Amazed by the electronic music of your favourite band? Mesmerized by the way suddenly an electric guitar goes as loud as the singer? Like most of know, there is an expert recording engineer present in the studio to make sure that music instruments and the voices make a perfect blend. A critical device that helps in creating this blend is an audio transformer. Works just like any other conventional transformer; audio transformers are designed to work at audio frequencies, i.e. between 20Hz to 20 KHz. Interestingly, the initial need to design to such transformer was to enhance long distancevoice communication. Now serving essentially for professional musicians, audio transformers stand as one electronic-audio component. This article will cover technical and applicative features of audio transformers explaining how they are different from the conventional counterparts.
1. Phase splitting
a) Power Levels
Fig. 7: Image Showing Fall of Frequency by 3dB when Inductive Reactance in Primary Impedance
Fig. 8: Image Showing Fall of Frequency by 1dB when Inductive Reactance in Primary Impedance
DESIGN OF AUDIO TRANSFORMERS
a) Core Size & Material
Choice of core size and its material depends upon frequency response, power levels, and harmonic distortion. Audio transformers should preferably use both high-permeability cores and the largest number of coil turns to create high primary inductance.
Voltage induced into the secondary winding depends on the turns ratio of the transformer. The turns ratio is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding.
Windings, primary as well as secondary, are the coils of conducting wires as a coil of conductors create a higher magnetic flux compared to the flux created by a single conductor.
When signal levels are low, it is often essential to keep external magnetic fields to the minimum. For this purpose, transformers are enclosed in a case of high permeability materials.
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.
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.
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.
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.
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).
Fig. 10: Telephone Directional Coupling in an Audio Transformer