In the previous tutorial, an audio equalizer was designed. In this tutorial, now an audio mixer will be designed.
The Audio Mixer is an electronic device which combines and modifies the audio signals. The audio signals can be either in digital form or analog form. In digital form, the analog audio signals are encoded so that the audio information in the signal becomes independent of the amplitude of the signal. Both analog and digital signals can be combined by different types of audio mixers. For mixing digital audio signals, digital signal processing techniques are used while for combining the analog audio signals, generally operational amplifiers are used. A basic audio mixer has a separate volume knob for each source and output volume knob (as seen in the image below).
Fig. 1: Typical Image of 2-Channel Audio Mixer
In this tutorial, a 2-Channel Audio Mixer will be designed. This audio mixer will take two audio signals and combine them to one. One of these two audio sources will be taken from a smart phone and the other audio source will be a microphone. The audio from the microphone will be amplified to line level by a preamplifier before it is combined with the audio from the smart phone by the operational amplifier circuit. The combined audio will be played on a speaker. So, by using this audio mixer, a user can actually mix her own voice with an audio track.
A preamplifier and power amplifier are the main building blocks of this mixer circuit. There are few terms associated with amplifiers used in this tutorial like voltage gain, line level, clipping effect, Zobel network etc. For learning the basics of audio amplifiers and terms associated with them, check out the following tutorial –
Components Required –
Fig. 2: List of components required for Audio Mixer
Block Diagram –
Fig. 3: Block Diagram of 2-Channel Audio Mixer
Circuit Connections –
The audio mixer combines the two or more audio signals using a summer. The summer takes multiple input signals and mix them together. The mixed signals are then amplified with the help of power amplifier and at the output, the combined audio signal is obtained. This is a 2-Channel Audio Mixer, so it combines audio from two sources.
This audio mixer circuit is built by assembling the following components together –
Power Supply – In the circuit, the preamp for the microphone requires a 5V DC while the power amplifier requires 9V DC. So, the preamp circuit is powered by a 5V battery while the amplifier circuit is powered by a 9V battery.
Audio Source from Smartphone – One of the audio input is provided from a smart phone. For receiving audio from the smart phone, an audio jack of 3.5 mm is plugged into the phone. The 3.5 mm audio jack has three wires – one for ground and two wires for left and right channel. The two wires for left and right channels are used for stereo systems. In this circuit, only one of the channels is used for audio source while the ground wire of the jack will be connected to the common ground of the circuit.
Fig. 4: Typical Image of Audio Jack
Volume Control for Audio from Smartphone – For controlling the volume input through the smart phone, a variable resistor (Shown as RV1 in the circuit diagram) is connected between the audio jack and the power amplifier. By adjusting this variable resistor, the volume of the audio from the smart phone can be adjusted.
Microphone – A Microphone is used to provide the other audio signal. An electret MIC has been taken as the input source. The electret MIC requires a biasing voltage between 1 to 5V to power the inbuilt FET buffer which is present in the MIC. Generally, this microphone is powered by 1 V to 5 V DC through a resistor having value from 1K ohms to 10K ohms. There must be sufficient voltage at the bias pin of the microphone so that it could sense the audio signal.
Fig. 5: Typical Image of Electret Microphone
The biasing circuit for this microphone has been shown below –
Fig. 6: Circuit Diagram of Electret Microphone Biasing
In the circuit diagram, a resistive divider network (Shown as R1 and R2 in the circuit diagram) is used to provide a fixed voltage to the MIC. A capacitor (Shown as C1 in the circuit diagram) of 10 uF is connected at the resistor’s junction point which helps in maintaining the voltage at microphone pin. The other pin of the electret microphone is connected to the common ground.
Learn more about microphones from the following tutorial –
MAX4468 Microphone Amplifier – MAX4468 is a low cost, micro power microphone pre-amplifier. This IC operates over a supply voltage in range from 2.4 V to 5.5 V and provides high voltage gain up to 125 dB.
