Radio today (Jan-Dec 1938)

WHAT AMPLIFIER RATINGS MEAN Facts that will help the sound man select his equipment * As new amplifiers continue to appear on the market the prospective customer and P.A. man find it increasingly difficult to compare ratings and values. Since equipment for checking amplifiers is not generally available, and even if it were comparative tests would be a lengthy procedure, the customer must rely entirely upon the manufacturer's ratings. These ratings, although generally authentic, are often based upon different standards of measurement, and in order to compare any two amplifiers, it is extremely important that the various ways of rating amplifiers be understood. The amplifier specifications on pages 42 & 44 have been compiled so as to list the data on a basis suitable for comparison. Normal vs. max. power At present the term "power" may be defined in a number of ways and it is important in comparing amplifiers that the different methods of power rating be understood. The terms "normal power" and "peak power" are misleading to the layman and to add further to the confusion some amplifiers are listed as capable of useful output over a certain power range. If the output of an amplifier is listed as 8 to 15 watts, the lower power of 8 watts is the normal output, and the wave form flattens abruptly for higher outputs, even though the amplifier may be capable of 15 watts of highlydistorted power. In all cases the normal rating is the true output of the amplifier, states S. Douglas Fortune of Thordarson Elec. Mfg. Co. Practically any amplifier is capable of an output of twice its normal value. Fig. 1A shows the sine wave output Fig. 1 — Waveform of various outputs. of an amplifier. The effective value of the peak value E is .707E, and the power output varies as (.707E)2 or 54 E2. If now the amplifier be overdriven so that Fig. 1C is the result, the effective value is very nearly E, so that the power varies as E2 which is twice the normal value of 'AE-. The distortion in a square wave is extremely high. Sometimes the value of "peak power" for a given amount of distortion is listed. For instance an amplifier may be rated at 20 watts normal and 25 watts peak. This peak rating usually contains from 10 to 15 per cent distortion. Although this method is somewhat more conservative than the previous method of peak power rating, it does give the power rating with a flat topped wave, which is similar to that of Fig. IB. In comparing power rating it is important to place all values of peak power secondary to the normal rating which is the true output of the amplifier. Naturally, the greater the peak power for a given normal poicer, the lower the distortion will be when overloading takes place. "Distortion" as well as "power" may be interpreted in a number of ways. Some manufacturers lacking the facilities for actually measuring distortion list the total amplifier distortion as that given by the tube manufacturer. The distortion figure listed by the tube manufacturer in no way accounts for practical limitations encountered in actual design or variation normally encountered in tube characteristics. Distortion figures may be accepted as authentic only if a distortion curve is run with a representative set of tubes, such as in chart on page 32. "Undistorted" output Some manufacturers list the output of their amplifiers as "undistorted power." Distortion under 5 per cent is usually considered negligible and an amplifier with 5 per cent distortion may be considered as having "undistorted output," although the term has been applied so loosely that it may now mean an output with practically any amount of distortion, and it should not be assumed that an amplifier with "undistorted power output" has necessarily less than 5 per cent distortion. In the amplifier specifications on pages 42 & 44, the distortion at normal rated output is given. (To page 46) Fig. 2 — Distortion vs. frequency characteristics of a typical amplifier at rated output. 38 Fig. 3 — Normal amplifier frequency response is center curve. Other curves show effect of tone controls. Radio Today ' % V/i' <fi<