Final Project: Bluetooth Speaker

For my final project, I created a simple audio amplifier using the NTE823 op-amp. My amplifier is powered by a nine-volt battery with a 10 microfarad capacitor in parallel. The capacitor is connected close to the Vcc pin to ensure that the amplifier receives the power it needs for loud bass notes. Additionally, I connected a green LED and a current-limiting resistor in series with the battery as a visual indicator that the circuit was live. The input for my audio amplifier is a male stereo headphone jack.

Since there is only one input pin for the op-amp, I had to convert the stereo signal to a mono signal by putting two 1k ohm resistors in series with the left and right signal wires. I also connected pins 1 and 8 together with a 10 microfarad capacitor in series with a 10k ohm potentiometer. By varying the resistance across pins 1 and 8, I am able to adjust the gain of the NTE823 from 20 to 200. However, in order to connect pins 1 and 8, we must short the bypass pin (7) with a 10 microfarad capacitor. I also connected a 0.1 microfarad capacitor in parallel with the output speaker for filtering out white noise. Similarly, I connected a 1000 microfarad decoupling capacitor in series with the output speaker to ensure that there is no DC offset present in the output audio signal. I also have a switch that allows me to alternate between aux mode and Bluetooth mode.

When the switch is moved, it sends the nine volts from the battery through an L4940V5 voltage regulator to power the USB Bluetooth dongle. I really like this Bluetooth dongle because it is simple to implement---i.e. just plugging the male audio jack into it. I also put a couple of 10 microfarad capacitors in parallel with the input and output of the 5V regulator to ensure that I was consistently receiving a five-volt output. Unfortunately, I learned the hard way that linear voltage regulators such as the one I used tend to have a lot of noise at the output and are therefore impractical for noise sensitive applications such as audio amplification. To remedy this issue, I inserted an emitter follower circuit using a BJT and a high pass filter to minimize the high-frequency signals I was getting from the voltage regulator. This circuit worked moderately well, but there is still some audible noise present in the output of the speaker. Another way I could have solved this issue is by using a high pass filter with an op-amp.

Note: This circuit could have also been done using the LM-386 op-amp. This is because they have the same pinout and function very similar to one another.