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Car Radio - Push-pull Output - Stereo Amplifier - Audio Pumps.. Electronics

Car Radio

Figure (132) is an amplification circuit for a car radio. The output transistor needs a high-intensity current so that it can operate on the voltage of the car battery with a voltage of 12 volts. This circuit uses only one transistor that amplifies the entire cycle. Therefore, it is called the "A" type amplifier, as the collector voltage remains present throughout the entire cycle and the process of connecting the loudspeaker to the collector circuit is done by an autotransformer that includes a single coil containing interconnection points instead of the connection transformer that contains two separate coils: primary and secondary.

132 - Class A output stage in a car radio.

Push-pull output

In this system, it is preferable to use a type B amplifier that works with transistors whose base current gives a lower collector current than in type A amplifier:
It is worth noting in Figure (133) that the two resistors M and M2 feed the bases of transistors T1 and T2 via the secondary coil of the coupling transformer. When the value of the input audio signal reaches the limit that leads to raising the bias of transistor T1 to the conduction region, during only half a cycle, the collector current intensity increases, then it is cut off during the other half cycle where transistor T conducts the current alone (134). This method leads to reducing battery consumption, especially when the sound is weak, and to increasing consumption only by the value necessary to raise the volume of the amplifier to the required level.

The circuit shown in Figure (135) is usually used in small transistor receivers. The transformer is characterized by two secondary coils, each of which is dedicated to supplying one of the transistors that transmit current in an alternating manner. This system does not need an output transformer, as it is possible to supply a 75 ohm loudspeaker or any other loudspeaker directly.

Stereo amplifier

Stereo amplifier (136) includes two channels, and in it the loudness regulator MM1 (sound regulator) obtains the required signal level from

133 - TR1 conducts when its base is fed.

134 - TR2 conducts when its base is fed.

135 - One-way push-pull output stage.

Stereo receiver includes two outputs: the right wire output, and the left wire output. The signals coming out of these two paths pass through the resistors M2 to the valves SAA and SAB which can be considered as a combination of two valves, one of which is a triple and the other a quadruple. The resistor is considered with the anode load resistance, while the capacitor SM is a connection capacitor with the valve network SAB. The two resistors M8, along with the two ampere capacitors S2 and SH, provide the necessary bias voltage for the cathode. The resistor M and the capacitor S4 reduce the high voltage used to feed the valve S1 and smooth it. The transformer connects this stage to the loudspeaker while the high frequencies pass through the capacitor S6 and the resistor M to the connection between the resistors M2 and M3, whose task is to provide the selected feedback to adjust the valve output. This adjustment in the feedback is done by the audio controller M2. The output of the left channel of the receiver passes to an amplification circuit completely similar to the previous circuit and is then connected to the left loudspeaker. The amplifiers are fed by a common power supply unit, and all components are assembled in one box. During the recording process using the left and right channels, the musical instruments must be placed and distributed according to their performance and intensity so that the listener to both loudspeakers (136) will have a sense of depth and liveliness in the recording (see also page 104). We also get the same feeling and sensation when listening to a stereo broadcast station.

136 - Stereo amplifier and its circuit system.