Fig. 7: Typical Image of MAX4468 IC
The MAX4468 IC has the following pin configuration –
Fig. 8: Table listing pin configuration of MAX4468 IC
The IC pins are just like the OPAM pins with the only difference of MIC bias pin. The IC comes in SMD package and has the following pin diagram –
Fig. 9: Pin Diagram of MAX4468 IC
The IC is used as inverting amplifier. The inverting amplifier has negative feedback which makes them better than non- inverting amplifiers. The inverting amplifier changes the phase of output (signal amplitude) by 180 degrees with the input (signal amplitude). However, this phase inversion does not affect the audio signal as human ears are response only to the intensity of the sound. The intensity is the energy flowing through an area in a given time, expressed in joule/s/m2. The energy of the wave is proportional to the square of its amplitude. So for a unit area, the intensity is also proportional to the square of the amplitude.
I A2
So, changing the sign of wave has no effect on I.
For providing the DC bias to the non-inverting pin of the amplifier, a capacitor (Shown as C3 in the circuit diagram) of 0.01 uF is connected along with a resistor divider network at pin 3 of the IC. A capacitor (Shown as C2 in the circuit diagram) of 0.1uF is connected at the inverting input pin of the IC to provide the decoupling of AC signal from DC. A capacitor (Shown as C4 in the circuit diagram) of 47 pF alters the frequency response of the amplifier. This capacitor acts like a short circuit at high frequency and allow signal to bypass through the resistor R2 so the gain of the amplifier is reduced at high frequency. At low frequency, this capacitor acts like an open circuit so it does not affect the gain. Therefore, this capacitor act as high pass filter in the circuit.
The typical application circuit as given in the datasheet of MAX4468 is used for designing this pre-amplifier.
Fig. 10: Circuit Diagram of MAX4468 IC based Microphone Audio Pre Amplifier
The line output is drawn from the pin 8 of the IC. The preamp circuit looks like as shown in the figure below –
Fig. 11: Prototype of MAX4468 IC based Microphone Audio Pre Amplifier
Volume Control for Microphone – For controlling volume input through the microphone, a variable resistor (Shown as RV2 in the circuit diagram) is connected between the preamp output pin and the power amplifier. By adjusting this variable resistor, the volume of the audio from the microphone can be adjusted.
Summer and LM386 Power Amplifier – The two audio sources (MIC and audio track) need a summer to get mixed with each other. For this, LM386 IC is used as a summer as well as power amplifier. LM386 is a low voltage audio power amplifier IC. It operates between a voltage range of 4 V to 12 V. In this circuit, the IC is provided a bias voltage of 9 V. This IC can drive a load having impedance in range from 4 ohms to 32 ohms. As the speaker used as load at the output of the amplifier has 8 ohms impedance, the IC is suitable to drive it well. Internally, the voltage gain of the IC is set to 20 (26 dB) but it can be set between 20 (26 dB) to 200 (46 dB) by connecting a suitable combination of resistor and capacitor between its pins 1 and 8. The IC has 8 pins in PDIP package with the following pin configuration –
Fig. 12: Table listing pin configuration of LM386 IC
The IC has the following pin diagram –
Fig. 13: Pin Diagram of LM386 IC
The IC has the following Internal Diagram –
Fig. 14: Internal Circuit Diagram of LM386 IC
Its internal circuitry can be represented by the following functional diagram –
Fig. 15: Functional Diagram of LM386 IC
This IC is basically an operational amplifier whose voltage gain can be adjusted by using a proper RC circuit between its gain setting pins. If the gain setting pins are left open, the voltage gain of the amplifier is internally set to 20 (26 dB). Since the pins are left open, the gain of the amplifier will be 20.
