Introduction to Summing Amplifiers

One of the applications of summing amplifiers is to mix or combine signals. You must have seen or heard the audio mixers used by DJs. The following diagram shows an audio mixer.
In this week, you will construct summing amplifiers using op-amps. You will calculate and observe the output signals and verify how the output is related to the inputs. Toward the end of this part of the project, you will complete the design and construction of a simplified audio mixer.
Procedures

Part 1:

Write the formula for the summing amplifier shown in Figure 1. You can find the formula (in terms of V1, V2, V3, R1, R2, R3, and Rf) from this week’s lesson or the textbook.
Build the summing amplifier shown in Figure 1.1. Notice that all three inputs are DC voltage sources. Notice that the input resistors and feedback resistor have the same value and that the inputs are connected to the negative terminal of the op-amp. Therefore, the gain for each input is -1.
Run the simulation. Observe the measurement on the multimeter. It should be the inverted sum of the input voltages since the gain for each input voltage is -1. Therefore, it should be close to -6.
Change the inputs to = -2 V, = -2 V, = -3 V. Run the simulation. Take a screenshot of the circuit with the multimeter display and paste it to ECT226 Project Deliverables Module 6. Please make sure that the multimeter does not block your circuit.

Increase the input voltages to = 6 V, = 7 V, = 8 V. Run the simulation. Take a screenshot of the circuit with the multimeter display and paste it to ECT226 Project Deliverables Module 6. Please make sure that the multimeter does not block your circuit.
Discuss the results obtained in Steps 4, and 5 and write the explanation in the ECT226 Project Deliverables Module 6. Explain why the voltage that you measure on the multimeter for Step 5 is not equal to the inverted sum of the input voltages. Please make sure to include an explanation of your result in Step 4.

In part 2,

we will build a simplified audio mixer. Remember to save the simulation circuit that you build (similar to Figure 2.1) and you will need to use it in the final project. The frequencies of audio signals typically range between 20 Hz to 20,000 Hz. For simplicity, we will use only three frequencies in the experiment.
Modify the schematic in Figure 1.1 to the schematic shown in Figure 2.1 below. Instead of using DC power inputs, we feed the summing amplifier three AC signals from three function generators. A 4-channel oscilloscope is used to display the three inputs and the output.
Set the three function generators as follows. Note that the 200 Hz frequency instead of the low boundary of audio (20 Hz) is used since 20-Hz waveform will be difficult to observe together with the higher 1 kHz and 20 kHz frequencies.

Run the simulation. Figure 2.3 shows the suggested settings for the vertical and horizontal scales of the 4-channel oscilloscope. Notice that you will need to click on the dial button to A position to set the vertical scale and offset voltage for channel A, B position for channel B, …, and so forth.
Take a screenshot of the simulation results (similar to figure 2.4) obtained up to Step 4 and paste it to ECT226 Project Deliverables Module 6.
So far, you have added (combining) signals, both in DC and AC. Now, you will experiment with mixing signals. The slide switches on the audio mixer are essentially used to change the gains for different frequencies.

We could have used the variable resistors for each of the R1, R2, and R3 as the slide switches in the design. But, the circuit will be more complex. The variation of gains can be directly obtained from the three function generators by adjusting the signals’ amplitudes.
Increase V1 (XFG1) to 1 Vp and keep V2 (XFG2) and V3 (XFG3) unchanged. Your oscilloscope screen should look similar to Figure 2.5 after adjusting the offset voltages for all channels. All three input signals are moved to the top. We want to concentrate on the output.
Keep all frequencies the same and V1 at 1 Vp. Change V2 to 250 mVp and V3 to 750 mVp. Run the simulation again. Paste the screenshot of the oscilloscope to ECT226 Project Deliverables Module 6.
Write your summary in ECT226 Project Deliverables Module 6. Your summer should include the questions below.

What did you learn from this lab project?
What problems did you encounter? How did you solve them?

The audio mixer output can be impacted by changing the input amplitudes. When the output waveform is changed, the sound we hear will be different.

Explain this by comparing the results from figure 2.4, figure 2.5 and the screenshot of oscilloscope in step 8.
Figure 1.1: Summing Amplifier

Part 2: Audio Mixer

Figure 2.1: Summing amplifier to combine three AC signals
Figure 2.2: Three AC signals input to the summing amplifier
Figure 2.3: Vertical setting for the three input signals on channels A, B, C,
and the output signal on channel D
Figure 2.4: Screen capture of the 4-channel oscilloscope
Figure 2.5: Screen capture of the oscilloscope after changing V1 to 1 Vp

Last Completed Projects