![]() Figure 7 shows this phenomenon on the lower end of the signal. ![]() In real amplifiers, the behavior is less idealized, and some soft clipping will occur before the voltage rails are actually reached. Since the amplifier cannot output voltages outside the rails, any input that will create an output outside of the rails will actually be set to the voltage of the rail. In amplifiers, clipping occurs when the output signal starts to get close to the supply and ground voltage (referred to as rails). While there can be many sources of distortion, our system is designed specifically to detect clipping, which is a common primary source of distortion. We build a simple DC offset circuit, shown in Figure 5, to drive the audio amplifier within the 0 to 5 V range using a 10 ♟ capacitor and two 100k resistors. From this point on, we were very careful about checking the voltage range with an oscilloscope before connecting any signal to the input of the ADC. ![]() Bruce Land speculated that this excessive negative voltage switches CMOS logic into an always-on latch, which can not be reset except by removing power from the circuit, allowing constant current to flow, and in our case destroying the device. After consultation with Bruce Land and an oscilloscope, we realized this was because the audio jack of the Pi oscillates around its ground, generating signals in the range of ☑ V, rather than being DC offset from ground as we assumed. When we first tried to drive the signal to the ADC using amplified audio from the Pi instead of a function generator, the ADC overheated and burned out. The output is also connected to a 3W, 4Ω speaker through a 100 ♟ capacitor. The output of the audio amplifier is connected to the MCP3008 ADC chip, which is configured to take inputs from 0 to 5 V. This helped ensure that the amplifier would produce some visible distortion at peak input from the audio jack of the Pi, simplified powering it, and eliminated the need for a level shifter circuit on the output. Initially, we powered the amplifier using ☙ V, but ended up powering the amplifier directly from the Pi. Using the TDA4052A audio amplifier with the circuit shown in Figure 4, we were able to generate a clean audio output signal with a maximum voltage gain of ~35 dB and good stability.
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