revised previous paper

Student’s Name
Professor’s Name
Course
Date
Abstract
The project was to help students in designing an audio mixer able, to sum up, two audio signals from different sources and combine them to one signal that can be listened to. The two audio signal sources, in this case, were the microphones and music from a mobile phone. The maximum and minimum gain allowed by the music signal was computed as 40- and 9.69-Volts peak to peak. On the other hand, the maximum and minimum gains within with a microphone could be operated to achieve the desired output was 95- and 82.75-Volts peak-to-peak. The cutoff frequency determined from the experiment for both the music and microphone were 100 kHz and 54 kHz respectively. It was also noted at the end of the test that the graph displayed a curve with a polynomial characteristic and attained the optimal peak value. Despite a few challenges experienced during the experiment and the complexity involved in the designing the circuit; high precision was observed. In the end, a mixer with the ability to sum up two audio signals was designed, and a music source and microphone signals were successfully tested on the mixer.
Keywords: mixer, signal, music, microphones, maximum, minimum, cutoff frequency
Lab Report: Music and Microphone Mixer
Introduction
The overall aim of the design project was for students to come up with a music and microphone mixer circuit that can support both signal input from the music source and the microphones and with a suitable balance in the signal parameter; process the signals and produce an output that can be listened to.

The activities involved included designing, implementing and testing the various design circuits for the music and microphone mixer circuit such that it may be possible to combine both the output signal from the microphone and the music source and with appropriate amplification values, the speaker may play both signals without any interference. The experience gained from the design project in the lab will help in understanding the various operations principles of the op-amp circuits and hence provide background knowledge for creating op-amp circuit mixers with gains and gains adjustments that meet a specific design requirement.
Objectives
The objectives of the experiment were;
To be able to design different op-amp circuits that can support multiple signal inputs and meet specific gains and gain adjustments.
To be able to characterize the frequency response of the electrical circuit using a function generator and oscilloscope
To learn how to simulate the frequency response of a circuit using PSpice.
To learn how to plot frequency, phase, amplitudes response plots.
To be able to carry out a comparison of the different response plots with their respective plots generated by the PSpice software.
Theory
The basic operation principle of signal mixers includes the transformation of electrical voltage signals to an alternative current then later be summed up to produce and alternative currents that are supplied to the collector circuit. In this case, the mixer involves a combination of power and summing amplifier voltage signals. The simple AC summing circuit consists of four primary parts, that is, a feedback resistor connected across the op-amp, two capacitors, a few input resistors, and an operation amplifier. The purpose of the summing amplifiers is to merge all the weighted voltages, in this case, two voltage from the music source and the microphone. The value of the output voltage is given by
V0=(-V1+V2+V3+…+Vn)Design
A capacitor’s main function in the op-amp circuits is to blocker any DC signal in the circuit hence eliminating any cases of noise in the AC signal. According to Phillips (16), the value of current flowing through a capacitor corresponds to the rate of voltage change across it. This given by the equation below;
i=C×(dVdt)Such that dVdt represent the instantaneous change in voltage output.
As for the case of DC voltage, the voltage remains constant and the value of DV
According to Phillips (17), a capacitor design consists of two parallel plates with the ability to store AC charges. As a result, the instantaneous voltage of in a DC circuit is given as
dVdt=0Hence, there is no DC allowed to pass through the capacitors. On the other hand, AC voltage signals vary in a regular manner and hence the value
dVdt≠0In the case of a mixer, the capacitor denoted in Figure 1 as C2 plays the role of removing any DC signals from the music sources and prevent the output AC signal from having any form of distortions. It applies the concept of instantaneous voltage. As a result, the final signal in the collector is formed without any signal interference or noise.

