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Nowadays, audio mixers are commonly used in many practical applications such as recording studios, public address systems, sound reinforcement systems, broadcasting, television, and film post-production. Audio mixer is an electronic device for mixing, routing, and changing the level, timbre or dynamics of audio signals in professional audio. With audio mixers, musicians, sound engineers, disk jockeys (DJs) are able to combine multiple audio sources into one seamless and harmonious output especially DJs will find them useful during performances at parties, concerts, and events where a continuous mix of music is necessary.
The objective of this project is to expose to engineering system development process and to enhance the skills and capability of designing and constructing circuit, troubleshooting, problem analysis and solving that required to design and implement the audio mixer project. Therefore, knowledge that gained in Circuit Theory, Electronics I, Electronics II, Electronics III, needed in order to understand the circuit and operation of the circuit. In this project, a basic audio mixer with human hearing range 20HZ to 20k HZ is attempted to build using fundamental knowledge on the operation of operational amplifiers and its application in cross fading, amplifying, and filtering.
1.3 Work Plan
Lab 1 Operation Amplifier Basic
Lab 2 Operation Amplifier Application 1
Lab 3 Operation Amplifier Application 2
Lab 4 Operation Amplifier Application 3
Lab 5 Operation Amplifier Application 4
Lab 6 Operation Amplifier Application 5
Lab 7 Audio Mixer
1.4 Design Methodology
Figure 1.1: Design methodology and flow
Figure 1.1 shows the design flow of the whole project. An overall block diagram of the design is drawn to have a general idea on the operation of the whole design. Next, appropriate circuits are studied and analyzed from the internet resources and related materials on their operations. A simple circuit schematic is drawn and checked by using Pspice. When the schematic check passed, circuit design will be generated. If the simulation is failed, troubleshooting and corrections need to be done then only repeat the simulation again.
Lastly, a complete report or thesis about the project will be written to record the details of the entire project.
1.5 Organization of Report
This report is organized into 5 chapters. In chapter 1, a brief introduction on the audio mixer is provided. Chapter 2 introduces the backgrounds and circuit construction of the basic amplifier circuits that need for audio mixer. Besides, its applications also discussed in detailed. Analysis and the discussion for each stage of the project before combining circuit will be discussed in chapter 3. Next, circuit design, circuit testing of basic audio mixer and problems occurred during construction will be discussed in chapter 4. Besides, chapter 4 also introduces possible improvements which could be carried out in the next stage of project. This report is concluded in chapter 5, which discussing the final outcome of project.
CHAPTER 2: BACKGROUND AND CIRCUIT CONSTRUCTION
2.1 Basic amplifier circuits
At first, operational amplifier (op amp) could be modified by external circuitry easily to perform mathematical operations such as addition, integration and so on in analog-computer applications. Nevertheless, op amps have become more reliable, miniaturized, temperature-stabilized and very useful in wide range of applications such as summing amplifier, voltage follower, comparator and active filter with the advent of solid-state technology.
Figure 2.1: Op amp 741 pin connections
Figure 2.1 above shows the pin connections of op amp 741.Notice that there is a dot in the corner, or notch at the end of the package which identifies the end from which to begin counting the pin numbers.
The ideal op-amp would be expected to have the following important characteristics:
1) Infinite input impedance, .
2) Zero output impedance, .
3) Infinite open-loop voltage gain, .
4) Infinite bandwidth, AOL remains unchanged from DC to very high frequency.
5) Zero offset voltage, zero input (V+=V-) produces zero output.
6) Infinite common-mode rejection only amplifies voltage difference between non-inverting and inverting input.
The characteristics above in turn form the basis for two fundamental rules of an ideal op-amp which is called as the golden rules. The golden rules are idealizations of op amp behavior.
However, they are very useful for describing overall performance nevertheless since they are applicable whenever op amps are configured with negative feedback. Consider the golden rules to be exact when ideal op amp behavior is assumed. Following are two simple statements of the golden rules:
No current flows into either of the input terminal
There is no voltage difference between the two input terminals
2.1.1 Voltage Follower/Buffer
Figure 2.2: Voltage Follower/Buffer
Voltage follower or buffer is shown in Figure 2.2. From the equation, closed loop gain ACL is only dependent on the ratio of RF and R1 .Besides, the closed loop gain ACL becomes one or unity when RF goes to zero and R1 approaches infinity from the above equation. Isolating properties including high input impedance and extremely low output impedance causing an amplifier of unity gain called as a buffer. In addition, it has the best bandwidth of any op amp circuit. Voltage follower provides effective isolation of the output from the signal source that causing very little power is drawn from the signal source. Therefore, loading effect is eliminated.
2.2 Operation Amplifier applications
2.2.1 Summing Amplifier
Summing amplifier multiple input signals in any proportion desired and is commonly called as a signal mixer since it is used to combine audio signal from several inputs such as microphones, guitar, tape recorders and so on to provide a single output. Summing amplifier can be categorized into two types that are inverting amplifier and non inverting amplifier.
