Audio Mixer Circuit using a Single Transistor

To begin with, this Audio Mixer Circuit using a Single Transistor is an small and easy circuit which mixes three sounds; it uses only one transistor which can mix two or three sounds into one.

Also, it is good for beginners who want to learn about sound and electronics

Circuit Working:

Parts List:

Components	Values	Quantity
Resistors	2.2k CFR 1/4 watt	1
	1M CFR 1/4 watt	1
	22k CFR 1/4 watt	3
	Potentiometer 10k	3
Capacitors	Electrolytic
	100uF 25V	1
	4.7uF 25V	2
Semiconductors	Transistor BC547	1

This sound mixer circuit is very simple like in the diagram above, which can mix three or more sound signals using only one transistor.

Also, the circuit diagram shows only three inputs, but we can add more if needed.

Each input have own knob to control sound level before mixing and if input sound is 50 mV this circuit make it louder up to 500 mV.

Hence, this output is good enough for most power amplifier to use.

Formulas:

Formulas for Audio Mixer Circuit Using One Transistor, also the small sound mixer circuit uses preamplifier.

Also, gain of this preamp tell how much sound can get louder.

Preamplifier gain formula:

Av = ΔVo / ΔVi

where:

• Av is the gain of amplifier
• ΔVo is the output voltage change
• ΔVi is the input voltage change

This shows how much larger the output is than the input and how the transistor connection changes the gain.

Here, different type of setup give different formula.

Common transistor setups and gain:

People use the common emitter configuration most often for sound applications.

Av = – (Rc / Re)

• Rc is the collector of the resistor
• Re is the emitter of the resistor

Common Base:

• Av is β
• β is the transistors current gain

Bias Resistors:

Check right resistor value for base and emitter and ensure transistor works good, so we can find resistor information in datasheet of transistor based on how much current we need.

Capacitors in Mixer Circuit:

Capacitors allow only AC sound signals to pass and block DC power and preamps use capacitors to get best sound result.

Coupling Capacitor Formula:

Cf = 1 / (2π × fL × Rin)

where:

• fL is the lowest sound frequency we want
• Rin is the input resistance
• Cf is the capacitor value to use

This capacitor sit between stages to pass sound and block DC.

Bypass Capacitor:

If required, put capacitor with resistor to pass AC sound around resistor and also use common values like 47µF or 10µF.

How to Build:

To build a Audio Mixer Circuit using a Single Transistor we need to follow the below steps for connections:

• First, connect + positive of battery to collector of transistor using 2.2k resistor.
• After that, connect emitter of transistor to ground – negative of battery.
• Then connect base of transistor to 10k pot and 22k resistor through a 4.7μF capacitor.
• Next, each input jack connects to one side of 10k pot and other side to ground and middle pin of wiper connects to base side of 22k resistor.
• Also, connect output jack from collector through 4.7μF capacitor to ground.
• Lastly, check all wires with open short circuit and ensure everything is correct.

Testing the Mixer:

• Here, use 9V battery or good power source to power the circuit and plug in audio sources to input jacks.
• Then turn the pots to control sound level and listen from output using amplifier or headphones.

Conclusion:

To conclude, If we do not know electronics well then ask someone expert or read good guide before starting; also try using different parts to see what sound is better.

Furthermore, this Audio Mixer Circuit using a Single Transistor is small project, is fun for learning and for mixing sounds.