CX 4 - Amplificateur audio QSC AUDIO - Free user manual and instructions

USER MANUAL CX 4 QSC AUDIO

AC Power . Operation Troubleshooting … Open Input Architecture” Level | Parallel, Stereo, or Bridged Operation Maximum Long-Term Output Power AC Current Consumption Heat Emissions

Designing the Distributed Sound System Loudspeaker Coverage and Placement Determining Power Levels Selecting the Amplifier Using Components with Different Voltages How Many Speakers? … Other Design Considerations Speaker Transformer Saturation Connecting Both a Distributed Line and a Direct Loudspeaker on One Channel Wire Loss AC Current Consumption Heat Emissions

EXPLANATION OF GRAPHICAL SYMBOLS The lightning flash with arrowhead symbol, within an equilateral triangle, is intended to alert the user to the presence of uninsulated “dangerous voltage” within the product's enclo- sure that may be of sufficient magni- tude to constitute a risk of electric shock to humans.

The exclamation point within an equi- lateral triangle is intended to alert the users to the presence of impor- tant operating and maintenance (ser-

vicing) instructions in the literature accompanying the product.

EXPLICATION DES SYMBOLES GRAPHIQUES Le symbole éclair avec point de flèche à l'intrérieur d'un triangle équilatéral est utilisé pour alerter l'utilisateur de la presence à l'intérieur du coffret de “voltage dangereux" non isolé d'ampleur suffisante pour constituer un risque d'elétrocution.

SYMBOLE Das Ausrufzeichen in einem gleichseitigen Dreieck weist den Benutzer auf wichtige Betriebs- und Wartungs- vorschriften in den beiliegenden Unterlagen des Gerâtes hin.

DECLARATION OF CONFORMITY for all CX and CXT models

We declare as our sole responsibility that this product is in compliance with the EMC Directive 89/336/EEC and con- forms to the requirements of the Har- monized Product Standards EN 55013 (Product Emissions), and EN 55020 {Product Immunity).

CAUTION RISKOF ELECTRIC SHOCK DO NOTOPEN CAUTION: To reduce the risk of electric shock, do not remove the cover. No user-serviceable parts inside. Refer servicing to qualified service personnel.

WARNING: To prevent fire or electric shock, do not expose this equipment to rain or moisture.

AVIS RISQUE DE CHOC ÉLECTRIQUE NE PAS OUVRIR ATTENTION: Pour eviter les risques de choc électrique, ne pas enlever le courvercle. Aucun entretien de pièces intérieures par l’usager. Confier l'entretien au personnel qualifié.

SAFEGUARDS Electrical energy can perform many useful func- tions. This unit has been engineered and manufac- tured to assure your personal safety. Improper use can result in potential electrical shock or fire hazards. In order not to defeat the safeguards, observe the following instructions for its installation, use and servicing.

PRECAUTIONS L'énergie électrique peut remplir de nombreuses fonctions utiles. Cet appariel à &té conçu et réalisé pour assurer une sécurité personnelle entiére. Une utilisation impropre peut entraîner des risques d'électrocution ou d'incendie. Dans le but de ne pas rendre inutiles les mesures de sécurité, bien ob- server les instructions suivantes pour l'installation, l'utilisation et l'entretien de l'appareil.

Optional rear rack mounting ears.

SPEAKER DAMAGE The CX Series amplifiers are among the most powerful professional amplifiers available and are capable of producing much more power than many loudspeakers can handle. It is the user's responsibility to use suitable speakers with the amplifier and to use them in a sensible way that will not cause damage.

QSC will not be responsible for blown speakers. Consult the speaker manufacturer for power-handling recommendations.

Even if you reduce the gain using the amplifier's rear panel attenuator controls, it is still possible to reach full output power if the input signal level is high enough.

À single high-power crescendo can blow high-frequency speaker drivers almost instantaneously, while low- frequency drivers can usually withstand very high, continuous power levels for à few seconds before they too die. Reduce power immediately if you hear any speaker “bottoming out"-harsh pops or cracking distortion that indicate that the speaker voice coil or diaphragm is striking the magnet assembly.

