4 Channel Portable Audio Mixer

High-quality modular design, 9V Battery powered - Very low current drawing

The target of this project was the design of a small portable mixer supplied by a 9V PP3 battery, keeping high quality performance. The mixer is formed assembling three main modules that can be varied in number and/or disposition to suit everyone needs. The three main modules are:

Input Amplifier Module: a low noise circuit equipped with a variable voltage-gain (10 - 100) preset, primarily intended as high quality microphone input, also suitable for low-level line input.

Tone Control Module: a three-band (Bass, Middle, Treble) tone control circuit providing unity-gain when its controls are set to flat frequency response. It can be inserted after one or more Input Amplifier Modules and/or after the Main Mixer Amplifiers.

Main Mixer Amplifier Module: a stereo circuit incorporating two virtual-earth mixers and showing the connection of one Main Fader and one Pan-Pot.

The image below shows a Block diagram of the entire mixer featuring four Input Amplifier Modules followed by four in-out switchable Tone Control Modules, one stereo Line input, four mono Main Faders, one stereo dual-ganged Main Fader, four Pan-Pots, a stereo Main Mixer Amplifier Module and two further Tone Control Modules switchable in and out for each channel, inserted before the main Left and Right outputs.

Obviously this layout can be rearranged at everyone wish. An astonishing feature of this design lies in the fact that a complete stereo mixer as shown below in the Block diagram draws less than 6mA current!

Block diagram:

Parts:

R1 = 22K - 1/4W Resistor
R2 = 22K - 1/4W Resistor
R3 = 47K - 1/4W Resistor
R4 = 47K - 1/4W Resistor
R5 = 47K - 1/4W Resistor
R6 = 4K7 - 1/4W Resistor
R7 = 22K - 1/4W Resistor
R8 = 220R - 1/4W Resistor
R9 = 2K - 1/2W Trimmer Cermet (See Notes)
R10 = 470K - 1/4W Resistor
R11 = 560R - 1/4W Resistor
R12 = 100K - 1/4W Resistor
R13 = 220R - 1/4W Resistor

C1 = 470nF - 63V Polyester Capacitor
C2 = 100µF - 25V Electrolytic Capacitor
C3 = 2µ2 - 63V Electrolytic Capacitor
C4 = 2µ2 - 63V Electrolytic Capacitor
C5 = 2µ2 - 63V Electrolytic Capacitor
C6 = 47pF - 63V Ceramic Capacitor
C7 = 4µ7 - 63V Electrolytic Capacitor
C8 = 100µF - 25V Electrolytic Capacitor
Q1 = BC560C - 45V 100mA Low noise High gain PNP Transistor
Q2 = BC550C - 45V 100mA Low noise High gain NPN Transistor
IC1 = TL061 - Low current BIFET Op-Amp

Circuit Description:

The basic arrangement of this circuit is derived from the old Quad magnetic pick-up cartridge module. The circuit was rearranged to cope with microphone input and a single-rail low voltage supply. This low-noise, fully symmetrical, two-transistor head amplifier layout, allows the use of a normal FET input Op-Amp as the second gain stage, even for very sensitive microphone inputs. The voltage-gain of this amplifier can be varied by means of R9 from 10 to 100, i.e. 20 to 40dB.

Notes:

• R9 can be a trimmer, a linear potentiometer or a fixed-value resistor at will.
• When voltage-gain is set to 10, the amplifier can cope with 800mV peak-to-peak maximum Line levels.
• Current drawing for one Input Amplifier Module is 600µA.
• Frequency response is 20Hz to 20KHz - 0.5dB.
• Total Harmonic Distortion measured with voltage-gain set to 100: 2V RMS output = <0.02%>
• Total Harmonic Distortion measured with voltage-gain set to 10 & 33: 2V RMS output = <0.02%>
• THD is much lower @ 1V RMS output.
• Maximum undistorted output voltage: 2.8V RMS.

