do I choose the right amplifier power for my speaker system?
you should pick an amplifier that can deliver power equal to
twice the speaker's continuous IEC power rating. This
means that a speaker with a "nominal impedance" of
8 ohms and a continuous IEC power rating of 350 watts will require
an amplifier that can produce 700 watts into an 8 ohm load.
For a stereo pair of speakers, the amplifier should be rated
at 700 watts per channel into 8 ohms.
- A quality
professional loudspeaker can handle transient peaks in excess
of its rated power if the amplifier can deliver those peaks
without distortion. Using an amp with some extra "headroom"
will help assure that only clean, undistorted power gets to
your speakers. Some professional amplifiers are designed so
they have additional headroom. These amps can cleanly reproduce
transient peaks that exceed the amplifier's rated power. In
this case select a model with an output power rating equal to
the continuous IEC power rating of the speaker. Consult the
amplifier manufacturer or owner's manual to learn more.
- In some
applications, such as critical listening in a studio environment,
it is important to maintain peak transient capability. For these
applications, use an amplifier that can deliver 6db (or four
times as much) more power than the continuous IEC power rating.
- If budget
restraints or legacy equipment force you to use an amplifier
with less power, extreme care should be taken to see that the
amplifier is not driven into clipping. It may surprise you to
learn that low power can result in damage to your speaker or
system. Download our
Danger Low Power (Adobe
204kB) tech note for more information.
How does JBL establish power
handling specifications? At JBL Pro,
we subject our designs to the most rigorous and demanding testing
in the industry. As a result, the power rating specification of
a JBL Professional product may be lower than that of
a competitive speaker which actually has less power handling
- JBL tests
speaker systems as systems. Some competitive speaker systems
are rated based on the power rating of the individual transducers.
Actually, the power handling of each individual component doesn't
tell the entire story. When a transducer is installed in an
enclosure, it may not be able to dissipate heat as well as it
did outside of a box. Or the cross-over network might fail long
before the transducers. When you select a JBL speaker system,
you know that the design has been tested as a complete system.
- JBL tests
speaker designs with long term testing at high power. A speaker
system typically doesn't reach its maximum operating temperature
for at least 2 or more hours. Yet some manufacturers make power
handling claims based on mere minutes of testing. At JBL, our
power tests subject each speaker design to the kind of stress
and strain it will get in years of actual use.
- JBL uses
the IEC spectrum for testing speakers. The IEC (International
Electrotechnical Commission) has established a standard for
a loudspeaker test signal. This method uses shaped noise in
a specified frequency range (50 Hz - 5 kHz), a specified high
and low-pass filter slope, and a specified "crest factor"
(the ratio between the average and peak signal level). IEC shaped
noise places greater demands on a speaker than real music.
makes speakers fail?
Transducers can fail due to excessive mechanical stress or excessive
- One form
of mechanical stress would be a mic stand through the speaker
cone. JBL has overcome this failure mode by using heavy gauge
steel grills on all live sound models. But there are more subtle
ways in which a transducer can fail mechanically. The mechanical
design of JBL Professional transducers has benefited from decades
of experience and the best engineers in the industry. As a result,
mechanical failure of a contemporary JBL speaker is a rare event.
- The speaker
surround and spider, known collectively as the "suspension",
must be flexible in order to accommodate the excursion of the
cone or diaphragm. At the same time, they have to keep the cone
or diaphragm from tipping or becoming "de-centered".
Suspensions can fail due to environmental factors such as exposure
to heat, UV rays, or humidity. It is also possible for the adhesives
that attach the suspension to the cone and the speaker basket
to fail or to be improperly applied at the time of manufacture.
- The separation
of tinsel leads is also a common failure. Conducting the electrical
signal from the speaker terminals to the voice coil is the job
of the tinsel leads. The leads must be light, flexible, and
immune from breakage due to fatigue. At the same time, the leads
need to be able to carry the full rated current load of the
is "Signal Burn"? Signal
burn is a failure mode where the voice coil is burnt across its
entire width, indicating uniform voice coil travel with respect
to the stationary magnet structure. Such a burn pattern is not
indicative of amplifier malfunction but instead is due to excessive
signal or program level. The cause is simply trying to get more
from the speaker than it is capable of delivering.
is "DC Burn"? DC (direct-current)
burn is a failure mode where the voice coil is burnt only
at one end. This is an indication that it has been traveling in
one direction more than the other. Since the transfer of heat
is from the voice coil to the adjacent magnet and metal parts,
the voice coil will be burnt on the end that stays the farthest
away from the top plate.