QSC recommends that you use amplifiers of this power range for more headroom (cleaner sound) rather than for increased volume.

SPEAKER OUTPUT SHOCK HAZARD A CX amplifieris capable of producing hazardous output voltages. To avoid electrical shock, make sure the cover is in place overthe outputterminals, and do nottouch any exposed speaker wiring while the amplifieris operating.

RACK MOUNTING PRECAUTIONS To minimize twisting stress on the front mounting ears, the amplifier's internally mounted output transformers

are located close to the front panel. Nevertheless, when rack mounting any CX amplifier, make sure it is well

supported at all four corners to avoid damage to either the amplifier mounting ears or the mounting rails. Contact the QSC Technical Services Department to order rear support brackets.

LIFTING PRECAUTIONS In order to safely move or install the amplifier, it is recom- mended that two persons share the weight when lifting and positioning the unit.

SICHERHEITSVORSCHRIFTEN BEIM HOCHHEBEN Um den Verstärker sicher zu verschieben oder einzubauen, wirdempfohlen,das Gewichtdes Verstärkers beimHochheben und VerschiebengleichmäBigaufzw ei Personenzuverteilen.

Il. Overall Description

The CX Series from QSC comprises six high-efficiency professional power amplifier models, each with two independent channels. The table below lists the range's audio power ratings.

MODEL 80! 40: 20? 25 volts® 70 volts! 100 volts! cx4 150watts 225watts 350 watts CXAT 150watts 225watts 350watts 150 watts 175 wattst 175 watts‘ Cx6 200 watts 300watts 450 watts

CX6T 200 watts 300watts 450watts 200 watts? 250 watts’ 250 watts’ Cx12 425 watts 600watts 900 watts CX12T 425 watts 600watts 900watts 400 watts? 500 watts’ 500 watts?

1: FTC watts per channel, 20 Hz-20 kHz, 0.1% THD 2: ElA watts per channel, 1 kHz, 1% THD

3: Watts per channel, 45 Hz-15 KHz, 0.25% THD

4: Watts per channel, 50 Hz-15 kHz, 0.25% THD Each channel has its own power transformer secondary. The CX4, CX4T, CX6 and CX6T utilize complementary

Class AB linear output circuitry. For improved efficiency, the CX12 and CX12T utilize Class H step linear

complementary output circuitry and have bipolar multi-rail power supplies. The power transformers, as are the

output transformers, are mounted in the front of the amplifier chassis, as close to the front mounting rails as

possible; this keeps the unit's center of gravity forward to minimize the twisting force on the front

mounting ears. Still, the amplifier should be supported at all four corners, especially if it is in à MODEL portable rack.

cxa The CX4 and CXAT are two rackspaces high;the CX6, CX6T, CX12, and CX12T are three rackspaces high. The adjacent table lists the amplifiers’ weights by model. CXAT The amplifiers require à rack depth of 18 inches (45.7 cm) to clear the rear support ears. Allow Cx6 some additional clearance for input and output connectors at the rear panel, which is 16.9 inches CX6T {42.9 cm) behind the plane of the frount mounting rails. The built-in cooling fan draws air in at

the rear of the chassis and exhausts it through vents in the front panel. The flow-through cooling scheme allows you to rack-mount the amplifiers one atop the other, with no clearance necessary in between. This mounting technique also helps support the weight of the upper amplifiers.

The CX Series derives much of its design from our award-winning M Xa Series amplifiers. They are engineered for stability and exceptional reliability, with protection for open or short circuits and mismatched loads. To protect your loudspeakers, the outputs mute during turn-on and turn-off, and also in the event of a DC fault. The CX4 and CX4T are AC coupled and cannot pass DC to the load. Therefore, the DC fault muting is not required. AII protection circuitry automatically resets to normal when conditions assure safe operation.

Every CX amplifier features thermal management and protection in the form of à two-speed fan and extruded aluminum heat sinks with direct-mounted output devices. If a heat sinkoverheats to 8522 C or higher, a protection circuit mutes the amplifier channel output.

