Audio Frequency Amplification

Mains Supply Units.

To avoid the difficulties concomitant with high and low tension batteries, particularly the former, a number of schemes have been devised for supplying receivers with the necessary current from either alternating or direct cur rent supply mains. This permits an adequate anode voltage being supplied to the power valve, so that a reasonable intensity of sound can be secured before "blasting" is encountered. A more extensive use of these supply units is advocated.

Radio Telegraphy.

Receivers for this purpose have been described in the appropriate place and it is only necessary to say here that although the principles involved are similar, the design technique is quite different. The low frequency portion of the amplifier is preceded as usual by radio frequency circuits and a detector, and a separate radio frequency oscillator is used to generate an audible beat note with the incoming radio frequency. Furthermore, the audio frequency consists of a series of circuits tuned to the beat note. Before being passed on to the recording instruments the audio note is rectified and smoothed, so that the result is a series of dots and dashes, in the Morse code, which can be considered as an interrupted direct current.

Loud Sounding Telephones (Loud-speakers).

The out put from a broadcast receiver is taken either to a telephone worn on the head or to some form of loud sounding telephone. The reception on a headphone cannot be realistic, since the original studio conditions and the degree of loudness are completely lost.

To obtain more realistic repro duction it is essential to employ some form of loud sounding tele phone—which is not applied to the ear. The reproduction ap proaches most closely to the or iginal conditions when the room where the loud-speaker is situated is heavily draped to damp the sound, and the intensity of sound from the loud-speaker is equal to that which would be heard by the listener were he or she situated at the same point in the studio as the microphone.

The subject of loud-speakers is wide and varied and can only be touched upon lightly; and though there are numerous forms on the market few reproduce with any degree of accuracy. The essential components of a loud-speaker are some form of dia phragm to agitate the air and a mechanism for driving the diaphragm to and fro under the stimulus of the electric currents from the power valve of the receiver. The motion of the dia phragm reproduces—in a degree—the sounds which are picked up by the microphone in the broadcasting studio or auditorium.

When a large diaphragm is used the most general form is the cone. When small conical or flat diaphragms are used, e.g., 2 in. diam eter, the loud-speaker is fitted with a horn to increase the in tensity of sound and provide the proper coupling between the diaphragm and the surrounding air. No horn is required with a

large diaphragm, e.g., 9 in. diameter, but to secure the requisite degree of intensity the two sides must be isolated as explained below. The material of the cone of a large diaphragm is usually some kind of paper, but impregnated fabric is also used, as in the author's cloth cone, fig. 5. When a large diaphragm is in action, at any particular instant the air at one side is compressed and on the other side rarefied (reduced in pressure). Now since one side is at a higher pressure than the other it is necessary to prevent a sound wave travelling from the side at higher to that at the lower pressure, and to avoid this effect the usual practice is to isolate the two sides of the diaphragm by mounting it either at the centre of a large flat board or in one face of a cabinet. Having mounted the diaphragm, it is essential to apply some form of driving force, the mechanism for which is actu ated by the electric currents from the receiver. There are two main forms of loud-speaker drive for large diaphragms : (a) reed drive, (b) coil drive. Type (a) is characterized by a reed of steel or other suitable magnetic material poised over two pole pieces on which are wound many turns of wire. The magnetic circuit of the pole pieces is completed through a strong permanent magnet. Since the reed has a natural fre quency and its response with fre quency becomes gradually less at low and at high frequencies, care is required in designing the diaphragm and its peripheral support to secure a good acoustic register. Where only one reed is used, it is difficult to get full strength in both the lower and the upper register. In general, it is better to get a good upper and middle register. At the same time, it is very desirable to reduce resonances and harmonic tones not present in the original sounds to a minimum. This is accomplished in a new form of reed drive loud-speaker, the Amplion Lion. The drive indicated as type (b) is capable of giving greater output in the bass register than the reed drive. A coil of wire, situated in the magnetic field produced by either a strong perma nent or an electromagnet, is attached to the back of a paper cone, fig. 6. The speech and music currents are passed through the coil and cause the diaphragm to move accordingly. This class of instrument is capable of reproducing orchestras at their original strength. A replica of the coil drive instrument designed by the author and now operating on weekdays and on Sundays in the South Kensington Science Museum, is shown in fig. 7.

See N. W. McLachlan, Loud Speakers. (N. W. McL.)