Simple Audio Mixer Circuit using a Single Transistor

The construction of a simple circuit which mixes two sounds is explained in this article.

This device uses a single transistor and a tiny switch for mixing two different sounds into one, much like a little DJ mixer.

This project is an excellent way to learn how to use electronics for blending sounds.

Circuit Working:

Parts List:

A simple audio signal mixer circuit as shown in the schematic above can be as simple as it seems.

Three or more input signals are easily mixed in this circuit which uses a single transistor.

Although just three inputs are shown in the diagram it can be modified to fit any number of inputs that is required.

The signal coming through the mixers inputs gets properly adjusted due to the individual level control knobs on each input.

Any input signal with an amplitude of 50 mV is amplified by this mixer circuit which uses a single transistor to produce an output signal of about 500 mV.

For the majority of power amplifiers set up at the output this output level is enough.

Formulas:

Following formulas are mentioned below for Simple Audio Mixer Circuit using a Single Transistor:

The preamplifiers gain is indicated by this formula:

A_v = ΔV_o / ΔV_i

where,

• A_v stands for the amplifiers voltage gain.
• ΔV_o indicates the output voltage variation.
• ΔV_i is the input voltage variation.

When comparing the input voltage change (ΔVi) to the output voltage change (ΔVo) it shows the ratio.

The kind and configuration of transistors in the preamp provides an exact process for calculating gain.

These calculations are often found in circuit analysis textbooks and transistor datasheets.

Some common configurations and their gain estimates are as follows:

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

Resistance to Bias:

The operating point of the transistor and the proper amplification depend on the values of biasing resistors such as base and emitter resistors.

Datasheets provide information on the selection of biasing resistors based on transistor features and the expected collector current.

How to Connect and Separate Capacitors:

These capacitors block DC voltages but allow AC audio signals to pass through.

While choosing their settings preamplifiers intended frequency response is considered.

Using the following formula to get started.

The component that lies between stages is called a coupling capacitor.

Choose one that is both large enough to block DC and small enough to allow the desired low frequency audio signal to pass through.

The basic 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 bypass capacitor and a resistor:

Choose a value that will enable the bypass of AC signals around the resistor.

47 or 10uF is a common value.

How to Build:

To build a Simple Audio Mixer Circuit using a Single Transistor one needs to follow the below steps for connections:

• 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.
• Check twice the connections and make sure there are no short circuits.

Testing:

• A 9V battery or other adequate power source are used to supply electricity to the circuit.
• Connect audio sources to the input jacks.
• Adjust the potentiometers to control the input levels.
• Monitor the output with an amplifier or headphones.

Conclusion:

If not familiar with electronics it is important to get help from a professional or consult the right sources.

One should experiment with different component combinations and values to obtain the necessary performance.

References:

Audio mixing