13,6 kg; 27 Ib 18.2 kg; 37 1b 20 kg; 42 lb 25 kg; 55 lb 22.7 kg; 50 lb

Front features of CX4 or CX4T amplifier

Rear features of CX4 amplifier

Rear features of CX4T amplifier

TON INT NITONTI TER . Front

| | IN,52 0 LE | features of - ] ][Il

5 — Protect Indicators (Ch1, Ch2)

6— Accessory Slot Cover Rear © features of 7— Gain Control CX6 or CX12

8 — Detachable Input Header amplifier 9 — Cooling Fan 10 — Shrouded Output Connector (Direct Outputs) 11 — Shrouded Output Connector Isolated Outputs) 12 — Mounting Holes for Optional Handles 13 — Rear Support Ear Rear features

14 — Parallel/Stereo/Bridge of CX6T or Switch CXLT

detachable header. Attach input signal wires as shown. If you are connecting an insertwire here, then tighten screw. unbalanced signal to an input, use the non-inverting (+) input and the ground NV terminals of the header, and also connect a wire jumper between the inverting (-)

input and the ground terminal. The wire jumper will prevent à reduction in gain à «3 : : TD3$S% caused by à floating input. ÉLBSE Es0e 8 + If you need XLR or 1/4" RTS inputs on your CX6, CX6T, CX12, CX12T, use the optional M X-1 66 96 È 5 8 input expander (available through QSC's Technical Services Department). Ê5

U Typical of QSC amplifiers, the input impedance of a CX Series amplifieris 20 kilohms balanced and 10 kilohms unbalanced.

Each CX Series (except CX4, CX4T) amplifier features a Level 1 Open Input Architecture slot. R See the Operation Chapter for more information about Open Input Architecture. Shield Balanced input

INPUT SENSITIVITIES Audio signals of these levels will produce full rated output power at 8 ohms.

Covered barrier strips located on the rear panel allow speaker cable connections to the amplifier outputs. See the diagrams for details on connecting speakers and/or distributed (25-, 70-, or 100- volt}lines. Forattachingwires, we recommendusinginsulated connectors ofthetype shown. Always make sure the amplifier is turned off before you change any output connections or lift the safety shroud.

If you are operating the amplifier in bridged mono mode to achieve higher output voltages, neither speaker wire is grounded. Therefore, use the same precautions in handling and dressing the wire that you would for the normal “hot” outputs. See the section on bridged mono operation for

PE . : Lift safety shroud. additional important instructions.

Loosen screw If you are driving a distributed line, make sure the sum of all the power demandés of the speakers terminale

je . . Insert wire connectors does not exceed the amplifier's power rating. In fact, itis good practice to allow about a 25% safety

under screws. margin. For example, if the load on the distributed line is 400 watts, use an amplifier with a power Tighten screw

ating of e the cati terminals. 500 watts. See the Application Notes in the back of this manual for more details on distributed lines. Lower safety shroud.

If your application calls for connecting an 8-ohm speaker and a distributed line to the same amplifier channel, the CXA4T, CX6T and CX12T can do that. How ever, since most of the audio power is drawn bythe direct-connected speaker, you must derate the distributed line; the distributed line should have a total power load of no more than one-fourth the amplifier's normal distributed line power rating. Make sure the direct-connected speaker can handle the 8-ohm power rating of the amplifier. See Applications Notes section for more details.

Output connections for CX, bridged mono mode. Output connections for CX, parallel or stereo mode.

Ë É LE Output connections for CXT, bridged mono mode. Output connections for CXT, direct low

HOME V.Controls & Displays

CX4 AND CXAT The channel attenuator controls, labeled in dB of attenuation , are located on the rear panel. They have 11 detents, adjustable from 0 dB (full gain) to x (full attenuation). The LED displays for each channel function as shown at right.

CX6, CX6T, CX12, AND CX12T The channel attenuator controls, labeled in dB of attenuation , are located on the rear panel. They have 11 detents, adjustable from 0 dB (full gain) to x (full attenuation). The LED displays for each channel function as shown at right.

