EXPERIMENT #1
AUDIO MONITOR
INTRODUCTION:
The last stage in many communications systems is an audio amplifier of some type. An audio amplifier provides both
voltage and current gain for signals from the detector stage so that a loudspeaker may be driven. The amplifier you'll be
constructing in this experiment will let you hear audio-frequency signals at many points within a system, and can be
used as a valuable troubleshooting tool. It is suggested that you build it in permanent form.
The active circuitry (resistors, diodes, and transistors) of this amplifier is contained entirely on a single IC (Integrated
Circuit) chip. Before the invention of ICs, even simple audio amplifiers required dozens of individual components and
took up a great deal of space. Today's IC technology is what enables miniaturization. Miniaturized electronics allows
us to build all sorts of neat "gadgets" such as hand-held portable telephones, pagers, personal digital assistants (PDAs),
and the like.
In this experiment, the following will be accomplished:
Ø You will build an audio amplifier using the LM386 provided in the lab kit.
Ø You'll get additional experience in the use of the signal generator and oscilloscope.
Ø You will measure the performance of the LM386, and learn how to measure audio power amplifier output
characteristics.
Lab 1 - Audio Monitor Page 1-1
© 2001-2010 Tom A. Wheeler
CIRCUIT ANALYSIS:
Figure 1 is the audio amplifier circuit. The LM386 IC audio amplifier chip does all the work!
Vcc
+12 V
R1
+
C1
470 uF 220 ohms
Volume U1
LM386
6
1
Audio Input (Gain)
3 C2
C3 +
5
+
R2 2 -
LS1
+
50K
1 uF R3 470 uF
10 ohms
4
7
8
C4 C6 8 Ohms 2"
+
47 pF C7
+
C5 47 uF 0.1 uF
47 uF
Figure 1: Audio Monitor Using the LM386
Input signals are applied to the amplifier at the audio input connection. The audio signal flows down into potentiometer
R2, which functions as a variable voltage divider. The percentage of AC input signal that leaves R2 is controlled by the
position of R2's wiper (the arrow). When the wiper is all the way down at ground, no signal leaves R2 and the
"volume" is at minimum. Sliding the wiper of R2 upwards (clockwise rotation of the physical potentiometer) increases
the percentage of signal that can escape R2, which in turn increases the effective gain and "volume" of the audio
circuit.
The AC signal passes through capacitor C3 after leaving R2. C3 is a DC block. It allows AC (information) to pass
through to the audio amplifier, but prevents DC from passing, which might disturb the DC bias on pin 3 of U1, the
LM386. C4 is an RF bypass capacitor. It is effectively an open-circuit at audio frequencies, but a short at radio
frequencies. (Recall that the reactance of a capacitor decreases as frequency increases). Its purpose is to short any stray
RF at the amplifier input to ground before it can get into the amplifier IC. We don't want to hear local radio stations,
CBers, or any other unintended signals coming out of our circuit!
The input signal is finally applied to pin 3 of U1. U1 is a special "power operational amplifier." It is built in a manner
similar to a regular op-amp, but has internal negative feedback already in place, and also has an extra stage to boost the
current for driving a loudspeaker. In other words, U1 provides both voltage and current gain -- U1 is a power amplifier.
U1 does all the real work in this circuit! R1 and C6 set the voltage gain of U1, C1 and C5 are filters for the power
supply, and C2 is a DC block for the output circuit. (Because the LM386 uses a push-pull output circuit, the DC voltage
on pin 5 is about one-half of Vcc. Applying 6 Volts to a speaker wouldn't be very healthy for the speaker!)
Lab 1 - Audio Monitor Page 1-2
© 2001-2010 Tom A. Wheeler
LABORATORY PROCEDURE:
Name _______________ Sign-off _________
1. Build the circuit of Figure 1. Note the following:
C1, C2, C3, C5, and C6 are electrolytic capacitors. These parts are polarized and must be installed the correct way!
(The longer lead is the (+) or positive lead. Observe the polarity markings.)
C4 is a ceramic disc capacitor. It is most likely marked "47", "47J", "47 pF", or "470." Putting the wrong capacitor
in this place can make your circuit inoperative!
Observe the pinout of U1 carefully. It is in an 8-pin DIP (dual inline package) and the pin numbers read like this:
8 7 6 5
Notch or
dot
1 2 3 4
TIP: It is best to solder two hookup wires to the speaker before connecting it into the circuit.
2. Connect the following test equipment to the circuit:
SIGNAL GENERATOR: Connect to the audio input terminal of the circuit (top of potentiometer R2). Connect the
generator "hot" (RED) lead to the audio input and the generator ground lead (BLACK) to the circuit ground.
DUAL TRACE OSCILLOSCOPE: Connect channel 1 to the audio input. Connect channel 2 to the audio output of
the circuit (Negative side of capacitor C2). Connect all ground (BLACK) leads to the circuit ground.
3. Apply power to the circuit and set the test equipment as follows:
SIGNAL GENERATOR: Frequency 1 KHz, waveshape sine, output voltage 50 mVpp (you may need to observe
the waveform on the scope to set the output voltage).