When a DC burn pattern appears on the voice coil of a woofer,
the problem will be due to a fault in the associated electronic
equipment. Most likely, the power amplifier has leaky or shorted
transistors that are allowing its internal power supply voltages
to be applied directly to the loudspeaker or loudspeaker system.
and Tweeter: When a DC burn pattern is observed on the voice
coil of such devices, it DOES NOT always mean that the amplifier
is faulty. In systems with passive crossovers, mid and high
frequency drivers are protected from DC by the cross-over. The
most likely cause of DC-like burns is an overdriven amplifier.
- When an
amplifier receives an input signal capable of driving
it beyond its power rating, the result is clipping. This means
that the negative and positive peaks of the amplifier's output
signal are "clipped" off. The amplifier may also clip
in an asymmetrical fashion, meaning that the positive side of
the signal is clipped more than the negative (or vice versa).
When subjected to an asymmetrical clipped waveform, one end
of the loudspeaker's voice coil is "on average" spending
more time outside of the gap (corresponding to the direction
that is clipped) than the other. The end of the coil that is
spending more time outside of the gap has poor heat transfer
to the magnet structure. As a result, it overheats and burns.
I fuse my speakers? JBL
does not recommend fusing loudspeakers. A fuse may blow with a
signal that would not damage the speaker, but it can also pass
a signal that can damage the speaker. To protect your system,
JBL advises using adequate, clean amplifier power and watch for
amplifier clipping. Add a limiter to your system to electronically
limit any potentially damaging transients. Some JBL Professional
speaker systems include SonicGuard, a system that actively
protects drivers from excess power.
is "power compression"?
Speaker voice coils are made of copper or aluminum. As these voice
coils increase in temperature during normal operation, the DC
resistance of the voice coil increases. Greater voice coil resistance
means less power transfer from the amplifier. As a result, the
speaker will not play as loud when it's "warmed up"
as it did when it was "cold". Some speakers may exhibit
3 to 6 dB of power compression. This means that power compression
can have the same effect as taking away half of your PA!
JBL has gone
to great lengths to reduce the effects of power compression. Our
VGC (Vented Gap Cooling) and TTM (Total Thermal Management)
technologies effectively dissipate heat for reduced power compression
and increased reliability. In addition, some models use liquid-cooled
(ferrofluid-cooled) voice coils.
do I select the correct wire gauge for my speakers?
Selection of the appropriate wire gauge is important to system
operation. A cable that's too "light" will result in
amplifier power being wasted due to the series resistance of the
cable. It will also result in the loss of low-frequency performance
due to a degraded damping factor. On the other hand, a cable that
is too "heavy" is unnecessarily awkward and costly.
If in doubt,
use the chart below as a guide. For a given length of run and
speaker load, use the chart to determine the minimum wire size
needed to keep your line losses ("insertion" losses)
A pair of High Positive
Current Coeffiecient Resistors (HPCCR) (LP1 and LP2) is connected in series
with the high-frequency sections of the crossover network. These HPCCR
(a specialized type of light-bulb) are wired in parallel with resistor
R1 to form an attenuator. This attenuator matches the higher sensitivity
of the high-frequency driver to the lower sensitivity of the low-frequency
driver, thus balancing the high-frequency and the low -frequency output
of the speaker system. As the input signal rises, more currect passes
through LP1 and LP2. This causes the elements in these devices to increase
in temperature. As their temperature rises, their resistance increases
resulting in a reduction of the signal going into the high-frequency section
of the network. When the overpowered condition creases, the elements will
cool and normal system operation is restored.
The SonicGuard systems
works very well but it can be overwhelmed. A very high-level transient
signal could be through LP1 and LP2 before their temperature increases
sufficiently to cause an increase in their resistance. In addition, extended
and extreme overpowering can still get through the system. Even if LP1
and LP2 were to go open, it is possible that R1 will pass enough signal
to damage the high-frequency driver if the system is grossly over-driven.
As previously mentioned,
SonicGuard will do an excellent job of protecting drivers from inadvertent
overpowering. The system can,
however, be overwhelmed by extreme conditions so good system operation
practices must be observed.
Only those speakers
that are specifically designed and designated for use in suspended (or
flying) applications should be suspended. Suspension of a speaker not
designed for suspension represents a safety hazard and could result in
death, serious injury, property damage and liability for the installer
and system owner. If you have any doubts as to which models are flyable,
please contact your local sales representative or JBL Professional Technical
- JBL models that
have no provision for suspension include JRX*, SoundFactor*, TR, EON1500,
MR, MPro, SR, SR4700X*, SRX700*.
- JBL portable models
designed for suspension include JRX i, SoundFactor i, SRX4700X/F, SRX700/F,
SRX712M (with accessory yoke bracket), VRX.
- All EON models
(except EON1500) may be suspended with available ESK10 or ESK15 suspension
- JBL Installed
& Tour Sound models including AE, PD, Marquis and VerTec series
models are designed for suspended (or flying) applications.
(install) or “F” (flying) versions of some models are available.
© 2001 JBL Professional.
All rights reserved.