Figure 1: Op Amp Simple Audio mixer with capacitors, op-amp and feedback resistor
Therefore, in the mixer circuit, the left source consists of the summed input voltages and the bias voltages. The left source voltage is more superior than the DC biasing sources. As a result, the left source imposes its full voltage at the input side of the op-amp. The feedback resistor denoted in figure one as R4 is connected across the op-amp and acts a feedback look. As a result, the op-amp functions as an inverting op-amp. The purpose of the feedback resistor according to Phillips (23) is to ensure the differential input voltage remains zero when the op-amp is in use.
Frequency response
Extracting the frequency response from the circuit provides the understanding of how circuits respond to any arbitrary input. At the time when there is an AC sinusoidal signal sent through the circuit, the steady-state output is created and takes the sinusoidal form of the input signal and the frequency. However, there are cases where the output signal’s amplitude and phase are higher or lower than then the amplitude and phase of the input signal. The analysis of the steady-state behavior of the circuit helps in determining the amplitude and the phased of the voltage signal at the output side of the circuit. The values can be found theoretically by applying the impedance and phasor concept. Once the frequency response has been computed, it is presented in two forms, that is, the amplitude response and the phase response. According to Phillips (24), the value of the normalized amplitude response is usually in decibels (dB) such that the quality plotted is given by
20log10VoutVinIn the equation, Vout represents the amplitude of the frequency response while Vin is the input signal’s amplitude. The values found are plotted against the function frequency.
Experimental Procedures
Before coming to the laboratory
In groups of four, we designed music and microphone mixer circuit that was as per the specifications indicated in Table DP1-A shown in the Appendix. The circuit design followed was that stated in the figure DP1-1 with the speaker, microphone, music source, and the power amplifier all connected in the circuit. The power amplifier used, in this case, had a voltage gain of 2/3 which was later used in the computation of the gain and gain amplifications then in the project. We used a ±5V voltage supply and included a capacitor in the circuit to block any DC voltage signals from interfering with the output signal. Moreover, the effect of the capacitor in the circuit was captured when making the frequency response measurements. However, we did not include any external component connected with the microphone that was not part of the design circuit in the computation of the frequency response.

Figure 2: External connection of audio mixer circuit
Since the solution for coming up with an appropriate mixer circuit is not uniquely stated anywhere, we implement several building blocks for the design indicated in Table DP1-B shown in the Appendix. To determine the frequency response characteristics of the circuit, we conducted a steady-state analysis of the circuit block we used and came up with the amplitude and phase response graphs. The expected graphs that we were to come up with from the steady state analysis were as shown in figure 3 below;

Figure 3: the General appearance of the amplitude response at the op and the phase response at the bottom plots for the given circuit.
We proceed to construct the circuit shown in figure 4 and simulate the circuit in PSpice. The circuit design implemented multiple voltage sources; a capacitor rated 1nF and resistor rates 1kΩ and a 50kΩ feedback resistor. We connected the resistors to an operational amplifier and an n-MOSFET and p-MOSFET transistors. Using the virtual probe application in the PSpice, we were able to simulate the circuits to test the workability of the circuit block we had selected, and if it met the specifications, we needed for the mixer. We also incorporated the oscilloscope and the function generated to get the readings and graphs for the amplitude and phase responses.
In the laboratory
Once we had verified which block to use, we proceeded to the laboratory to construct the circuit. We adjusted the mixer to meet the specification indicated in Table DP1-A. Using a 2kHz sinusoid, we determined the range of the adjustable gains of the music input while having the input of the microphone grounded. In this case, we were able to verify the maximum and the minimum gains for the channel. We did the same for the microphone input while having the music input grounded. We then grounded the microphones and disconnected the power amplifier and the speaker to measure and plot the frequency response for the music channel. We then tabulated the results. We repeated the process for up to 10 different frequencies with over three decades of frequency. We then ground the music input and ensured the power amplifies, and the speakers were disconnected and then plotted the frequency response for the microphone input. We repeated the same procedure for ten different frequency values. From the behavior of the waves, we saw displayed on each channel and computed the cutoff frequency and phase shift. According to Phillips (31), the cutoff frequency refers to the frequency values at which the amplitude of the input signal is higher than that of the output signal. When we were done, we tested the mixer qualitatively using the output from the sound care on the computer and the microphone.

Figure 4: Audio mixer PSpice Circuit
Result and Discussion
Frequency Hz Gain Phase
100 186 0
200 186 7
500 186 7.6
1000 186 8.2
2000 128 6.4
5000 146 8.3
10000 98 30.4
20000 55.23 34
50000 23.8 50
100000 14 70
Measurements
Music
Minimum Vin = 320 mVpp , Vout = 3.10 VppMinimum gain=3.100.32=9.69Maximum Vin = 120 mVpp, Vout =4.8 VppMaximum Gain=4.80.12=40Microphone
Minimum Vin = 80 mVpp Vout = 6.62 VppMinimum gain=6.620.080=82.75Maximum Vin = 80 mVpp Vout = 7.60 VppMaximum gain=7.60.080=95Frequency (Hz) Gain Phase
100 7.33 0
200 7.33 0
500 7.33 0
1000 7.33 0
2000 7 -23.6
5000 5.33 43.1
10000 4.03 74
20000 2.5 79.9
50000 1.04 87.3
100000 0.444 94.1

Figure 5: Voltage phase (mic)

