Mini Mixer

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.

Parts:

• 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_________1µF 63V Polyester or Electrolytic Capacitors
• C2,C4,C7,C9____100µF 25V Electrolytic Capacitors
• C3,C5__________220µF 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

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.

Technical data (Level controls in mid position)

Sensitivity:

Low level inputs:

• 2mV RMS for 316mV RMS output [-10dB(V)]
• 6mV RMS for 0.775V RMS output [0dB(V)]

High level inputs:

• 130mV RMS for 316mV RMS output [-10dB(V)]
• 400mV RMS for 0.775V RMS output [0dB(V)]

Maximum Input sensitivity: 1.3mV RMS for 0.775V RMS output [0dB(V)]

Maximum undistorted output: 2.4V RMS [+10dB(V)]

Frequency response: flat from 20Hz to 20KHz

Total harmonic distortion @ 1KHz: <0.1%>