A complete stereo system can consist of a number of modules, each with its own box, and each has a different function. Figure 6.62 shows a modular diagram of a stereo system. In general, it consists of four groups, namely: signal source, processor, amplifier, and audio transducer. However, there are two additional modules that also need to be considered, namely the power supply and the inter-module connection system.
1. Signal source
The signal source is anything that produces a signal that is processed, amplified, and then converted into audio. There are two things to consider, first the signal quality. If the signal source has a low frequency response, the resulting signal will be deformed (cut off or distorted) and experience a phase shift, so that at the end the system cannot be expected to produce as desired. The second problem to consider is that the signal must be noise-free. If the signal source contains noise, then the noise will be processed and amplified simultaneously by the system.
Figure 6.63: Some Examples of Parts of an Audio System
2. Processor
Processors come in many forms, generally functioning to select and modify signals from other sources or processors without including noise or inaccurate signals. Processors can be tested by removing the module from the system, then seeing if the same problem still exists after the module is removed. However, the front preamp amplifier should always be checked, to see and compare the input signal with its output signal.
3. Amplifier
Most systems have only one stereo amplifier, and it is usually combined with a front-end amplifier integrated with the amplifier. This part will amplify the signal (including noise and distorted signals) it receives, to drive the transducer output.
Problems that can occur with the amplifier are:
- Signal cut off,
- Loss of output signal,
- Over temperature,
- Volume not working,
- Poor frequency response. Use the amplifier within its limits.
4. Transducer
The transducer will convert the electrical signal into audible sound. You may think that there is only one transducer, which is the speaker. In general this assumption is correct, but do not assume that the speaker is simple. This system can consist of standard permanent magnets, tweeters, electrostatic loudspeakers and so on. All parts will receive signals and manage the power sent by the amplifier to it.
Speakers can cause:
- Sound distortion,
- Additional noise from speakers
- Gain problems due to impedance mismatch.
The way to check the speaker is to try the speaker on the left and right amplifier outputs alternately. If the problem follows then the speaker is damaged.
5. Power supply
Almost every module has its own power supply. This part should be able to provide dc supply (noise and hum free) and can maintain dc levels within the limits acceptable to the components in the module without being affected by changes in load or line voltage.
- Impure DC supply will cause hum or buzzing in the audio.
- If the dc level is low, the module will lose one or more of its specifications.
6. Connections between modules
The problem in the modular system is the need for electrical connections between modules in the form of cables and connectors. This usually depends on wiring and physical connection problems. The function of the connection between modules is to carry signals (including ground) from one point to another.
- Corroded or oxidized connections/connectors can cause intermittent signal loss or periodic bursts of noise.
- Wires without good insulation can produce noise (hum)
- Closely spaced wires will increase capacitance, causing the impedance to become mismatched, especially for high frequency and high impedance effects.
- For this, use a good connector, and usually use a special coaxial cable for audio.
In a modular system to produce audio sounds that are more pleasant to hear, usually before the processor, several other modules are added, namely equalizers and expanders.
7. Equalizer
An equalizer separates audio information into different frequency bands and controls the strength of each band as the user sets it. A good equalizer allows the user to select the desired band width by adjusting the sliders on the panel. And in fact, the equalizer circuit is an active filter circuit that can be set to which frequency areas will be removed or displayed. So here because it is an active filter, there must be an element of reinforcement if desired at a certain frequency. But there are also equalizers that use passive filters and additional reinforcement at the end. What needs to be remembered is that the equalizer cannot improve the quality of the incoming signal, if the signal does not produce high / low frequencies, of course the equalizer will not make those frequencies appear. Moreover, it contains noise / hiss, this will still be carried over even for standard equalizers that will further strengthen the noise.
8. Expander
The basic of expander is shown in Figure 6.64, this tool will detect the input signal level. It reacts by increasing the gain on the expander for large inputs and reducing the gain on the expander for small inputs.
Figure 6.64: Expander Block Diagram
The filter circuit isolates some parts that represent the audio spectrum (700 Hz to 7KHz) that are detected. Then, the rectifier and detector circuits convert the filtered audio into a variable dc voltage that changes in its part according to the input level (ac audio). This variable dc (in the form of current) is used to control a voltage or current in both the left and right channels through transconductor amplifiers 1 and 2, which process the audio signal. Inside the expander circuit there is a capacitor that determines how fast the gain can change. And this change is what our ears hear. However, if the gain change is too slow then there will be no sound, and if it is too fast there will be noise. Usually the most frequently damaged are amplifiers 1 and 2.
A simple expander like this has several drawbacks; a loud single note in a recording can increase the gain of the entire spectrum, making all notes louder.
Stereo Amplifier System
An amplifier is a device with a small input signal that can be used to control a large output power.
This is shown in Figure 6.30. The signal input here is used to control the electric current flowing in the active equipment. Then this electric current causes a change in voltage on the load resistance, so that the output power becomes:
Power Amplifier (Ap), produced by the ratio of output power to input power:
A more general symbol is shown in Figure 6.31. Each amplifier increases the amount of voltage from its input signal.
Figure 6.30 
Basic Block Diagram of Amplifier
Figure 6.31: General Amplifier Symbol
The classification of an amplifier can be for voltage amplifier, current amplifier or power amplifier.
The use of these amplifiers is shown in table 6-2.
Table 6-2. General Classification of Amplifier Circuits
There are three basic classes of amplifier operation, namely:
Class A: Active devices (transistors) are biased so that there is always an average current flow (always on). This current also fluctuates around its average value depending on the input signal. This class is the most commonly used, an example of the type is a small signal amplifier (figure 6.32).
Class B: The active device is biased to the cut-off position and will be turned on by a 1/2 cycle input signal. This class of operation is widely used in push-pull power amplifiers (figure 6.33).
Class C: The active device is biased beyond its cut-off point, so that the input signal must exceed a relatively high value before the device can be made to conduct. This class is used in oscillator circuits and transmitter circuits (figure 6.34).
Figure 6.32: Class A Single Stage Amplifier
Figure 6.32: Class A Single Stage Amplifier
Figure 6.34: Oscillator Circuit