University of Portland
School of Engineering
5000 N. Willamette Blvd.
Portland, OR 97203-5798
Phone 503 943 7314
Fax 503 943 7316
Theory of Operations
Project Brown Trout: In-Line
Headphone Amplifier
Contributors:
Jim Bosak
Jeff Sharp
Casey Hughes
Approvals
Name
Dr. Inan
Date
Name
Date
Dr. Lillevik
Insert checkmark (√) next to name and add the date when approved.
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Revision History
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THEORY OF OPERATIONS
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Author
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Reason for Changes
Initial draft
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Table of Contents
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Summary.......................................................................................................................
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Introduction ..................................................................................................................
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Background .................................................................................................................. 3
Architecture .................................................................................................................. 4
General Description ................................................................................................................................4
Hardware Architecture ............................................................................................................................5
CD Player........................................................................................................................................................5
Amplifiers ........................................................................................................................................................5
Gain Switch .....................................................................................................................................................5
Battery Pack....................................................................................................................................................5
Headphones ...................................................................................................................................................5
Design Overview.......................................................................................................... 6
System Block Diagram............................................................................................................................6
Analog Circuits ........................................................................................................................................6
Unity Gain Buffer ............................................................................................................................................7
Low Pass Amplifier .........................................................................................................................................7
High Pass Amplifier ........................................................................................................................................8
Full Circuit .................................................................................................................................................... 10
Conclusions ...............................................................................................................11
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List of Figures.
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Figure 1. Hardware Architecture
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Figure 2. Brown Trout Schematic Block Diagram…………………………………………………………...4
Figure 3. Brown Trout’s Unity Gain Buffer. ...................................................................................................4
Figure 4. Brown Trout’s Low-Pass Fliter. ......................................................................................................4
Figure 5. Brown Trout’s High-Pass Filter. .....................................................................................................4
Figure 6. Brown Trout’s Full Circuit (one channel). ......................................................................................4
UNIVERSITY OF PORTLAND
SCHOOL OF ENGINEERING
CONTACT: JIM BOSAK
THEORY OF OPERATIONS
PROJECT BROWN TROUT
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Summary
Project Brown Trout is an in-line headphone amplifier that will allow a user to increase the
amount of sound coming out of a personal listening device to any of three additional
settings as well as being able to have the device on and being able to hear the sound as it
comes directly from the listening device.
The design of the Brown Trout Amplifier is simple. The input signal can be either stereo or
mono sound. The signals will be split into their left and right channels and amplified
separately. In essence the Brown Trout Amplifier is really two separate amplifiers. The
first stage of each amplifier is a unity gain buffer used so that input impedances are high.
The next stage is a high-pass filter with a gain of 1. This is used to filter out any sound
below 20 Hz. The final stage is a low-pass filter designed to break at 20 kHz. This low
pass filter has three gain resistors, one of which the user will choose according to how
much louder they want the signal. With the low and high pass filters in series, the amplifier
is basically a variable gain band pass filter with user defined gain. After the signal has
been amplified, it will be output to a user’s headphones, which again can be stereo or
mono.
The key concern in the production of the Brown Trout Amplifier is sound quality. There is
no need for a loud noisy signal. The second main concern is power consumption. Once
again, there is probably little desire to have to carry a couple pounds of batteries in order
to be able to listen for only a short period of time. With that in mind the design of Brown
Trout is constructed of components that minimize noise and power consumption. All of
Brown Trout’s resistors are metal film resistors which are less susceptible to Johnson
Noise. Resistor values were chosen to reduce the amount of current drawn by the circuit
and minimize resistor. Whenever possible electrolytic capacitors are used in order to
reduce noise produced by the amplifier, however, when unable to find certain values of
electrolytic capacitors, ceramic capacitors will suffice. The op amps chosen for this design
are low power low noise in design, helping the overall concerns of the amplifier.
UNIVERSITY OF PORTLAND
SCHOOL OF ENGINEERING
CONTACT: JIM BOSAK
THEORY OF OPERATIONS
PROJECT BROWN TROUT
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Introduction
This Theory of Operations document has been developed to inform the reader,
presumably higher ups in a corporation, or our advisors, of how the Brown Trout design is
being implemented. This document will help you to better understand the functions behind
our design and what each part of the system does in its own respect and why it needs to
do its specified function. This document will start from the top and work its way into the
inner workings of the design, outlining why each component is there and what it does.
The first portion of the document will describe the high level architecture of the Brown
Trout design. Each major component will be introduced in this chapter, and the preceding
chapter will specifically explain the amplifier portion of Brown Trout.
UNIVERSITY OF PORTLAND
SCHOOL OF ENGINEERING
CONTACT: JIM BOSAK
THEORY OF OPERATIONS
PROJECT BROWN TROUT
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Background
Portable audio devices have been around for many years now. Technology has
progressed from portable cassette players to CD players to MP3 machines. In a society
where people are constantly on the move, the demand for advanced technology in the
portable audio realm will undoubtedly continue to increase. Unfortunately, a disturbing
trend has developed with the advancement of portable audio technology. While new
players handle different audio formats and offer improvements in usability and aesthetics,
the option of internal amplification often gets pushed aside for power efficiency. At high
volumes, the signal quality degrades to a point that is not tolerable by most people’s
standards.