AC POWER The amplifier is available for 100, 120, or 220-240 VAC, 50 or 60 Hz operation. Unless it is specially ordered otherwise, each amplifier is configured for the line voltage in the market it is sold in. For example, those sold in Japan are 100 VAC:; for North and Central America, 120 VAC:; for Europe and most of Asia, 220-240 VAC.

The power cord has a plug for connecting to a standard AC source. For 120-volt operation, the plug is a standard 15-ampere grounded NEM A fitting; it meets safety agency requirements for current consumption of less than 12 amperes during “normal” operation, but peak current draw under heavy usage can be higher.

M odels built for 220 to 240-volt use have a standard Schuko (continental Europe) connector on the power cable.

OPERATION CX4 and CX4T When first powering up, start with the gain controls off until proper operation is verified (upon turning on the AC switch, the “POW ER" LED should come on, and after three seconds, the muting will stop and sound can be heard). The amp should now be working.

Gain should be kept in the upper half of its range for full performance. "POW ER" LED indicators are provided to monitor the operation of each channel. Each channel has a red “CLIP” indicator that will show any distortion in the amplifier. The mute circuit should cut the sound off as soon as you turn off the amp, and mute for three seconds before restoring power to the speakers. This blocks turn-on and turn-off thumps.

LED Display for CX4 and Cx4T: NORMAL OPERATION CLIP ©

POWER OCCASIONAL CLIPPING CLIP POWER STEADY CLIPPING CLIP POWER LED Display for CX6, CXGT, CX12, and CXI2T:

2O% Full brightness Flashing Not illuminated

KEY CX6, CX6T, CX12 and CX12T When the amplifier is first turned on, the red “PROTECT” LED on each channel will light for about two or three seconds, during which the output relays will stay open to mute the speakers. Afterthe turn-on muting interval, the “PROTECT” LEDS turn off, the green “POWER” LEDs light, and the output relays close to enable the speaker outputs. Even during the muting interval, the yellow “SIGNAL” and red “CLIP” LEDs operate normally if there is a signal present. If the “CLIP” LED is on while the amplifier is muted, cut the gain back immediately to avoid à full-power blast of sound when the output relays close. If a channel stays muted with"PROTECT" lit, orifits "SIGNAL" or “CLIP” indicators light up whenthe gain is turned allthe way down, it may be defective; see the troubleshooting segment for more information.

The “SIGNAL” LED indicates signal levels that are -30 dB (referenced to full rated output power) or higher.

Whe the amplifier is shut off, the amplifier should mute both channels virtually instantaneously, with the “PROTECT" indicators lit until the power supplies are discharged.

TROUBLESHOOTING Problem: Channel will not come out of muting

+ If reducing the gain control to x attenuation does not release muting, the channel is defective or overheated (see “Overheating," below).

*__ lfreducingthe gain releases the muting, raise the gain back up slowly while you watch the "SIGNAL" and “CLIP” indicators; the problem may be an abnormal signal (with excessive ultrasonic energy, for example) that could otherwise damage your speakers.

PROBLEM: No sound (CX6/CX12) + Isthe channel muted? (If the “PROTECT" indicator is lit, the channel is muted: see below.)

+ Isthe "SIGNAL" LED lit orflashing? (If so, the speaker is open or blown, there is an open circuit in the speaker wiring, or there is an open circuit in the internal output wiring of the amplifier.)

+ lfthe "SIGNAL" indicator is dark, there is probably not enough signal, or even none at all. Try turning up the rear panel attenuators or boosting the signal level at the input.

+ Ifthe"SIGNAL"indicator shows little orno activity butthe “CLIP” LED is litorflashing, there is probably à short circuit in the speaker wiring, especially if the “PROTECT” indicator starts flashing. It is also possible, but less likely, that the channel's output relay is defective and will not open, thereby short- circuiting the channel output and producing the same symptoms.

PROBLEM: Channel goes into muting, with “PROTECT” LED on (CX6/CX12)

+ lfthe fan is running full speed, the channel probably suffers from severe overheating. Unless there is a blockage in the flow of cooling air, the channel should return to normal within à minute or so (see “Overheating," below).