Figure 2.3: Inverting summer
Figure 2.3 above shows an inverting summer. From the equation above, if , then the output is the sum of both the inputs inverted where .
Comparator is a device which compares two voltages or currents and switches its output to indicate which is larger. In other words, it provides an output signal that stays high while the input signal is higher than the reference signal or threshold. When the input signal changes slightly, comparator is useful
in providing a large change in output signal. Besides, it can be used to convert an analogue signal into a digital signal.
Figure 2.4: Comparator
Crossfader is used to implement smooth transitions between two music sources. On a DJ mixer, crossfader is basically functions like two faders connected side-by-side, but in opposite directions to allow DJ to fade one source out while fading another source at the same moment. The crossfader would keep constant output level in the perfect case but there is no customary on how this should be achieved. Most crossfaders are variable resistors where only two terminals of potentiometer which are one side and the wiper are being used. At the end of variable resistor, the input signals are connected. The wiper which is third contact slides between the end contacts as the crossfader knob is tuned. Crossfader is simply a summing amplifier.
The output voltage of crossfader is expressed as above where as in below Figure 2.5.
Figure 2.5: Crossfader
2.2.4 3-band Graphic Equalizer
Figure 2.6: 3-band Graphic Equalizer
There are many kinds of equalization which each of them has a different pattern of attenuation or boost. 3-band Graphic Equalizer is shown in the Figure 2.6. The graphic equalizer is developed to equalize the sound of a room and
a sound system's frequency response. They are widely seen in middle and high end stereo sound systems and other consumer products. Graphic equalizer consists of a bank of sliders for boosting and cutting different bands or frequency ranges of sound and they are used to correct the reduced level of high frequencies in early telephone systems as well as in modern digital telephone systems and in individual channels of a mixing board, where the sound waveform is being applied with equalization.
2.2.5 Voltage Level Indicator
Figure 2.7: Voltage level indicator
Voltage indicator is a voltage comparator which is shown in the Figure 2.7. It is used to compare two signals or in the case here it is used to know whether it is exceeds the thresholds voltage. Several comparators share a common input and each of the comparator is then supplied with a different reference or triggering voltage. The reference voltage to each comparator is simply a simple voltage divider. When the inputs exceed the reference voltage, the LED will be turned ON.
CHAPTER 3: Analysis and Discussion
Before combining all the circuits, each stage of the basic audio mixer has been tested so that errors and noises can be reduced when combining the circuit.
3.1 Voltage Follower/Buffer
Figure 3.1: Input and output waveform of buffer
For CH1: Vin = 2 - 0.5V/div = 1V
For CH2: Vout = 2 - 0.5V/div = 1V
Voltage gain, Av = Vout / Vin = 1
Percentage of error = 0%
The result is satisfied as the percentage of error between theoretical and experimental result is zero.
3.2 Inverting Summer
Figure 3.2: Input and output waveform of inverting summer
For CH1: Vin = 2 - 0.5V/div = 1V
For CH2: Vout = 5.2 - 1V/div = 5.2V
Theoretical result for Vout = (-Rf / R1) *Vin1 + (-Rf / R2)* Vin2
= (-5k / 1k)(1) + (-5k / 10k)(1)
= 5.5 V
Percentage of error = [ | 5.2 - 5.5 | / 5.5] - 100%
The result is quite satisfied as the percentage of error between the theoretical and experimental result is only 5.45%.
Figure 3.3: Output waveform of crossfader when connect both input Vin1 and Vin2
For CH1: Vin = 1 - 1V/div = 1V
For CH2: Vout = 3.8 - 5/div = 19V
Av = 20 log (Vout / Vin) = 20 log 19
Figure 3.4: Output waveform of crossfader when V- is set to center position
3.4 3-band Graphic Equalizer
Vin, pp (V)
Table 3.1 Frequency response of 3-band graphic equalizer (Rf = 0Ω)
Vin, pp (V)
Table 3.2 Frequency response of 3-band graphic equalizer (Rf =10kΩ)
Graph 3.1: Frequency response of 3-band graphic equalizer (Rf = 0Ω)
Graph 3.2: Frequency response of 3-band graphic equalizer (Rf =10kΩ)
3.5 Voltage Level Indicator
Figure 3.5: Voltage level indicator
The flag denotes the cathode. During circuit construction, the anode and cathode pin of LED have to connect correctly onto the breadboard. During the experiment, the anode and cathode pin have been connect wrongly onto the breadboard and it was found that all LEDs are ON once connect to the power supply. After troubleshooting and reconstructing the circuit, the problem been solved. The LED was ON one by one by increasing the input voltage.
