Voice and Music Audio Mixer Circuit using IC LM3900

This circuit is a complex of Voice and Music Audio Mixer that uses the LM3900 quad operational amplifier.

It is designed to handle different types of audio inputs, like signals from microphones and line level audio and combines them into one mixed audio output.

The LM3900 is a great choice because it has a high gain bandwidth product and can work with a single power supply across a wide voltage range from +5V to 30V.

This makes the circuit very adaptable for various audio mixing situations such as sound reinforcement public address systems and musical instrument setups.

Each channel of the mixer has a special adjustable knob called a preset that lets users change the input gain.

This gives a lot of flexibility making it easy to balance the audio levels from different sources.

The circuit does a great job of mixing the audio signals while keeping the sound clear and true to the original quality.

Circuit Working:

Parts List:

The above circuit we are talking about has four different input channels.

There are two microphone inputs called Mic input1 and Mic input2 and two line inputs named Line input1 and Line input2

Each of these channels is connected through variable resistors labeled VR1, VR2, VR3 and VR4 all rated at 1k.

These resistors are really important because they let users adjust the volume of each input signal to their liking.

To make sure only the right audio frequencies get through and to block any direct current DC, there are coupling capacitors, C1, C2 and C3 placed at the start of each channel.

These capacitors have values of 0.1µF and 0.47µF.

The audio signals are amplified and mixed using the LM3900 quad op-amp, which is set up so that each channel uses one part of the op-amp in a summing amplifier setup.

The gain for each channel is carefully set by feedback resistors R3, R4, R5 and R6 each with a rating of 1M.

This ensures that the audio signals are boosted enough for the next steps.

All the outputs from the op-amps come together at one point where the signals mix.

After mixing the combined audio signal goes through a coupling capacitor C3 which removes any DC offset to avoid problems in the next stages of audio processing.

This mixed audio signal can then be sent either to a pre amplifier or directly to an audio amplifier depending on what is needed.

The circuit can work with a power supply ranging from +5V to +30V making it flexible for different setups.

Plus, it only needs a single power supply which simplifies the design since it does not require a negative voltage supply.

Good grounding helps keep everything running smoothly and without any annoying noise.

The current limiting resistors R11, R12, R13, R14 which are 470Ω make sure the op-amp outputs stay steady and stop too much current from flowing.

Formulas with Calculations:

Below mentioned are the formulas with calculations for Voice and Music Audio Mixer Circuit using IC LM3900:

Gain of each op-amp:

Gain = (Feedback Resistor) / (Input Resistor)

Example: Gain = R3 / R1 (for Mic input1 channel)

Output voltage of the mixer:

Vout = (V1 × Gain1) + (V2 × Gain2) + (V3 × Gain3) + (V4 × Gain4)

where,

V1, V2, V3 and V4 are the input voltages of Mic input1, Mic input2, Line input1 and Line input2 channels respectively.

Gain1, Gain2, Gain3 and Gain4 are the respective gains of each channel.

Input impedance of each channel:

Zin = Input Resistor

where,

• R1, R2 are the input resistors in this circuit

Cutoff frequency of input capacitors:

Fc = 1 / (2 × π × R × C)

where,

R is the Input resistor R1, R2
C is the coupling capacitor C1, C2

Calculations:

For R1 = 1Ω and C1 = 0.1μF:
Fc = 1 / (2 × 3.1416 × 1 × 0.1 × 10^-6)
Fc = 1.59 MHz

Total current consumption:

I = (Vcc – Vout) / R

where,

• Vcc is the supply voltage
• Vout is the output voltage of the mixer
• R is the total resistance in the circuit

How to Build:

To build a Voice and Music Audio Mixer Circuit using IC LM3900 follow the below steps for connections:

• Gather all the components as mentioned in the above circuit diagram
• Connect pin 1 of IC1 LM3900 to pin 2, 12, 13 and GND.
• Connect pin 3 of IC1 LM3900 to pin 4 through a resistor R3, connect from pin 1 to one end of capacitor C1 and other end to resistor R1, connect the resistor R1 other end to center leg of VR1 preset, one leg to Mic input1 and third leg to GND.
• Connect pin 4 of IC1 LM3900 to +5V to +30V positive supply.
• Connect pin 5 of IC1 LM3900 to pin 6 through resistor R4.
• Connect pin 6 of IC1 LM3900 to one end of capacitor C2 and other end to resistor R2, connect the resistor R2 other end to center leg of VR2 preset, one leg to Mic input2 and third leg to GND.
• Connect pin 7 of IC1 LM3900 to GND.
• Connect pin 8 of IC1 LM3900 to pin 9 through resistor R5, connect from pin 8 to one end of capacitor C4 and other end connect center leg of VR3 preset, one leg to Line input1 and third leg to GND.
• Connect a resistor R7 from pin 8 and GND.
• Connect from pin 9 of IC1 LM3900 a resistor R8 and GND.
• Connect pin 10 of IC1 LM3900 to pin 11 through resistor R6, connect a resistor R10 from pin 10 and GND.
• Connect pin 11 of IC1 LM3900 to one end of capacitor C5 and other end connect center leg of VR4 preset, one leg to Line input2 and third leg to GND.
• Connect a resistor R9 from pin 11 to GND.
• Connect resistors R11, R12, R13 and R14 from pin 4, 5, 9 and 10 of IC1 LM3900 respectively.
• Connect capacitor C3 as pre amp output between resistor R12 and R13

Conclusion:

The Voice and Music Audio Mixer Circuit using IC LM3900 is a smart and budget friendly way to combine different audio sources into one output.

Thanks to the LM3900s strong amplification and ability to work over a wide range of voltages, this circuit performs well in many audio situations, like public speaking events, musical performances and general sound mixing.

Each channel has adjustable gain control which allows users to easily balance the input levels.

Plus, the circuit uses coupling capacitors to remove any DC offset making sure the audio signal stays clear.

The design is simple and has fewer parts which makes it easy to put together and keep in good shape.

To conclude, this mixer circuit is perfect for both hobbyists and professionals who want an easy and effective way to mix audio signals.

References:

LM3900 Audio Mixer questions

Datasheet IC LM3900