Yamaha P-2200 - Manual (Page 45)

range, the clipping may sound like an irritating sizzle
that only happens on certain sounds. Similarly, clipping
in the low frequencies can cause bass notes to sound
fuzzy or muddy, or it can cause mid-range frequencies
to be harsh. Yet because the clipping only takes place
on certain sounds, it may not be immediately apparent
that clipping is the source of the problem. With graphic
equalization, the choice of a cut-only device (rather than
a boost and cut device) may help solve the problem
because, since boost is not available, clipping problems
are reduced. With cut and boost graphics, parametrics, or
other types of equalizers, the system operator must be
aware of the potential clipping problem, and attempt to
avoid it.
SPEAKER PROTECTION
The maximum sustained output power of the P-2200 in-
to an 8-ohm load is at least 230 watts. Few, if any, single
speaker systems are capable of absorbing that much
power on a continuous basis. Most speaker systems,
however, are capable of absorbing short duration peaks
of considerably higher power than their rated continuous
power capacity. The ability to produce these peaks with-
out distortion is a major advantage of a large power
amplifier like the P-2200. The speaker, however, must
be protected against the abuses of excessive average
power, sudden large peaks, DC current, and frequencies
outside its range. The following are methods of achieving
some degree of protection against these abuses.
Fuses
Yamaha does not recommend the use of any type of
fuse as speaker protection. Fuses are slow-acting devices
of inconsistent quality, and do not offer adequate pro-
tection for speaker systems. They are mentioned here
only because they are used in some systems. Standard
fuses may be capable of protecting a speaker against
excessive average power, but they are too slow to
successfully protect a speaker against sudden peaks.
Fast-blow, instrumentation fuses, with improved time
response, may blow on normal program peaks and
needlessly disrupt the program. Slo-blo fuses, on the
other hand, may not blow quickly enough to prevent
loudspeaker damage due to voice coil overheating. If
fuses are used, whenever possible fuse each loudspeaker
separately so that a single fuse failure will not stop the
show.
A fuse will protect a loudspeaker against one common
fault of a DC coupled amplifier: DC at the output. The
slightest DC offset from a direct coupled preamplifier
will be amplified and appear at the power amplifier's out-
put as a larger voltage with the power amplifier's large cur-
rent capacity behind it. Even though there is no immediate
audible affect (the extra power draw may cause some
amplifiers to hum slightly), the loudspeaker is forced to
absorb the DC power output of the amplifier. Since it
cannot convert this DC power into acoustic power, the
speaker overheats. Small amounts of DC voltage can
shorten the life of a loudspeaker, and any large amount
of DC will cause sudden, catastrophic failure. Fortunate-
ly, the input of the P-2200 is not DC coupled so any DC
voltages from preamplifiers, etc. are not amplified, and
cannot reach the speaker. The only time DC voltage
could appear at the P-2200's output would be in the
event of a severe electronic failure inside the amplifier,
a very unlikely event.
Capacitors
Inserting a non-polarized capacitor in series with a
high frequency driver can protect it against excessive
low frequency energy. The capacitor acts as a 6dB/
octave high pass filter. Especially on a biamplified
system, this kind of protection is desirable. For a bi-
amplified system (or triamplified system), choose a
protection capacitor by the following formula:
Value (in microfarads) =
(Where " p " =3.14, "f" is the crossover frequency
divided by two, and Z is the nominal impedance of
the driver.)
The same formula can be used to choose a
capacitor to insert in series with a low quality 70-volt
speaker transformer to avoid excessive current flow at
low frequencies (see Page SIX 13). Measure the imped-
ance of the transformer primary at the lowest frequency
of interest (which will probably be somewhere around
100Hz) with a speaker load connected to the secondary.
Choose the protection capacitor by the above formula
with Z = the measured impedance of the transformer,
and f = the lowest frequency of interest divided by two.
The voltage rating of the capacitor chosen must be
greater than the maximum expected total peak to peak
voltage that will ever appear at the driver's terminals.
For the P-2200, this is equal to the sum of its positive
and negative supply voltages, which is 160 volts. The
most common types of capacitors used for driver pro-
tection are non-polarized electrolytics. Because of the
inductance associated with an electrolytic capacitor, it
may be paralleled with a mylar capacitor of about 1 / 1 0
the value in microfarads to reduce high frequency losses.
Limiters
A limiter is not normally considered a loudspeaker
protection device, but it may be one of the best and most
practical. A "squared up" or "clipped" waveform causes
a loudspeaker cone or driver diaphragm to move to one
position and stay there, then move back to the extreme
opposite position, and stay there, etc. Because there is
still power flowing through the voice coil, but there is no
voice coil movement, the power is converted to heat. If
a limiter is placed before the power amplifier in a system,
it can be adjusted to prevent peaks from reaching a level
that would cause the power amplifier to clip, which may
avoid burned out loudspeakers.
Transformers
The 70-volt transformers used in "constant voltage"
commercial sound systems lend a certain amount of
protection to a loudspeaker. They will not pass DC
current, and most of them will not even pass subsonic
Fig. 71 - Typical use of Auto-Transformer for Speaker
Impedance Matching which also helps protect the Speaker
from damage caused by DC at the Amplifier's Output.