,R12,R16_______2M2 1/4W Resistors
R3,R13,R17_____750R 1/4W Resistors (See Notes)
R4,R14,R18_______1K 1/4W Resistors
R5______________15K 1/4W Resistor
R6_____________220R 1/4W Resistor
R7_______________1K5 1/4W Resistor
R8_____________820R 1/4W Resistor
R9_____________150R 1/4W Resistor
R10____________100K 1/4W Resistor

C1,C6,C8_________1uF 63V Polyester or Electrolytic Capacitors
C2,C4,C7,C9____100uF 25V Electrolytic Capacitors
C3,C5__________220uF 25V Electrolytic Capacitors

Q1,Q4,Q5______BC550C 45V 100mA Low noise High gain NPN Transistors
Q2____________BC547 45V 100mA General purpose NPN Transistor
Q3____________BC557 45V 100mA General purpose PNP Transistor

J1,J2,J3________3mm or 6mm Mono Jack sockets

SW1,2,3,4______SPST Toggle or Slider Switches

B1_______________9V PP3 Battery

Clip for PP3 Battery