AUDIO-TRANSFORMERS

Radio Record, Volume I, Issue 13, 14 October 1927, Page 3

AUDIO-TRANSFORMERS

HOW THEY FUNCTION here are many brands of audio frequency amplifying transformers on the market, but so much information and misinformation has been published that it is difficult at times to decide what braud one wants. With an understanding of what happens in an audio frequency amplifying transformer you can judge for yourself whether statements regarding this or that brand are logical. The broadcasting station sends out a large amount of power in the form of an electro-magnetic wave oscillating at a frequency of from 1,500,000 to 500,000 cycles per second, depending on the wave-length given to the station. This large amount of energy varies in intensity, is modulated proportionately to the frequency of the singer’s voice, the violin’s notes or whatever form of sound or noise is made in front of the microphone. ‘These sounds vary from 50 to 6000 cycles per second and are called audio frequencies because they are audible to the human ear. RADIO FREQUENCY ENERGY. This energy at radio frequencies, varying in intensity at audio frequeucies, goes out into space in all directions and travels on and on until completely absorbed by trees, wires, buildings, aerials aud loops. Every radio receiving set must have some form of aerial to pick-up from space .some of this radio frequency, which has in it the undistorted voice of the singer or the note of the violin, but in such a form that it cannot be made audible until stripped of its carrier or radio frequency component. This is accomplished by the detector found in every radio set even if radio frequency amplification has been employed first. The detector reduces the energy to a pulsated direct current, but some detectors do not do a clean job, and the voice and notes become distorted, that is, are not the same as the ones sent into the microphone. After detection some of the energy from the broadcasting station is in a form easily put into head phones for hearing, or into an audio amplifier for even louder hearing. An audio amplifier consists of audio frequency amplifying transformers, valves and accessories for the control and operation of the tubes. The number of stages of amplification is determined by the number of valves or tranisformers used, but for the broadcast listener should never exceed three audio stages on account of engineering difficulties even when using efficient transformrs. TRANSFORMER A LEVER. The valve and B battery may be considered as a pipe line with a pump and shut-off valve in the line. The battery is the pump and the grid of the tube is the shut-off valve. The more pressure applied to the valve the more water the

pump will push through. Therefore, if you use a lever or gear on the valye, you can with the same pressure release more water. The transformer is the lever or gear; this is why they have ratios, that is, the primary or mput has less turns than the secondary or output. This audio frequency energy coming from the detector has so much pressure and is fed into the primary of the transformer, or gear box, and stepped up or increased. Naturally, oie would think a transformer with as high a ratio as possible would be the best, and so it would if the pressure didn’t vary in frequency. Being an alternating pressure of from 50 to 5000 cycles per second other factors enter into the picture and become of great importance both for reproduction and efficiency. As the ratio of transformation is inincreased until the ratio reaches 4} to torted pressure on the grid or valve is’ increased until the ratio reaches 44 to 1, when maximum amplification without distortion is obtained. Further increases in ratio increase tlle ampiification slightly with a greater and greater amount of distortion until ratios of 10 to 1 are reached, when both amplitication and reproduction suffer.

EFFECT OF RESONANCE. Why? you ask. Resonance and distributed capacity. Too techriical, you say. Resonance is the relatively free passage of energy at one frequency and the suppression of energy at other frequencies. When ratios of over 43 to 1 are used resonance creeps in and at higher ratios distributed capacitv, that is, the condenser effect of the windings causes energy to be absorbed in the insulating materials used. But ratio is not the whole story of an audio transformer. There are other technical factors. There are impedance, iron, air gaps, size wire, short circuited turns, burn outs, losses, etc. Impedance is the resistance a winding has to alternating currents, and in a properly designed audio transformer is the same as that between the plate and filament of the tube. With this fixed ratios inean nothing, GOOD IRON NECESSARY. Iron, good iron, must be used, or things happen in it which cause losses and distortion. An air gap in the wire acfs as a valve for energy. When signals are very loud the air gap prevents overloading the frear-box with consequent distortion. The radio valves of to-day use plenty of B battery current and most trans- formers leave such small wire that these currents are crowded for space. One short circuited turn in over twenty thousand reduces the amplification 10 per cent., so a few short circuited turns make the transformer useless except as a paper weirht. The only way to prevent short circuited turns is to test the coils and throw out the shorted ones. Burn outs in amplifving transformers. occur with different degrees of frequency with different brands, but the only thine to do when it happens is to send it back to the manufacturer for repair or replacement, Thev ought to do it because thev can’t convince you that it was your fault, To keep losses a minimum in transformers it takes transformer engineers to desien them, ‘They know where they can get in and how to get them out.