For many users, improved amplification is unimportant considering the conditions under
which the audio device is used. However, with today’s lightweight and durable audio
devices, some users want to listen to music while performing extreme activities such as
snowboarding down a mountain, or simply in loud environments like the busy streets of
New York.
Today’s audio devices do not produce a powerful enough signal to offer a clear sound in
accordance with the needs of many users. In loud environments, users will peg the
volume control on their device and receive a somewhat loud and highly distorted signal.
The In-line Headphone Amplifier proposed in this report, is a simple and inexpensive
solution that can be purchased by those who desire stronger signals for higher intensity
usage of their portable device.
With the purchase of a Brown Trout Amplifier, a person can simply insert two nine-volt
batteries, plug it into their portable listening device, and plug their headphones into the
amplifier. The user will then have the option of adjusting the sound between three levels
of gain as well as letting an un-amplified signal pass right through. The user will be able to
make the sound 2, 3, or 4 times louder than they would experience without the Brown
Trout Amplifier, and the signal will remain strong and clear.
UNIVERSITY OF PORTLAND
SCHOOL OF ENGINEERING
CONTACT: JIM BOSAK
THEORY OF OPERATIONS
PROJECT BROWN TROUT
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Architecture
This Chapter provides the architectural foundation for project Brown Trout. It provides a
high-level description of our system components which will be described in greater detail
in the following chapter.
General Description
Figure 1. Hardware Architecture of Brown Trout System.
Brown Trout will simply plug into the headphone jack on a portable audio device, and inturn, headphones will be plugged into Brown Trout. On the amplifier device, the user will
be able to select from gain levels of low, medium, and high. Alternatively, the device can
remain turned off for no amplification. After the gain level is selected, the user will be able
to fine-tune the volume with the standard volume control on the CD player unit.
Figure 1 shows a high-level diagram of the Brown Trout device. The stereo signal will
originate in the CD player, be split into right and left channels, and then these two
channels will be amplified independently with two separate amplifiers. The signal will then
be available for listening through stereo headphones. Independent of the amplifier layout,
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CONTACT: JIM BOSAK
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there will be a battery pack and a gain control switch that will serve both amplifiers
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simultaneously. The gain switch, battery pack, and both of the amplifiers will all be
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contained in a device housing with a permanently attached male-ended cable to connect
. and a female jack for the headphone plug.
to the CD player,
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Hardware Architecture
CD Player
The CD player will be the source of the signal upon which our design will operate. Brown
Trout will have a stereo cable originating in its circuitry that will plug directly into the CD
player headphone input jack.
Amplifiers
Each amplifier path in the Brown Trout design passes a band of 20 Hz to 20 kHz. The
gain within this band may take on three levels depending upon the position of the gain
switch including: 2, 3, or 4 times amplification of the original source signal. Two amplifiers
are needed because the signal from the CD player is in stereo. Thus, two separate
channels must be amplified simultaneously within the Brown Trout device.
Gain Switch
The gain switch selects between a parallel combination of feedback resistors in order for
the desired amount of amplification to be established (low, medium, high). Careful
attention was placed on the switching of the gain to avoid current spikes and other
unpleasant noises from momentary open circuit positions. There is also an “off” position
on the gain switch that will instigate the signal to bypass the amplifier circuitry and
disconnect the battery to eliminate power loss when the device amplification is undesired.
This feature allows the user to listen to the CD player without having to unplug the device
completely.
Battery Pack
The battery pack will house two 9 V batteries that will be used to supply power to both of
the Brown Trout amplifiers.
Headphones
The headphones chosen for test and demonstration purposes are common 25 ohm stereo
headphones. The headphones will plug directly into the Brown Trout device through a
standard stereo headphone jack.
UNIVERSITY OF PORTLAND
SCHOOL OF ENGINEERING
CONTACT: JIM BOSAK
THEORY OF OPERATIONS
PROJECT BROWN TROUT
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Design Overview
This chapter provides a low-level description of the amplifier in Brown Trout.
System Block Diagram
Figure 2. Brown Trout Schematic Block Diagram.
Above is the system block diagram of Brown Trout. As you can see the audio device
plugs into the first amplifier which is a unity gain amplifier. Second, it is passed to the
second amplifier which also serves as a low pass filter. The signal then goes into an
amplifier that also acts as a high pass filter. Once the signal has traveled through the two
amplifiers it is outputted to the headphones. A gain of two, three, or four is implemented in
the low pass amplifier. There is also a bypass around the whole circuit. This diagram is
implemented twice in the circuit since there is a signal for each ear piece on the
headphones.
