Crown OC-150A - Page 20

for smadsthing out the effect of frequency on im-
pedance, For instance, by careful design of the
swa ker enclosure it is wssi ble to smooth out some
sf the wide variations in impdance, But speaker im-
pedance ss still not linear with frequency. Many
variables a&ect I mpedanse, such as cabinet
resonance and C~QSSOY~~ characteristics. These and
sther factors lead 40 the Qpe of impedance curve it-
lustrated in Figure 2-4,
Speaker impedance is usuaily given as a nominal
value, such as 8 ohms or4 ohm%. This is a rough ap-
prsxsmatzon of an average immdance fsr the range
sf audsa frequencies.
The overall effect af uneven pswer response [Ae ta
varylnp smpedance) may not be as dismaying as it
appears. Here by the application of inverse Murphy's
Law, we end lap w&t-1 a beeseficia! tw~st sf fate,
A taudspeager ss largeiy voltage sensitive ra"ther than
power sensitlve Since an ampiifier WI fl defiver a con-
staant voltage over a range of impedaoces, the audi ble
effect oft he cbnging power is not at ways notices ble,
Even though the impedance of a speaker may drapat
a cedain frequency, in many speakers, tbeefficieracy
drops at that frequency as well, The net resuit is th%t
while more power is produced at that frequency,
rnore of it is lost as heat, and the acoustical output
level is unchanged,
All sf the preceding indicates the diRicul%ies involved
in aeeuaately interpreting 06- 150A voltage readings,
using a magic pr~gram $S $OUTC$, with 8n amplifier
driving a campiex load* It is apparent that you must
be satisfied with relative ar comparative inter-
pretations. For specific computations you must use a
test bench set-up. (Sine wave and resistive laad),
Each depa~uie from this test situation inlrduces
new variables which require very carefadli $and Gem-
plicated) analysis+ By the time you have tuned in an
FM stereo signal and hookd up your favorcite
speakers, the voft~ge readings an the QC-1SOA are
chiefly usefut a$ a camparabive analysis tmf*
Ysu might try computing power lasing the oaminal
rated impedance af the speaker system, Ihshoanld be
clear from the above discussion, however, that the
resu its may be far from accurate, If the laad is a pure
resistance, then the graph of Figure 2-5 may be used
ta compute pswer, Look again at Figure 2-3, Nstiee
that the mare reactive the Isad becomes, the fa@her
the graph depa@s from a straight line. Because the
reactance produces negative power, the ampiifier
loses efficiency, This means less of the total pclwer is
defivered to the laad.
Now 4x1 examine the usefulness sf the OC-JSOA
voltage readings* In Figu~e 2-5 the veeicai calumn
repre%enXs voltage, The horizontal ca9umn repres-
ents power. We have drawn three laad lines sn this
graph, Yacr must pick the line that comes the cfasest
to your load impedance*
IF ysur impedance is a pure resistance, you will kx? ap-
proaching 100% eeff iciency. (This won't praduce
I
much acatdsticai energy, but it's great far heating
your room.) As your load impedance becomes reac-
tive:, your amgIifier efficiency goes down. However, it
is f t785 reactive component which prg3duee~
acoustical energy, So WE are looking at a trade-off
between high inductance fgosd enera transfer) and
the corresponding high reactance (amplifier inef-
ficiency),
If we $16uid take an 8 ohm speaker, and vary the ratias
of reactance to resistance from pure resistarace to
pure reactance, the amptlfier wa~Id supply the same
Power through the entire range. At tkteextremee~ld af
pure resjslanee, the load would dissipate all the
power, with 1QO'3& amplifier e^EFiciency, At the other
extreme of pure reactance, the amplifier wauld dis-
sipate all the pswer for an efficiency of 0%
Somewhere in between these extremes, we would
find a campromise between amplifier efficiency and
maxi mum acsadsfi~al energy,
As yau use the graph of Figure 2-5, keep the a bove
discussian in mind and you will not be enticed into
making erroneous sbtements about how much
power your speake~ are pradueing.