Crown OC-150A - Page 19

$4) %me power is last due to inefficiencies in the
doudspeaker, Interaction of the voice coil with
the magnetic field causes heating af the
magnetic care. nki sheat is all wasted, producing
no acokastica t effect,
It can be seen fram this, that perhaps anty a
smaif paWPian of the 12-5 WBBS total has been
transiated into actual aeaugtical energy, HOW
much reactance is present, how march resist-
ance, and ather factars such 3% speaker ef-
ficiency dstermine the nst result, !a: should be
clear the6 highly reactive speakers (such as clec-
trortatic system%;;) rgpresent a difficult load and
severely limit the amplifier's efficiency,
Motim the V-l auwuf graph, Figure 2-3. The
straight line through %he middle represents the
toad fine for an 8 ohm resistive toad, Notice that
no negative power is produced (voltage and
currgnt are in phase). "I"@ ellipse in the center
represents an 8 ohm impedance laad line, (The
8 ohm impedance consists of 3 ohms re~i~fance
and 9.42 ahms inductive reactance at 1KHz.)
Notice that because af the phase shrift, 68"n this
case, the straight fine has became an ellipse,
and then far WO p% riod5 af time through the 36Q0
cycie, negative power is produced.
UP
FIG, 2-3
It should be clear that tb ideal laad in terms of
producing aceustiea9 enerw would be one which
minimize% the reactive camwnant (high amplifier
efficiency) while optimizing the desired dissipation
campsnsnt , $Isw inductance high efficiency
spaker).Aiss the resistance sf the load shagaid h
kept at a minimum. Iln practical terms this ideal is difw
ficult to achieve and amplifiers shoufd therefore !x
designed to handle high reactances without aver
heating,
At this win.%: the obsgwant raader has probably notie-
ed ahat wrs have employed only osle frequaasey in
daaribing complex laads. WC have one pa&ing poiut
to mnder kfore W@ can be satisfied in knowing what
the OC.150B"s voitags rgading~ are &bring us* (You
didn? texpeet anfiRing simple with so wphisticated a
system , , , 2)
Notice the*graph sf Figure 2-4. This is a plot of a 3ypp-
icaf" 'speaker system impedance aver the audi~
bandwidth,
Since reactance varies wi"r frequency, the ratia af
reactance to resistance, and thus immdal-rce, varies
with frequency also.
When a music program is used as a wurce and is
applied ts speakerterminals, the impedance seen by
the amplifier becomes vey complex, king siightly
different far each frequency component present.
Since
the impedance varies, the amaunt of ww@r at
difzfecent frequencies atso varies.
Swaker manufacturers have "Eied many schemes