Final Pa System
Final Pa System
Final Pa System
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Table of Contents
Pre-lab Preparation 2
Parts List 3
Background information 6
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Pre-lab Preparation
each group should obtain a parts kit from the ECE Shop.
flow =______________Hz
week #2 may be used to finish the audio amp. All breadboarding and
testing can and should be done in lab. Soldering and hardwiring can
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Pre-lab Preparation
each group should obtain a parts kit from the ECE Shop.
flow =______________Hz
audio amp. All bread boarding and testing can and should be done in
lab. Soldering and hardwiring can and should be done outside lab.
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Qty Description Circuit
1 Rubber feet
1 9V battery leads
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1 9V battery holder (adhesive backed)
1 9V battery
2 flea clips
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Background information Audio Speakers
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If you look through catalogs of audio-speakers, you will find many
different sizes and price ranges. For example, check out http://www.tb-
such as the ones used in this lab, have poor low-frequency response
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D. B. Weems, Designing, Building & Testing your Own Speaker
System, Tab Books: Blue Ridge Summit, PA, 1984 Band of operation
from the voice coil itself. The resistance term is largely the DC resistance
circuit model for any speaker is shown in Figure 1-4 and can be created
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Free-air resonant frequency, fs
resonance)
R ). The Qfactor
is then Qms= fs /f
can be found as 2
2 (2 )
ms
mtcmm
sm s m
RRRCQL
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fR fC
so that the voice coils and cones will move in phase with each other.
symbol.
In-Lab Procedure
drive an audio speaker, right? Why not just use a simple op-amp gain
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a peak AC current of around 350 mA with an 8-Ohm load. This is a lot
LM358 and LF353 (two op-amps used in this lab) can source around
went with a simpler solution, and chose a chip that was specifically
This is an old chip that has been a popular choice for low-power audio
applications for many years. There are many other audio amp ICs on
the market (LM380 and LM383 are similar but higher-power amps
from National), but the LM386 is sufficient for our purposes. It comes
provide 0.3W, 0.5W, and 0.7W respectively, more than adequate for
this lab. Another nice thing about the LM386 is that the gain-frequency
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curve can be shaped with some external feedback components, so it is
find many examples of clever circuits that people have come up with
over the years. The one in this lab is a minor modification of a circuit
quiescent current and hence help prolong the life of the battery. The
possibilities that could be used, but this one is very cheap! We also
chose a dual device in case you choose to do the optional tone- control
circuit. It doesnt matter which of the two internal op-amps you use for
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1.2 Speaker Equivalent Circuit
Using the R-L-C meter in the lab, measure the coil resistance and
coil inductance (the meter uses a frequency that is well above the
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parallel, seems to work well). This creates a voltage divider, so
can map out the variation of impedance with frequency using the
We will first construct this amplifier using your plastic solder less
breadboard and bench power supply. Once you debug the circuit and
board and power it from a 9V battery. The idea of bread boarding the
since it allows you to debug your design more easily, allows you to
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experiment with component values, and most importantly will help
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First study the schematic carefully, and locate all the necessary
parts in your kit. Find the LM386 chip and compare it with Figure
1-5 and the schematic in Figure 1-7 to correlate pin numbers with
supply and appropriate wiring to the power busses, then add the
bias/bypass capacitors.
elements as shown.
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4. 10k resistor and 33nF capacitor between pins 1-5: bass-boost
feedback circuit (see data sheet), helps compensate for the poor
inductive loading
device. These diodes clamp the input voltage on pin 3 at +/- 0.7
Add the speaker. You will need to first solder wire jumper leads
to the speaker terminals. You are now ready to test the circuit as
and adjust for a 1kHz sine wave with a 0.2 V amplitude, and
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verify on the oscilloscope. Then apply this signal to your amplifier
along with the +9V bias. If all goes well, you should hear a tone.
happening?
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DC current flow into the amplifier. Record the current under the
operating conditions.
