APPLICATIONS of transformer

Audio output transformers are used when load impedances are low, as in line drivers, while audio input transformers are used when load impedances are high, as in line receivers.

Output transformers have very low leakage inductance in order to maintain high-frequency bandwidth with capacitive loads. For low insertion loss, they use relatively few turns of large wire to decrease winding resistances. They use fewer turns and operate at relatively high signal levels.

On the other hand, input transformers directly drive the usually high-resistance, low capacitance input of amplifier circuitry. Many input transformers operate at relatively low signal levels, often have a Faraday shield, and are enclosed in magnetic shields.

 

1.      Microphone Input

A microphone input transformer is driven by the nominal 150 W source impedance of professional microphones. One of its most important functions is to transform this impedance to a generally higher one more suited to optimum noise performance. The optimum impedance may range from 500 W to over 15 kW, depending on the amplifier. For this reason, microphone input transformers are made withturns ratios ranging from 1:2 to 1:10 or higher.

High CMRR is desirable form an input transformer. To achieve this, they must have two attributes. First, the capacitances of its two inputs (to ground) must be very well matched and as low as possible. Second, it must have minimal capacitance between its primary and secondary windings.

 

2.      Line Input

Line input transformers are generally driven by balanced line and drives an unbalanced line. They also transform the impedance as microphone input transformers and have high CMRRs.

3.      Moving Coil Phono Input
Moving coil phonographs are very low impedance devices (~3W). Due to this, it is very difficult to achieve good noise performance in an amplifier. Hence, the transformer is used in step-up configuration so that amplifier sees an impedance of 600 W in order to achieve good noise performance.

4.      Line Output
Typically, audio transformer is driven by an amplifier and loaded by several thousand pF cable capacitance and high input impedance of line receiver.Therefore, a line output transformer should have low output impedance which remains low at high frequencies. This requires both low resistance windings and very low leakage inductance.

5.      Interstage and Power Output
Interstage coupling transformers used to be popular in vacuum tube based designs. They used 1:1 to 1:3 turns ratio and classical push pull power amplifier in the output stage.

 

6.      Microphone output

Ribbon microphones& Dynamic microphones use step up transformers whereas condenser microphones use step-down transformers. Ribbon elements have impedance of the order of 1 W and hence need a step up transformer with turns ratio of 1:12 or more to transform its impedance to somewhere around 150 W. Similarly, step-up is required for dynamic microphones having impedances of the order of 10-30W. On the other hand, condenser microphones have high impedance and use step down transformer so that amplifier sees smaller input impedances.

 

7.      Speaker Distribution

When number of low impedance speakers are located at a distance from the  power amplifiers, the speakers are connected with the help of technique used in grid electricity supply, i.e., use of step-up transformers at transmission end and step-down transformer at receiving end.

 

8.      Telephone directional coupling or “Hybrid”

Telephone “hybrid” circuits use bridge nulling principles to separate signals which may be transmitted and received simultaneously on a 2-wire line. This nulling depends critically on well-controlled impedances in all branches of the circuits. This nulling is what suppresses the transmit signal (your ownvoice) in the receiver of your phone while allowing you to hear the receivesignal (the other party).