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Portable Mixer

High-quality modular design

9V Battery powered - Very low current drawing

Design description:

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) pre-set, 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:

Mixer Block diagram

Input Amplifier Module


Input Amplifier


R1,R2,R7_______22K   1/4W Resistors
R3,R4,R5_______47K   1/4W Resistors
R6______________4K7  1/4W Resistor
R8,R13________220R   1/4W Resistors
R9______________2K   1/2W Trimmer Cermet (See Notes)
R10___________470K   1/4W Resistor
R11___________560R   1/4W Resistor
R12___________100K   1/4W Resistor

C1____________470nF   63V Polyester Capacitor
C2,C8_________100µF   25V Electrolytic Capacitors
C3,C4,C5________2µ2   63V Electrolytic Capacitors
C6_____________47pF   63V Ceramic Capacitor
C7______________4µ7   63V 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.


Tone Control Module


Tone Control


P1,P2_________100K   Linear Potentiometers
P3____________470K   Linear Potentiometer

R1,R2,R3_______12K   1/4W Resistors
R4,R5___________3K9  1/4W Resistors
R6,R7___________1K8  1/4W Resistors
R8,R9__________22K   1/4W Resistors
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,C5___________4n7   63V Polyester Capacitors
C4_____________22nF   63V Polyester Capacitor
C6,C8_________100µF   25V Electrolytic Capacitors
C7______________4µ7   63V 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.


Main Mixer Amplifier Module

Main Mixer


P1,___________100K   Linear Potentiometer
P2_____________10K   Linear Potentiometer

R1,R2,_________15K   1/4W Resistors
R3,R4,R11,R12_100K   1/4W Resistors
R5,R6__________22K   1/4W Resistors
R7,R8_________390K   1/4W Resistors
R9,R10________560R   1/4W Resistors
R13___________220R   1/4W Resistor

C1,C2_________330nF   63V Polyester Capacitors
C3,C8_________100µF   25V Electrolytic Capacitors
C4,C5__________10pF   63V Ceramic Capacitors
C6,C7___________4µ7   63V 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 in 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.


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.

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