The next step is to add the summing network, which is used to combine
and right stereo channels together so that the amplifier can amplify both
Add the summing circuit as shown in Figure 1-9. Just put in the
AUX channel for now (C3-R3), dont worry about the stereo jack
hardwired version
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There are two important points to understand here, both of which are
related to the use of a single (battery) supply. First, in the textbook dual-
voltage for the input signals, which is the desired average or DC output
V), but for our 9V battery supply, it is half the supply voltage, or 4.5
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Volts. This is accomplished using the two 100k resistors in a voltage
divider (we choose 100k resistors here to minimize the current draw
from the battery). The 10 F bypass capacitor helps prevent this voltage
potential and hence a good AC ground .Secondly, since the input and
This is what R5 is for. Youll have to wait until ECE 137AB to fully
appreciate the details. Again, were choosing a large value resistor here
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the frequency to determine the cutoff and compare this with your
prelab calculation.
Why not use it? This is not a required element of your project, but is a
quality.
Figure 1-10 (a) Single-knob tone control circuit. This makes use of the
second op-amp in the LM358. You can use the same Vref as for the
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summing circuit. (b) Bode plots for the tone-control circuit at opposite
There are many possibilities for tone-control circuits. The basic idea is
When the wiper is at its midpoint, both sides of the feedback network
are identical, and the system has unity gain for all frequencies
attenuation of about 1/3 (-10dB). When the wiper is at the bottom, the
action reverses so that the low-frequencies are cut and the high-
frequencies are boosted. Figure 1-10b shows the Bode plots for the
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1.6 Hardwire the Amplifier
Now we are ready to put everything together, and add the stereo
connectors. They look like the picture in Figure 1-11, and are
Figure 1-
insulator. The largest of these is the ground connection. The other two
carry the left and right channels of the stereo signal. The 3.5mm
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designation refers to the diameter of the cylindrical conductors. There
are other diameters on the market, but 3.5mm is the standard for audio.
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In your kit you should have been provided a stereo jack (or female)
jacks also have more than three pins which are connected in a way that
removed from the jack. The one we will use is just a simple 3-terminal
PCB stereo jack. On the backside you will see each pin marked by a
Use the IC socket provided in the parts kit so that if anything goes
Add the 9V battery leads supplied in your kits. If you were given
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Attach the stereo jack to the edge of the vector board. Connect
ground.
flea clip.
working, add the battery and test again under battery operation.
Using the patch-cord in your kits, you can now test your amplifier
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Congratulations: you now have a portable audio amplifier!
The circuit youve just built is a nice building block for many projects
not very efficient in terms of battery life; and 4) the small speaker size
limits the low-frequency response. These are all areas that can be easily
improved upon if you are interested. For a true stereo output we would
need separate amplifiers for the left and right channels, not to mention
set of speakers.
Nowadays you can often pick up a cheap set of speakers designed for
for this lab. In terms of power and efficiency, there are now several
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much high powers with very little power consumption. As one of many
contained 2W stereo amplifier chip that can directly drive two external
be much more attractive in terms of size and cost than piecing together
a system from several discrete parts, as we did in this lab. On the other
hand, you most certainly learned more about the circuit building it from
scratch then you would have by simply hooking up the LM4663 circuit
below!
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Another possibility for future work is to create a more sophisticated tone
search will turn up many possibilities, and you can also come up with
Lastly, you may wish to put your circuit in some kind of enclosure to
protect it, with shaft & knob potentiometers for the volume and tone
controls, and perhaps a jack for headphones (note that there are
Congratulations!
You have now completed Lab 1a
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16 Audio Amplifier Circuit
Bob York 16
Lab 1 Record
Section____________ Names:
_________________________________________
Equivalent Circuit modeling for the Speaker
Lab 1 Record 17
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17 Bob York
Are the limits above related to limits on your hearing or are they due
speaker disconnected:
From your answer above, estimate the battery life for this circuit
assuming a 500 mA-hr battery rating: ________________ hrs
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