Fig. 16: Typical Image of LM-386 IC
The Pin 2 and 3 are the Input pins of IC. The pin 2 is the inverting input pin and it is grounded. The pin 3 is the non-inverting input pin and is used for feeding the audio signal from both the sources. The pin 4 is the ground pin and is connected to the common ground. The pin 6 is the power supply pin of IC and it is connected to 9V DC. A filter capacitor (Shown as C7 in the circuit diagram) of 100 uF is used for removing any high-frequency ripples at the input. At the pin 5 which is the output pin of the IC, a capacitor (Shown as C11 in the circuit diagram) of 1000 uF is connected to block any DC components. The DC components (as are appeared in case of clipping effect) can damage the speaker connected at the output of the circuit.
Along with this capacitor, an RC filter circuit consisting of a resistor (Shown as R7 in the circuit diagram) of 10 ohms and a capacitor (Shown as C10 in the circuit diagram) of 0.05 uF is used at the output pin. This is called a ‘Zobel network’. It ensures the impedance of speaker appears as a steady resistance for the amplifier after output. So it helps in stabilizing the frequency and oscillations at the output. If the capacitor C10 and resistor R7 are interchanged then it would be no longer a Zobel network but still, the output impedance will remain constant. The pin 7 which is the Bypass Terminal pin is grounded with a capacitor for improving the stability of the amplifier output.
Fig. 17: Circuit Diagram of 1 Watt Audio Power Amplifier
The amplifier circuit looks like as shown in the figure below –
Fig. 17: Prototype of 1 Watt Audio Power Amplifier
The LM386 also decides the output power of the mixer circuit. It will provide a power of approximately 700 mW for a load of 8 E at a supply voltage of 9V.
Speakers – A speaker of 10 Watt power rating and 8 ohms impedance is used as load at the output of the amplifier. The speaker is connected at pin 5 of the IC which is the output pin of the LM386 and the ground wire of the speaker is connected to the common ground. Since, the output power of the circuit is approximately 1 Watt, so, a speaker of minimum 1W or more wattage should be used. As per the availability, 8E 10 Watt speaker are used. A variable resistor (Shown as RV3 in the circuit diagram) is connected before the speaker to adjust its output volume.
Learn more about speakers from the following tutorial –
While assembling this circuit the following precautions must be taken care of –
1. Always use the filtering capacitor at the input terminal of power supply to avoid the unwanted ripples.
2. Use the speaker of equivalent or high power rating as amplifier output power.
3. Always use a series capacitor at the output of the amplifier to block any DC component.
4. Use Zobel network for frequency stability.
5. Always calculate the maximum power rating of the amplifier before connecting it to the speaker. The practical value may differ from theoretical one.
6. Always check the power rating of LM386 IC in its datasheet, as different companies have different ratings.
7. Avoid clipping of the output signal as it may damage the speaker.
8. Always place the components as close as possible to reduce the noise in the circuit.
How the circuit works –
The mixer circuit takes audio from two different sources of which one is the smart phone and other is the microphone. The audio input from the microphone needs to amplified to line level before directing to the power amplifier, so a MAX4468 Preamp is used. The audio from both the sources is feed to the non-inverting pin of the power amplifier IC. The amplifier also works as summer and combines the two audio signals. The audio waveforms from the microphone, audio jack and power amplifier output pin can be observed by connecting the CRO probes.
The audio waveform from the microphone is observed to be as follow –
Fig. 18: Graph of Audio Waveform from Microphone as observed on DSO
The audio waveform from the smart phone is observed to be as follow –
Fig. 19: Graph of Audio Waveform from Smart Phone as observed on DSO
The mixed waveform from the power amplifier is observed to be as follow –
Fig. 20: Graph of Audio Waveform from Audio Mixer as observed on DSO
The audio mixer designed in this tutorial is a 2-Channel audio mixer. It has an output power of 1 Watt approximately. The audio mixer is provided volume control at both input and output. This audio mixer circuit can be used in public addressing systems, audio recording and for DJ and musical concerts.
Filed Under: Electronic Projects