Analog Circuits
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CONTACT: JIM BOSAK
THEORY OF OPERATIONS
PROJECT BROWN TROUT
Unity Gain Buffer
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Figure 3. Brown Trout’s Unity Gain Buffer
The purpose of the unity gain buffer is to isolate the amplifier in Brown Trout from
whatever audio device is attached to it. Brown Trout could potentially be connected to a
variety of devices that have varying outputs. With a unity gain buffer Vout is approximately
equal to Vin, which is our signal. For calculation purposes, Rin approaches infinity and
Rout is about zero. This is ideal for us because the amplifier will essentially not see the
buffer since Rout is very low compared to Rin of the low pass amplifier. It will be as if the
signal is being directly inputted into Brown Trout with a resistance close to zero no matter
what type of audio device is attached to it.
Low Pass Amplifier
Figure 4. Brown Trout’s Low-Pass Filter
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The signal first goes into a low pass amplifier that is designed to have a gain of negative
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one for all frequencies
are passed through. This is achieved by keeping R2 and R1 at
. since thethatgain
the same value
of that specific low pass amplifier has the equation:
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G = -(R2/R1).
Where G stands for gain and R2 and R1 are the resistor values on the schematic.
This amplifier acts as a low pass filter due to the frequency response of the feedback
elements. The cutoff frequency is designed so that it is 20 kHz, which is the highest
frequency at which audible sound can be heard. Due to resistor constraints the closest we
could get by hand was 19.5 kHz, however this is close enough to 20 kHz. The equation
used for the filter is as follows:
Wh = 1/(R2*C).
Where W h is equal to the 3dB cutoff frequency in radians per second and R2 and C
correspond to feedback elements in the above schematic.
High Pass Amplifier
Figure 5. Brown Trout’s High-Pass
Filter
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After passing through the low pass amplifier, the signal is then sent to the high pass
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amplifier for amplification and filtering. In our design we wanted to engineer Brown Trout
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to have an amplification the user can define as two, three, or four times the original signal.
. by using resistor R1 in conjunction with resistors R2, R3, and R4. The
This is achieved
different gains. of the circuit are accomplished by switching between resistors R2, R3, and
R4 by using .the gain control on the user interface. To keep the circuit from becoming
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unstable and sending feedback to the listener, there is always current going through R2. If
we switched between resistors without keeping current in at least one resistor, there would
be an open circuit in our design for a second before the connection was re-made. This is
what would lead the circuit to become unstable for a moment. The gains of three and four
are achieved by putting a resistor in parallel with R2 to lower the net resistance of the
combination. Specifically the switch can be set to R2, R2//R3, R2//R4, or to bypass the
entire circuit. The equations are as follows:
Gain of 2:
G = R2/R1
Gain of 3:
G = (R2//R3)/R1
Gain of 4:
G = (R4//R2)/R1
Where G stands for Gain and R1, R2, R3, and R4 are resistor values corresponding to the
schematic.
The cutoff frequency ideally would be set to 20 Hz since that is the lowest audible
frequency to humans. Due to resistor constraints Brown Trout is set at 16 Hz. This is
acceptable because it is close enough to 20 Hz. Furthermore, the device that inputs the
signal should have a frequency range of around 20 Hz to 20 kHz. Our device will simply
filter the signal more. The filter is achieved through resistor R1 and C. The equation is as
follows:
WL = 1/(R1*C)
Where WL is the 3dB frequency and R1 and C correspond to the above schematic.
UNIVERSITY OF PORTLAND
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CONTACT: JIM BOSAK
THEORY OF OPERATIONS
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Full Circuit
Figure 6. Brown Trout’s Full Circuit (one channel)
UNIVERSITY OF PORTLAND
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CONTACT: JIM BOSAK
THEORY OF OPERATIONS
PROJECT BROWN TROUT
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Conclusions
In conclusion, this document has gone over both high-level and low-level features of the
Brown Trout Amplifier. To recap, the Brown Trout Amplifier will be plugged into a portable
CD player, a battery pack consisting of two nine-volt batteries, and a pair of headphones.
The user interface will consist of a switch which will vary the gain from either 2, 3, or 4
times as loud. The amplifier itself, for each channel, is comprised of a unity gain buffer, a
low-pass filter, and a high-pass filter that will have three gain resistors that can be varied to
create the desired gain levels.
Power consumption and noise are two things that are undesirable in a portable audio
device. In order to overcome these obstacles priority was given to certain design
components such as the resistors, capacitors, and op amps. The resistors are the metal
film type which will reduce Johnson noise along with electrolytic capacitors. The op amps
used in Brown Trout are low power and low noise by design. Additionally, the values for
the resistors were chosen in such a manner as to reduce current drawn by the op amp
and therefore power consumption. One way to improve the design would be to use more
capacitors in series with each other to get closer to our designed values and achieve a
bandwidth that exactly matches that in our functional spec. However, this would make our
design larger and heavier, which is not ideal in a portable device.
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CONTACT: JIM BOSAK