Before tracking the damage to an amplifier, especially a stereo amplifier, it must first be known what measurements must be made to determine the specifications of an audio amplifier. The specifications that must be measured on an amplifier are:
1. Measurement of Strengthening
The block diagram of the measurement circuit is shown in Figure 6.35. Suppose that a voltage amplification is required in an amplifier with a frequency of 1 KHz. First a signal generator is set up to give an output of, say, 500 mV at 1 KHz, with the attenuator switched to zero dB. This signal at the amplifier input (point A) is connected to the Y input of the oscilloscope and the oscilloscope control is adjusted so that its image appears on the available part of the screen.
Figure 6.35: Voltage Gain Measurement in an Amplifier Circuit
The oscilloscope cable is then connected to the amplifier output (point B) and the attenuator is then turned up until the output has the same height (peak) as the first measurement. The amplifier gain is now the same as using the attenuator that was installed. The advantage of this method is that the measurement is independent of the accuracy of the oscilloscope.
2. Frequency response and Band Width measurements
By still using a set of tools as in figure 6.35. can be obtained amplifier gain at each frequency. The gain is depicted against the frequency on linear/log graph paper, for audio amplifiers it takes 4 log cycles to reach the frequency limit of 10 Hz to 100 kHz can be determined quickly by recording 2 bandwidth frequencies, where the gain drops by 3dB from the gain of the center frequency.
3. Input Impedance Measurement
The circuit for measuring input impedance is given in figure 6.36, by giving a signal generator at 1 KHz. The resistance is set to zero and the amplifier output is connected to a measuring instrument, namely an oscilloscope or ac meter. The adjustment can be made so that large deviations can be seen.
The resistance of the decade box is then adjusted to be larger until the output signal drops exactly to half. As long as the resistance box (variable) and the input impedance of the amplifier form a voltage divider, if the output is half, then the resistance in the box is the same as the input resistance.
Figure 6.36: Input Impedance Measurement of an Audio Voltage Amplifier
4. Output Impedance Measurement
The circuit used for this measurement is shown in Figure 6.37 with the front as in Figure 6.33 without the box resistor.
Figure 6.37: Output Impedance Measurement of an Audio Voltage Amplifier
The measurement technique is the same as the input impedance measurement technique. The signal frequency used is 1 KHz and first RL is removed and a large deflection is observed on the oscilloscope. Then the external load RL is installed and the load value is reduced until the output drops to half the initial value. The RL value at that time is the same as the output resistance value.
5. Measuring output power, efficiency and sensitivity for an audio amplifier
For these measurements the loudspeaker can be replaced by a wire-wound resistor as a load whose value is equal to the loudspeaker impedance, and the tests can be carried out at frequencies where the loudspeaker impedance is generally resistive, for example approximately 1 kHz.
The diagram for the measurement is shown in Figure 6.38. The wattage value of the resistance load must be greater than the maximum output power. The input voltage can be adjusted until the output signal on the oscilloscope shows the maximum level without distortion.
Figure 6.38: Measuring Output Power, Efficiency and Sensitivity of an Audio Output Amplifier
This occurs when nothing is cut off from the positive input signal and the negative input signal. Usually if a distortion meter is available, a more thorough check for distortion levels can be carried out. Then the maximum output power should be recorded without exceeding the harmonic distortion value specified by the amplifier manufacturer.
The efficiency of the amplifier can be checked by measuring the dc power taken by the amplifier from the supply.
Amplifier sensitivity is the amount of input voltage required to produce maximum output power without distortion.