OSCILLOSCOPE: Timebase 200 µS/division ; Channel 1, AC coupling, 10 mV/division; Channel 2, AC coupling,
1 V/division.
4. On the audio monitor circuit, adjust potentiometer R2 for maximum volume. This should be at full clockwise
rotation of the control.
Lab 1 - Audio Monitor Page 1-3
© 2001-2010 Tom A. Wheeler
5. You should now hear the 1 KHz tone from the signal generator playing through the loudspeaker. If it gets to be too
annoying, just disconnect the speaker and replace it with an 8 to 10 Ohm fixed resistor!
6. Record the input and output waveforms below. Accurately record all important voltage and time dimensions for each
waveform.
AUDIO t
INPUT
AUDIO
OUTPUT t
7. What is the voltage gain of the circuit according to the data you collected in step 6?
TIP: Voltage gain is measured in Volts/Volt (V/V) and is calculated by:
Vout
Av = Where Vout and Vin are the AUDIO OUTPUT and AUDIO INPUT signals of this circuit.
Vin
When you measure Vout and Vin , remember that they must be in the same dimensions (Vpk, Vpp, Vrms, etc).
Show your calculation:
Voltage Gain (V/V): _________________
8. What is the voltage gain of the circuit in decibels?
TIP: The formula for calculating voltage decibels is: dB = 20 log( Av )
Show your calculation:
Voltage Gain (dB): _________________
Lab 1 - Audio Monitor Page 1-4
© 2001-2010 Tom A. Wheeler
9. Let's measure the maximum undistorted output power of this amplifier. (You may want to replace the loudspeaker
with an 8 to 10 Ohm fixed resistor in this step!)
a) Increase the voltage from the SIGNAL GENERATOR until the AUDIO OUTPUT signal
Clipping
begins to "clip" or distort, as might appear below:
b) Back down the input voltage from the SIGNAL GENERATOR until the output signal
cleans up again.
c) Record the peak-to-peak voltage at the AUDIO OUTPUT. This is the maximum undistorted
audio output signal voltage:
Maximum audio output signal voltage ___________ Vpp
d) Convert the signal voltage into RMS for power calculation.
Vpp Vpp
Recall that: Vrms = ≈
2 2 2.82
Maximum audio output signal voltage ___________ Vrms
e) Calculate the power being delivered to the loudspeaker by Ohm's law.
Write the equation for power (given Voltage and Resistance) here: _____________________
The RMS voltage you calculated in (d), and the resistance of the loudspeaker is about 8 Ohms.
Show your calculation for maximum output power.
Maximum undistorted output power: ____________ (Include appropriate units)
10. Get a sign off for your working circuit. You may want to save this circuit for later use, if you have a spare
breadboard.
Lab 1 - Audio Monitor Page 1-5
© 2001-2010 Tom A. Wheeler
Troubleshooting Hints
Most troubles with the audio monitor circuit can be traced to wiring errors or power supply troubles. In case of trouble,
try the following:
1. Perform a careful visual inspection of the circuit, comparing it to the schematic diagram. Make sure each component
value matches what is called for on the schematic diagram.
2. Check the DC voltage at the following locations:
Circuit Location DC voltage reading Comments
and tolerance
U1 pin 6 12 V +/- 0.5 V Vcc power supply for the power operational
amplifier IC
U1 pin 4 0 V (Ground) Ground terminal of LM386
U1 pin 5 6 V +/- 1 V The potential of 6V represents a centered DC
operating point for the amplifier circuit. This is
determined internally within U1. If this voltage is
out of range, make sure C2 is installed in the
correct direction, and that nothing except C2 is
connected to U1 pin 5.
U1 pin 2 0 V (Ground) Inverting input terminal.
U1 pin 3 0 V to 0.1 V Non-inverting input terminal. If the voltage is
greater than the limit, check for a wiring error on
pin 3.
3. Check the AC voltages in the order listed at the following locations, using an oscilloscope. The test points are listed
in order of signal flow, from input to output.
Circuit Location AC signal reading Comments
Top of R2 50 mV pp @ 1 KHz Input signal from SIGNAL GENERATOR
U1 pin 3 50 mV pp @ 1 KHz Depends on input signal level. R2 must be set for
maximum volume. This is the input to the
amplifier chip.
U1 Pin 5 At least 6 Vpp @ 1 KHz Output signal of amplifier circuit. Depends on
input signal level and setting of R2.
Lab 1 - Audio Monitor Page 1-6
© 2001-2010 Tom A. Wheeler
QUESTIONS
1. What component in the circuit of Figure 1 contains the active amplifier circuitry?
_______________________________________________________________________
2. What is the function of potentiometer R2? Explain how R2 performs its function.
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
3. What two points must be measured to determine the voltage gain of an amplifier?
1. _______________________________________________________________________
2. _______________________________________________________________________
4. Explain how to measure the maximum undistorted output power of an amplifier.
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
5. What else have you learned in this lab?
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
Lab 1 - Audio Monitor Page 1-7
© 2001-2010 Tom A. Wheeler
