Portable mixer

Design Description:

The goal of this project is to design a small, portable mixer powered by a 9V PP3 battery while maintaining high-quality performance.

The mixer is formed by assembling three main modules, which can vary in number and/or layout to suit everyone's needs.

The three main modules are:

Input amplifier module: A low-noise circuit with a variable voltage gain (10-100) preset, primarily used for high-quality microphone input, but also suitable for low-level line input.

An optional balanced input amplifier module with similar characteristics can be added upon request.

Tone Control Module: A three-band (bass, midrange, treble) tone control circuit that provides unity gain when the control is set to a flat frequency response. It can be inserted after one or more input amplifier modules and/or after the main mixer amplifier.

Main Mixer Module: A stereo circuit containing two virtual ground mixers and showing the connection of a main fader and a pan potentiometer.

The diagram below shows the block diagram of the entire mixer, which includes four input amplifier modules, followed by four tone control modules with switchable input and output, one stereo line input, four mono master faders, one stereo dual master fader, four pan potentiometers, one stereo master mixer module, and two additional tone control modules with switchable input and output for each channel, inserted before the main left and right outputs.

Clearly, this layout can be rearranged according to everyone's wishes.

One of the amazing features of this design is that the complete stereo mixer, as shown in the block diagram below, consumes less than 6mA of current!

Block diagram:

Input amplifier module

Circuit diagram:

part:

R1, R2, R7 _______22K 1/4W resistors

R3, R4, R5 _______47K 1/4W resistors

R6______________4K7 1/4W resistor

R8, R13________220R 1/4W resistor

R9______________2K 1/2W Fine-tuning ceramic potentiometer (see note)

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 capacitor

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 Operational Amplifier

Circuit description:

The basic layout of this circuit is derived from the old Quad magnetic pickup cartridge module. The circuit has been rearranged to handle microphone input and single-supply low-voltage operation.

This low-noise, fully symmetrical, dual-transistor preamplifier layout allows the use of a common FET-input operational amplifier as a second gain stage, even for very sensitive microphone inputs.

The voltage gain of the amplifier can be varied from 10 to 100 via R9, which is 20 to 40 dB.

Note:

• R9 can be a trimmer, a linear potentiometer, or a fixed resistor, depending on the need.

• When the voltage gain is set to 10, the amplifier can handle a maximum line level of 800mV peak-to-peak.

• The current consumption of 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 =