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Slowly decrease the frequency of the generator until
the output drops to .7 of its original reading, 1.4Vpp or
2.8 divisions.
This frequency is called the low
frequency 3dB corner - the low frequency 3dB corner or
(f high 3dB) - (f low 3dB). Your calculated answer
should be greater than 30kHz.
DISTORTION
Connect the generator and oscilloscope as shown in
Figure 8. Set the generator at a frequency of 1kHz, turn
the power ON and turn the volume to maximum. Adjust
the generator output until the peaks of the sinewave at
TP1 are clipped as shown in Figure 9A. One side of the
sinewave may clip before the other depending on the
DC centering at TP1. If oscillations are seen, connect
a clip lead from the GND of your generator to the GND
of the circuit.
Clipped
A
Figure 9
Measure the maximum voltage peak to peak when
clipping first occurs and record that value here:
Vclp = _______ Vpp.
Using a wire short out diode D5 and resistor R47 as
shown in Figure 10. The waveform should resemble
Figure 9B. The "flat spots" near the center of each
sinewave demonstrate what is called crossover
distortion. Most of this distortion should disappear when
you remove the shorting lead. Turn the power OFF.
Wire Lead
or Clip Lead
Figure 10
MAXIMUM POWER OUTPUT
The maximum power output before distortion due to
"clipping" can be calculated using the voltage Vclp
obtained in step 4 as follows:
Vpeak (Vp) = Vclp/2
Vroot mean squared (Vrms) = Vp x .7
Max power out = (Vrms)
Maximum power output should be greater than 350
milliwatts.
EFFICIENCY
By measuring the DC power taken from the battery at
the maximum power output level, the efficiency to the
audio amplifier can be calculated.
battery is equal to the current taken from the battery
times the voltage of the battery during maximum power
output. Efficiency can then be calculated as follows: Eff
= Max audio power/Battery power. It is best to use a
Crossover
Distortion
power supply (if available) to prevent supply voltage
from changing during these measurements. Connect
the generator, oscilloscope and current meter as shown
in Figure 11. Set your current meter to read 1 amp DC.
Turn the power ON and rotate the volume control to
maximum. Slowly increase the amplitude of the audio
generator until the output is clipped as shown in Figure
9A. Record Vclp here:
B
This should be equal to Vclp in step 4. Record the DC
current drawn from the 9 volt supply here:
Measure the supply voltage and record the V supply
here:
Turn the power OFF. Calculate the maximum power
output as done in the Maximum Power Output Step.
-13-
Vclp = _________ Vpp.
Current (I) max = ________ Amps.
V supply = ________ volts.
Record your answers on the next page.
2
/8 ohms = (Vclp x .35)
Power from the
2
/8
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