Crown OC-150A - Peak Catch and Hold Mode Explained

only relative voltage readings, and airnost
meaningless For accurate pawr eomputatians.
This hefps us to make an observation: (Rule of thumb
# 1 .) Unless the input is a 5teady sine ansave, you can
farget a bout taking a reading and deciding precisely
how much ~wer autpbot your ampiifier is delivering,
This abbemation aSss applies ta VU meters (and the
so-called peak reading VU met~rs since wcke
already dceided that a meter can? ~QSS~ btyfoIIaw the
ma ks in a complex wave farm). AVU meter is simply
a BC meter with a s~ciat fudge factor built into the
calibratisn ss that it rgads Volume Units. A Volume
Unit: is essentially the saw as a db. VU's are always
ref~r@ne@d to a defioed wwer 1ev@/, (One mEiliwat
across a 606 ohm load,) I6 addition, a VU meter must
canform to a defined r@spstse time.
At this wint you are pprobabiy wondering what useful
purpose mcetefs cauld ~ssibiy have, As Iang as you
keep rule af thumb WJ, clearly in mind, the meters
can sew@ 3% a valualsls aid in program comparison.
Far examp%@, it is generally safe ta assume that a
higher average voltage reading means mare gswr*
Notice we didn't say &ow much power! This assump-
tion esufd be used for comparing amplifier outputs,
with different toad cglmbinatianr;, sr different types of
music
The 0C-1568 aH~rs this type sf relative cornparisan
with the meters functioning in the Vlb mode. And this
is only the &ginning, "Th QC-I$OA also offers a
PEAK mde, or $0 describe this Functian compj~tely,
a peak ""ckeh and tsoBdv"ml@,
If there were same way ts actually measenre peaks,
even with comp tex wave forms, we would have a lot of
US~~UI infarmation on our hands, Far exsmgte, the
peaks tell us whether the amplifier is clippirrmg and
thus what [eve t af di6tofiisn to expect. Accadrats peak
vaffages help us to determine the peak power, if
the load is known,
The OC-15OA in the peak catch and hold mode wit1
pravide accurate starage of peak voltages. The cir-
i
cuit is designed sa that all $he inherent limitations af
a meter movemsnt are by-passed, Here" how it
works,
The waveform is sampied efectronieafty using a sen-
sing circuit which measuqes even the fastest. peaks.
"T^is measurement is stored aleetr~nically, and then
sesrt to %he meters, The hafd ti me controls determi me
haw long this sts~ed informatian 1s held at the meter
terminals, "Pe bald lime is adjustable from nrerafo in-
finity, so the slow meter movement can take all the
time in the world getting up to the peak reading.
When the pointer $taps movement, you are observing
a vattag@ peak that occurred some fraction sf a see-
ond previously. The meter scales are calibrated to
yead urns equiva8ent values, SO if you want a peak
value, muitiply the meter reading by 1.414,
Once a peak is caught and held, the meter wi 19 re-
main a$ that level until a peak of higher vatu@ comas
afsng, If you want to see as many peaks, at different
levels, as possibie, a sRo& hs\d time is necessary.
Remember, 8s long as the metercowlestoa stop, it is
reading an rms equivalent to a true peak. In essence
we have trickily outwitted the pr&~ve~biaJ ofd slow
meter movement. Rather neit. eh?
Up $0 this paint we have assumed a test-bench situa-
tion when relati~lgvsltage readingo ts pswer, That is,
we have assumed that th@ toad was naa-reactive;
and the resistance vatu@ was known, Power
calculations fsllow wjth Ohm's law; P = E?.
R
What happens ts power output if we substitute a
swaker (reactance and resistance) far our resistive
load?
When an amplifier supplies power to a purely
~esistiwe laad, "re current through the resistance,
and the valtage across it areexactly rn phase& That is,
waks and zeros sf the current and voltage wave
forms occur simultaneously. If a pure reactance is
substituted for the resistance, the GWTTEM~ is
longer in phage with the voltage, but leads sr lags by
90"depending on whether the reactance is
capacitive or inductive,
In any practical Isad, such as a loudspeaker,"ehe toad
is camplex, consisting of both resistance and reac-
tance. These two quantities are added by a technique
using phasors (quanti"ries which include both an
amplitude and a phase angle) and the resultant,
called impedar-ice, ix the real bad the amplifier
" sees".