Circuit Diagram:
Parts:

P1 = 100K - Linear Potentiometer
P2 = 100K - Linear Potentiometer
P3 = 470K - Linear Potentiometer

R1 = 12K - 1/4W Resistor
R2 = 12K - 1/4W Resistor
R3 = 12K - 1/4W Resistor
R4 = 3K9 - 1/4W Resistor
R5 = 3K9 - 1/4W Resistor
R6 = 1K8 - 1/4W Resistor
R7 = 1K8 - 1/4W Resistor
R8 = 22K - 1/4W Resistor
R9 = 22K - 1/4W Resistor
R10 = 560R - 1/4W Resistor
R11 = 100K - 1/4W Resistor
R12 = 220R - 1/4W Resistor

C1 = 1µF - 63V Polyester Capacitor
C2 = 47nF - 63V Polyester Capacitor
C3 = 4n7 - 63V Polyester Capacitor
C4 = 22nF - 63V Polyester Capacitor
C5 = 4n7 - 63V Polyester Capacitors
C6 = 100µF - 25V Electrolytic Capacitor
C7 = 4µ7 - 63V Electrolytic Capacitor
C8 = 100µF - 25V Electrolytic Capacitor
IC1 = TL061 - Low current BIFET Op-Amp

Circuit Description:

This is a straightforward design using the Baxandall-type active circuitry slightly modified to obtain a three-band control. Total voltage gain of this module is 1 when controls are set in their center position.

Notes:

• Current drawing for one Tone Control Module is 400µA.
• Frequency response is 20Hz to 20KHz - 0.5dB, controls flat.
• Tone control frequency range: ±15dB @ 30Hz; ±19dB @ 1KHz; ±16dB @ 10KHz.
• Total Harmonic Distortion measured @ 2V RMS output = <0.012%>
• THD is below 0.01% @ 1V RMS output.
• Maximum undistorted output voltage: 2.5V RMS.

Circuit Diagram:
Parts:

P1 = 100K - Linear Potentiometer
P2 = 10K - Linear Potentiometer
R1 = 15K - 1/4W Resistor
R2 = 15K - 1/4W Resistor
R3 = 100K - 1/4W Resistor
R4 = 100K - 1/4W Resistor
R5 = 22K - 1/4W Resistors
R6 = 22K - 1/4W Resistors
R7 = 390K - 1/4W Resistor
R8 = 390K - 1/4W Resistor
R9 = 560R - 1/4W Resistor
R10 = 560R - 1/4W Resistor
R11 = 100K - 1/4W Resistor
R12 = 100K - 1/4W Resistor
R13 = 220R - 1/4W Resistor

C1 = 330nF - 63V Polyester Capacitors
C2 = 330nF - 63V Polyester Capacitors
C3 = 100µF - 25V Electrolytic Capacitors
C4 = 10pF - 63V Ceramic Capacitors
C5 = 10pF - 63V Ceramic Capacitors
C6 = 4µ7 - 63V Electrolytic Capacitors
C7 = 4µ7 - 63V Electrolytic Capacitors
C8 = 100µF - 25V Electrolytic Capacitors
IC1 = TL062 - Low current BIFET Dual Op-Amp

Circuit Description:

The schematic of this circuit is drawn as a stereo unit to better show the input Main Fader and Pan-Pot connections. The TL062 chip contains two TL061 op-amps into the same 8 pin case and is wired as two virtual-earth mixer amplifiers having a voltage gain of about 4, to compensate for losses introduced in the passive Pan-Pot circuitry. Therefore, total voltage-gain is 1.
Each channel added to the mixer must include the following additional parts:
P1, P2, R1, R2, R3, R4, C1 and C2.
These parts must be wired as shown in the above circuit diagram, connecting R3 and R4 to pin #2 and pin #6 of IC1 for Right and Left channel respectively. These IC1 pins are the "virtual-earth mixing points" and can sum together a great number of channels.

Notes:

• Current drawing for one stereo Main Mixer Amplifier Module is 800µA.
• Frequency response is 20Hz to 20KHz - 0.5dB.
• Total Harmonic Distortion measured @ 2V RMS output = <0.008%>
• THD is 0.005% @ 1V RMS output.
• Maximum undistorted output voltage: 2.8V RMS.

Further Parts:

To parts listed above should be added: one Main on-off SPST switch, a LED used as pilot-light with its dropping 2K2 1/4W series-resistor, DPDT switches to enable or omit Tone Control Modules as shown in the Block diagram, input and output connectors of the type preferred, one stereo dual-gang 100K potentiometer to fade the Stereo Line Input as shown in the Block diagram, battery clip, PP3 9V battery, knobs etc.