Audio Amplifier - Nathan Castek`s Portfolio

EE 201 Lab Report
Lab Final Project – Two-Channel Headphone Amplifier
By: Austin Benson and Nathan Castek
Lab section C – Tues at 3:00 p.m.
Lab Instructor: Bob Sheldon
In this lab, our primary objective was to design a two-channel audio amplifier with the
following requirements: an amplitude gain of 20 (±1) for each of the channels, a band
pass filter with an upper limit of 15 kHz and a lower limit of 150 Hz for each of the
channels, and a volume control for each of the channels in the form of a potentiometer.
This experiment involved many topics we learned throughout the course including: opamps, filters, voltage dividers, capacitors and potentiometers.
Experimental Design
To begin, we drew up the following block-diagram for our two-channel stereo amp:
In order to attach the input to the breadboard, we used a male, 1/8” stereo connector. In
order to connect the breadboard output back to the headphones we used a female, 1/8”
stereo connector. In order to simplify the design of the circuit, we used active filters to
amplify the signal as well as filter the input at given frequencies. For the volume control,
we used non-inverting amplifiers and potentiometers. This setup resulted in the
following implementation for one channel of the amplifier:
The following is the diagram for the filters with the resistors labeled:
In designing the filters, we wanted to ensure that our band-pass filter allowed has a lower
corner frequency of 150 Hz, and an upper corner frequency of 15 kHz:
In achieving the desired audio gain, we used the following combination of resistors to get
close to the desired gain of 20:
Next, we designed the volume controls using a simple non-inverting amplifier using a
resistor and a potentiometer as shown below:
We then built a second channel using the exact same design. The final design for the
circuit after all the calculations were completed is shown below:
We then implemented our design in the lab.
Results and discussion
In order to test whether or not our amplifier worked, we hooked our circuit up to an iPod
and played the song “Back in Black” by ACDC. We had the following observation:
1. The sound output was definitely amplified, as it was much louder when compared
to just hooking the circuit up to the headphones.
2. There was one noticeable difference between the sound quality of the amplified
sound wave and the unamplified sound wave. The sound coming from the
amplifier had a slight humming or static sound in the background. This may have
been due to the fact that these sound frequencies were amplified, making them
much more noticeable than when they were not amplified.
3. The volume controls for the channels worked so that you could control the
volume of the music in each channel.
Overall, the circuit worked as it was supposed to and we completed all of the necessary
requirements of building an effective two-channel amplifier.
This project was a good culmination of the topics we learned throughout EE 201. We
took the knowledge we gained from lectures and lab experiments and used it to design a
real-world, useful circuit. Though our circuit was not perfect due to the impreciseness of
our parts, we were still able to develop an effective and working operational amplifier.
Hopefully in the future we will be able to build upon all that we have learned and use it to
design bigger, better, and more efficient circuits in the future.