Figure 6: Voltage mic trace

Figure 7: Voltage phase phone

Figure 8: Voltage phase trace
lefttopGraph 1: 20log10VoutVin as a function of the frequency log scale
Mixers are used mainly for frequency conversion of various input signals to appropriate level that both signals can be heard without any distortion. The ability to develop a mixer based on constructing a nonlinear or time variant circuit since linear time-variant channels are incapable of creating new frequencies. In this case, the input signal ends up being varied such that it develops a time-varying gain as a result of the influence of another input signal. The experiment was carried out to determine the possibility of summing up to signals using both summing and power amplifier within the same circuit. For the summing circuit, the components included were feedback and input resistors. By varying the different voltages in the summing circuit, different weighted voltages were obtained equal to the summation of the voltages in the circuit.
The design of the circuit was done such that it could support a minimum of 2 audio devices. In this case, ensuring the amplifier operates without being saturated. The components making up the circuit consisted of two inputs, that is one for the microphone and the other for the speaker. A power-amp was connected to the speaker since it is usually used in driving the speaker other than the op-amp. Besides, the circuit included a pre-amp to normalize the signal inputs. From the Figure 4, the circuit consisted of optional pre-amp, potentiometer, and a capacitor. The capacitor was used in the circuit to filter DC signals that might originate from any of the sources. According to Phillips (24), the presence of DC signals in the circuit tends to cause the output signals in the circuit to be distorted. As a result, it was mandatory to have a capacitor include right after the music source. On the other hand, a potentiometer was included in both input op-amps to control the voltage of each channel. Additionally, the circuit included a series resistor meant to maintain the op-amp not to operate as a differentiator.
From the results obtained in the PSpice, the graphical representation of the gain verse frequency, it was noted that the gain of the music remains constant upto 1kHz before it gradually started to drop until reaching zero. The phase, on the other hand, remains relatively constant before beginning to make a considerable drop at around 3kHz. Meanwhile, the gain for the microphone slowly rises, levelize and starts dropping the when the frequency exceeds 3kHz. As for music source gain, it can be noted that the signal VRF(t) undergoes a shift in its frequency while still maintaining a spectral shape. As a result, the frequency conversation produced a sinusoid component controllable at a specific frequency.
There was a gradual drop in the phase voltage of the microphone with the increase in frequency as shown in figure 5. In figure 6, the voltage trace for the microphone gradually rose and remained constant between a frequency range of 800Hz and 30kHz. The frequency then dropped steadily to zero. The voltage phase of the phone as shown in Figure 7 indicates a relatively constant change in phase until slightly beyond 1kHz where the phase drops exponentially. In figure 8, the phase shift and frequency were fairly constant until 3kHz where is gradually decreased to zero.
From the signal activity of the mixer as observed in the oscilloscope, it was possible to attain the circuit characteristics. The experiment data indicated the maximum and minimum gains that the microphone could operate were 95 and 82.75 peak-to-peak voltages. On the other hand, that for the music was 40 and 9.69 peak-to-peak. There was a cutoff frequency for the microphone at 54kHz that made the signal rise to 3.12 peak-to-peak volts.
As for the music source, the cutoff frequency was 100kHz and caused the signal to rise to 1.28 peak-to-peak volts. At the end of the experiment, the signal coupling was achieved successfully the same as the summations and amplification of the signals. Graph 1, on the other hand, indicated a polynomial trend with both the gain and the frequency going as high as the optimal peak value.
Care was taken throughout the experiment to ensure that both channels were of opposite polarity to prevent the clipping or to distort the signal. By computing the gains from each input, that is, the mic and the music gains, each individually, it was possible to create a mixer that had an end product of the sound output of either the phone and the microphone or a combination of both.
Conclusion
The project provided us with the experience of using different signal sources, summing them up and amplifying the product using a power amplifier. As a result, we successfully assembled an audio mixer with the abilities, to sum up, audio signals. Other notable features during the experiment were that there was a significant change in the output signal when the frequency of the input signal was changed. Hence by manipulating the frequencies, one can be able to alter the output signal strength and the boundaries.
Work Cited
Phillips, Peter. Electrical Principles. South Melbourne, Vic: Cengage Learning Australia, 2012. Print.

Appendix
Table DP1-A Specifications
Supply Voltage ±5VMusic (sound card) input 1V peak-to-peak
Microphone input 10 mV peak-to-peak
Maximum voltage gains The maximum voltage gains should be such that 350 mW peak power will be delivered to an 8 Ohm speak with either channel adjusted to maximum gain and the other channel set to zero
Range of music channel gain Adjustable from zero to maximum
Range of microphone channel gain Adjustable from zero to maximum
Table DP1-B Some possible building blocks for the Mixer Circuit