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y CTOR ROBINSON    /The meaning and measuremenł of steady-state, peak. and

chief Eng.. paco Electronic* Co., inc. / musie power along with discussion of power-bandwidłh.

One of the first steps the prospective purchaser of a hi-fi amplifier undertakes is a comparison of ad-vertised power output ratings. It be-comes immediately obvious to him that the higher-priced amplifiers are almost invariably associated with higher output ratings and although there may be a number of apparent irregularities in a “graph” of power output vs selling price, the prospective buyer can be quite surę that morę watts in any manufacturer‘s amplifier linę will mean morę dollars.

The answer to the ąuestion "How much output power do I really need?” is complex and involves many factors such as speaker efliciency, room acous-tics and size, and maximum permissible distortion. Before any of these morę refined considerations can be tackled, the apparently simple wattage specifi-cation must be examined and defined, inasmuch as several types of wattage rating expressions are currently being employed by amplifier manufacturers in their advertised specifications.

Any power-output rating must have an implied or stated relationship to a distortion rating. A 20-watt rating for an amplifier carries within itself a distortion limit even if this limit is not clearly defined in the manufacturer‘s specifications. If a distortion limit is listed, for example, as “no greater than 17r total harmonie distortion” and power is listed as ”20 watts,” it can be expected that as a standard test signal at the input of the amplifier is inereased, the graphical crossover point between rising power and rising distortion will be reached at 1% distortion and 20 watts of output power.

This point is an important one inasmuch as relatively low-priced amplifiers can show a considerable power advantage if the distortion limit is specified a bit higher than the figurę usually expected. This condition exists in this price class of amplifiers wherein little negative feedback may be employed, producing a gently sloping power    distortion characteristic.

Medium- and higher-priced amplifiers, however, usually use appreciable in-verse feedback, resulting in a sharp rise in distortion above l^rA or 2^r/t. Little advantage is gained horę if the maximum distortion limit is specified a bit higher.

When making tests for power output, a non-inductive output load re-sistance must be substituted for the loudspeaker. This condition for test eliminates the complications which would be introduced by the varied impedance vs frequency character-istics of speakers and the various efli-ciencies and characteristics of speaker Systems.

With these thoughts in mind, we can proceed to discuss the four types of expressions for output power com-monly used: 1. steady-state (r.m.s. > power; 2. “peak” power; 3. musie power (often referred to as musie waveform power output, or "musie waveforms,” or “program materiał” power, or “short bursts” power 1, and 4. power-bandwidth output.

Steady-Słałe Power Output

This may be defined as “the power output measured across the load re-sistor at the output of an amplifier with 1000-cps sine-wave signal im-pressed at the input of the amplifier.” This type of test reveals the power capabilities at 1000 cps only; it does not reveal the performance at either the high- or low-frequency ends. It does subject the amplifier to a rela-tively heavy duty test cycle because of the fact that a sine-wave cycle has a heavier duty characteristic as com-pared to the morę pulse-like character of musie or other types of program materiał. Because of this latter factor and the fact that this type of test can be easily duplicated through use of a sine-wave generator, a.c. voltmeter, distortion analyzer, or scope. the steady-state output specification is used fairly universally and is con-sidered quite revealing.

A typical test setup for measuring steady-state power output is shown in Fig. 1.

If the amplifier includes bass and treble Controls, these Controls are set to the “fiat" positions. The signal is inereased to the point just below the level wherein clipping begins to occur. By using P — E?/R, the power can be calculated from the voltage reading or by use of the graph shown in Fig. 2. To be as fair as possible to both the amplifier manufacturer and to the amplifier owner, the linę voltage should be checked and set, if necessary, to the standard value of 117 volts.

This scope method for checking steady-state power output can be used only to determine maiimum output values. If power output tests at lower distortion points are desired. a distortion analyzer must be used in place of the scope.

It is important to notę that if the ELECTRONICS WORLO

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