![]() If 20 volts is applied to the primary winding the voltage across the load impedance will be:Īnother Audio transformer, used for multi loudspeaker public address systems is the 100V line transformer used for connecting multiple speakers in public address systems to a single amplifier. Suppose a transformer has a primary to secondary ratio of 10:1 and a load impedance Z L of 8Ω is connected across the secondary winding. It can make a low impedance appear much higher, or a high impedance much lower. This is because a transformer has the ability to "transform" or change the apparent impedance of a circuit input or output. Where it is required to transfer the maximum amount of AC POWER between the circuits, transformers may be used. ![]() In most cases impedance matching requires that the maximum VOLTAGE is transferred from one circuit or device to the next and for the transfer of maximum voltage this is achieved using simple resistance networks. When the output of one circuit or device is feeding an AC signal to the input of another circuit or device, it is important that the input and output impedances are properly matched. 11.4.5 A Microphone Matching Transformer. The transformer within the metal screening case acts as a balun, an impedance matching device and an input isolator for the microphone.įig. The XLR socket at the other end of the device allows a low impedance microphone to be connected via a long lead. 11.4.5 shows a typical microphone transformer that plugs directly into an unbalanced high impedance amplifier input. A simplified Schematic of a balun is shown in Fig. This is a type of transformer for matching balanced transmission lines or cables to, or from an unbalanced input or output, (the device is reversible). The combination of signals at the amplifier can be carried out either by using a differential amplifier (an amplifier with two anti-phase inputs), or by using a balun (BALanced to UNbalanced) device. Any noise that has been externally induced into the cable after the transformer will be identical in phase on both conductors, so the subtraction (difference) combination occurring at the amplifier input cancels out these noise signals. The amplifier input uses the difference between these two signals to produce a signal that is doubled in amplitude. Because the cable is fed from a centre-tapped transformer, the signals on the two conductors are in anti-phase. They can match the low input impedance of the loudspeaker (typically a few ohms) to the much higher output impedance of an amplifier, allowing maximum power to be transferred from the amplifier to the speaker.Ī transformer with a single primary and a centre-tapped secondary is used to connect the microphone (a two wire unbalanced device) to the balanced cable.They provide an isolated external connection for the loudspeakers, improving safety.Where used, they allow the AC audio signal to reach the loudspeaker whilst preventing any DC from the amplifier affecting the operation of the loudspeaker.The dots near the windings on schematic diagrams indicate the relative polarity of the signals on different windings, and in this example show that the signal from the upper secondary winding (A) will be in phase with the primary signal, while the lower secondary winding (B) will provide a signal in anti phase with the primary signal.Įxample b.) shows two output transformers, used to couple the power output stage of an audio amplifier to the loudspeaker.Īudio transformers often perform several functions at once: ![]() In audio amplifiers, the phase/anti phase of signals can be important and phase splitting transformers with centre tapped secondary windings can be used to provide two anti phase signals. Some transformers may also have tapped primaries for an even wider range of ratios. Some common arrangements of audio transformer windings are shown in Fig 11.4.2.Įxample a.) shows a centre tapped secondary winding that can be used to provide a selection of different turns ratios. Fig 11.4.2 AF transformer schematic diagrams showing phase indicators.
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