BEGINNING OF OVERLOAO 0ISTORTION

amplifier spec sheet simply lists "power output" as “10 watts" or "20 watts" and does not further describe the "typc" of walls. it can bc assumed ihat the manufacturer is talking about "steady-state (based on r.m.s. values of voltage and current) watts."

this commonly used e.\pression infers a "maximum" rating. apparently re-lating to peak progmm conditions. when actually such is not the case.

Musie Power Output In an attempt to standardize on a power output rating which bears a reasonable relationship to an ampli-

Peak-Power Output

This expression in no way relatcs to the concept of peaks of program materiał. It is merely a convcnient mathe-matical conversion of r.m.s. power to the a.c. expression for peak of root-mean-square (r.m.s.). This corwersion reveals that lor a sine-wave stgnal Peak Power — 2 X r.m.s. Power, do-rived as fołlows:

Pr„," -^E&—. ^also 1414 XE,~.=

Ep0*k Or    E rm»

E

remk

P rmś

Ppeek

Pprmk

E rm* X Ę r n >

R

/JXĘ rm § X 2 X Ę rm* ~R

/2~X /2 X E\~.

R

Pl-tmk

2 E*

rm i

R

Therefore: Pfemk — 2 X Prm$

Fiq. 2. The qraph at the riqht shows the r.m.s. roltaqes as read on an output me-ter or a v.t.v.m. lor various power output levels. Values are shown lor lour commonly used output impedances. As an example oł the use ol the qraph. assume you are measurinq the volt-aqe across an 8-ohm load resistor connect-ed to the 8-ohm output terminals. II the r.m.s. voltaqe reading is 5 volts. the steady-state power output is just a bit over 3 watts as indicated by qraph.

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Steady-state (r.m.s.) power can be conveniently measured and compared. Peak power couhi be measured through use of peak-indicating voltmoters. but it is somewhat easier to obtain peak power simply by mulliplying r.m.s. power by 2. For practical purposes, "peak power" is not significanl in evaluating amplifier performance. It is unfortunate that the lerm "peak" in

October, 1959

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