Simple Audio Mixer Circuit using a Single Transistor

This post shows you how to build a simple circuit that mixes two sounds together.

This is like a mini DJ mixer, but it only uses one transistor a tiny switch to combine two different sounds into one.

This is a great project to learn about how electronics can mix sounds together.

Circuit Working:

Parts List:

The diagram above illustrates a simple audio signal mixer circuit which can be as uncomplicated as indicated.

This circuit employs a single transistor and accommodates the mixing of three input signals or more.

While the diagram displays only three inputs, it is not limited to this number and can be expanded to accommodate any desired number of inputs.

Each input on the mixer features individual level control pots allowing independent adjustment of the signal entering through the inputs.

This mixer circuit utilizing a single transistor, is designed to amplify any input signal with a 50mV amplitude to an output signal of approximately 500mV.

This output level is sufficient for most power amplifiers configured at the output.

Formula:

The preamplifiers gain is expressed by this formula:

A_v = ΔV_o / ΔV_i

where,

• A_v: This stands for the amplifiers voltage gain.
• It is a unitless number that indicates how much the input voltage signal is amplified by the amplifier.
• ΔV_o: This indicates the output voltage variation.
• In essence, it is the difference between the output voltage (Vo) that is produced when a signal is supplied and the output voltage (typically zero) that is produced when no signal is present.
• ΔV_i: This is the input voltage variation.
• It represents the difference between zero voltage and the input voltage signal (Vi).

It shows the ratio of the input voltage change (ΔVi) to the output voltage change (ΔVo).

The kind and arrangement of transistors utilized in the preamp determine the precise formula for determining gain.

Transistor datasheets and circuit analysis textbooks contain these formulae.

The following are some typical setups along with their gain calculations:

Common Emitter (most common for audio preamps):

A_v = – (R_c / R_e)

where,

• R_c is collector resistor
• R_e is emitter resistor

Common-Base:

A_v = β

where,

• β is the current gain of the transistor

Resistors for Biasing:

The values of biasing resistors, such as base and emitter resistors, are essential for determining the transistors operating point and guaranteeing appropriate amplification.

Based on the intended collector current and transistor properties, datasheets offer advise on the selection of biasing resistors.

Connecting and Isolating Capacitors:

These capacitors let through AC audio signals but block DC voltages.

The preamplifiers intended frequency response is taken into consideration while selecting their settings. To help you get started consider these formula:

Select a coupling capacitor the component that sits between stages: that is both big enough to stop DC and tiny enough to let through the low frequency audio signal you want.

The general formula is:

Cf = 1 / (2π * fL * Rin)

where,

• fL: is the lowest desirable frequency
• Rin: is the input impedance and
• Cf: is the coupling capacitor value.

Through a resistor and a bypass capacitor: Select a value that will allow AC signals surrounding the resistor to be bypassed.

A typical value is 47 or 10uF.

How to Build:

Building the audio mixer circuit described requires a basic understanding of electronics and soldering skills.

Circuit Construction:

• Connect the positive terminal of the power supply to the collector of the transistor through 2.2k resistor.
• Connect the emitter of the transistor to the ground negative terminal of the power supply.
• Connect the base of the transistor to the junction of a pot 10k through capacitor 4.7μF and individual 22k resistor.
• Connect individual potentiometers 10k for each input: one end to the input jack, the other end to the ground, and the wiper to the base resistor junction.
• Connect the audio output jack between the collector of the transistor through a 4.7uF capacitor and the ground terminal of the power supply.
• Double check connections and make sure there are no short circuits.

Testing:

• Power the circuit using a 9V battery or another appropriate power source.
• Connect audio sources to the input jacks.
• Adjust the potentiometers to control the input levels.
• Monitor the output with an amplifier or headphones.

Conclusion:

Remember to use proper safety measures and if you are not familiar with electronics, it is advisable to seek assistance from someone experienced or consult relevant resources.

Additionally, you might want to experiment with component values and configurations to achieve the desired performance.

References:

Audio mixing