CHAPTER 4: BASIC AUDIO MIXER DESIGN
4.1 Circuit Design
Voltage Level Indicator 3-band Graphic Equalizer Voltage Follower/ Buffer
Figure 4.1: Circuit construction for the audio mixer
The circuit was design where the voltage level indicator do not connected to the buffer which has connected to the speaker as well because the output voltage from the buffer could not support both of the voltage level indicator and speaker at the same time. This may due to not enough power supplying to the connection. So, the voltage level indicator was connected directly to the output terminal of the inverting summer.
Figure 4.2: Audio mixer design block diagram
4.2 Circuit Testing
Lower Cutoff Frequency (Hz)
Centre Frequency (Hz)
Upper Cutoff Frequency (Hz)
Table 4.1: The frequency response of the equalizer bands
The treble controls the high tones, middle controls the mid-range tones and bass controls the low tones. The mixing of two songs sounded nicer when bass, middle and treble were tuned to the minimum level. Once tuning larger the three bands, quality of the sound is acceptable at the beginning until certain range, the sound became noisy and especially tuning the treble into maximum, the songs were not sounded in perfect pitch and were very noisy.
Figure 4.3: Output waveform when bass, middle and treble were tuned to minimum
Figure 4.3 shows the output waveform when bass, middle and treble were tuned to minimum. There was no clipping for output waveform when the bass, middle frequency and treble were tuned off.
Figure 4.4: Output waveform when only bass was tuned to the maximum level
The output waveform where only bass was tuned to maximum level is shown in Figure 4.4.There was clipping at upper and lower part of the output waveform. When an amplifier overdriven and attempts to deliver an output voltage or current beyond its maximum capability, clipping is occurs and it is a form of waveform distortion. Bass band consists of more clipping compared to middle band and treble band as the range of frequency covered by the bass band is smaller which is shown in Table 4.1.
Figure 4.5: Output waveform when only middle was tuned to the maximum level
Figure 4.5 shows the output when only middle was tuned to the maximum level and it was observed that there was clipping at upper part of the waveform. However, there was lesser clipping at the bottom part of the output waveform compared to the output waveform of bass band. This may due to the middle band covers range of frequencies from 1989.4368Hz till 5894.6275Hz which is much higher than bass band. Therefore, the clipping has become smaller.
Figure 4.6: Output waveform when only treble was tuned to the maximum level
It was observed that there was no clipping at the lower part of the waveform only treble was tuned to the maximum level which is shown in Figure 4.6. This may due to treble band cover higher range of frequencies from 4232.8442Hz to 12541.7607Hz which is shown in Table 4.1 compared to bass and middle.
At the beginning stage of the project, the circuit constructed was quite messy, this made troubleshooting more difficult. After reconstructing the circuit by reducing the length of jumper wires, the error can be traced easier. As the project going on, more experience gained from the error and careless mistake that had been made. Therefore, extra care will be taken during constructing the circuit.
During circuit testing, it was found that noises, distortion and loading effects were occurred. In order to reduce the noise and distortion, circuit may be modified by adding higher band from original 3-band to more band in next stage of the project so that clearer sound with human hearing range 20HZ to 20k HZ can be produced.
Besides that, throughout the project, it was found that some of the outputs of the potentiometers were not consistent causing the correct result could not be achieved. It was observed that the output waveform of the crossfader was distorted when combining the circuit. However, there was no any connection fault after troubleshooting the circuit many times until the potentiometer of the crossfader has been soldered wires. The problem has been solved after soldering the wires and desired results were managed to be achieved. Therefore, enforcement to the soldered wires were done as to ensure that more stable output and robust structure can be achieved due to the inconsistency in the output of quality of the potentiometers.
During the testing stage, it is observed that there are some connection faults in the breadboard when combining the circuit .Besides, the components' legs and jumper wires did not inserted properly into breadboard causing no sound produced from speaker. However, with patience in troubleshooting and extra attention, the connection fault would not be a major obstacle to proceed with the testing procedures and assembly steps.
Future attempts of this project is to construct the circuit on the PCB board to increase the reliability of the circuit as PCB does not use the jumper wire and the connection loss problem can be eliminated and the messy circuit problem can be solved. As a result, size of the circuit can be reduced by using PCB for the circuit construction. Unlike a breadboard, PCB is designed as smaller as we can and will reduce the cost of the circuit. Furthermore, the next stage of this project is trying to design power supply so that audio mixer can be operated easily in home or where else.
CHAPTER 5: CONCLUSION
The project of basic audio mixer was able to be completed within 7 weeks. Although there were a lot of barrier met, we had successful to overcome them with effort and patient. Skills such as troubleshooting the hardware, circuit designing and implementing skill are gained throughout the project. Therefore, the objective of this project which expose to engineering system development process and to enhance the skills and capability of designing and constructing circuit, troubleshooting, problem analysis and solving that required to design and implement the audio mixer project has been achieved. Overall, the design specifications of the mixer has been met and surpassed in certain areas.