*__lfthefanis notrunningat full speed, orthe channel does not reset to normal after a cool-down period, the muting is probably because of a DC fault or other amplifier failure, especially if the "SIGNAL" or “CLIP” indicators are lit even with the attenuator turned all the way down.

PROBLEM: Overheating

+ The thermal management system on a CX Series amplifier features à two-speed fan that modulates the cooling air flow over the heatsinks in response to the cooling needs. nr If the air flow is blocked, however, or if the amplifier is overdriven into very low LA impedance loads, the amplifier could overheat even though the fan is running at full speed.

+ At85%C both channels will mute. The “POWER” LED will remain lit, althoughthe “CLIP” indicator will be dark. The amplifier will remain this way until the temperature drops to a safe level.

CX6, CXGT, CX12 & CXI2T

*__At85%C, the channel's output relay will mute the output. The channel will remain muted until the temperature drops to a safe level. Even whilethe channel is muted, the "SIGNAL" and "CLiP”indicators will function normally. If the “CLIP” indicator is flashing or continuously lit, reduce the gain to hasten the cool-down and prevent repeated thermal shutdowns.

PROBLEM: Hum in the audio

+ Becauseofits grounded-collector outputtransistor configuration, which maximizes thermal efficiency, the signal ground on à CX amplifier cannot be lifted. The amplifier's balanced inputs are meant to reject hum, but if hum remains à problem, checkthe tightness of the two mounting screws on the input panel (except CX4 and CXAT); if they are loose, the panel itself might not be well grounded to the chassis. If you have any problems with these screws, contact the QSC Technical Services Department. In some cases, such as whenthe audio signal cables are routed near lighting dimmers that use triacs, you may need to use input isolation transformers because of the extremely high noise field produced by the dimmer circuitry. See the paragraphs on Open Input Architecture for further information on input transformers.

OPEN INPUT ARCHITECTURE” LEVEL I (CX6, CX6T, CX12 and CX12T only)

Onthe rear of a CX amplifier are an upper and a lower input panel; both are easilyremoveable for future upgrades. This is the concept of Open Input Architecture, and the opening in which the panels mount is called the “slot.” As installed at the factory, the lower is the CX standard input panel and the upper one is à blank. À ribbon cable connects the “slot” panels to the rest of the amplifier. It carries input signals, speaker output monitor signals, muting status, and clip activity; these are for supporting future accessories and remote control systems. Although the physical dimensions of the CX, M Xa, and EX panels are the same, their internal cabling is different and the CX platform supports most (but not all) of the M Xa functions but doesn't support all of the EX functions. The CX and M Xa Open Input Architecture slot thus is designated as “Level l” and the EX as “Level 1."

Tightening input panel mounting screws

The standard input panel installed on the CX Series amplifier has balanced Euro-style detachable header inputs, gain controls, and the Parallel-Stereo-Bridge switch. The panel's circuit board has solder footprints for passive roll-off circuit components, input isolation transformers, and other special customizations.

The M X-1 Input Expander panel may be installed in the upper panel position. It features XLR inputs and 1/4" RTS inputs. Installation instructions are included with the expansion kit.

Other available accessories include a universal filter and a stereo power limiter: more are in development. Contact QSC for more details before using accessories with CX amplifiers. See the CX Application Notes also.

PARALLEL, STEREO, OR BRIDGED OPERATION (CX6, CX6T, CX12 and CX12T only)

= | The Parallel-Stereo-Bridge switch is located on the input panel. Always turn off the amplifier before changing the position of this switch.

— | The most commonly used operating mode for à power amplifier is “Stereo,” in which the two channels are independent and separate all the way from input to output. Set the switch in the center position for stereo operation.

— | “Parallel” ties the two channel inputs together so that both will be driven by the same signal, without } the need for external jumpers or wiring. After the inputs both channels operate independentiy: though they carry the same signal, their gain controls affect only their respective channels, and they must use separate speakers. Never parallel the speaker outputs!

The “Bridge” position uses both channels to provide about three times the power to a single speaker — 4 load that a single channel does. In this position, the switch feeds channel 2 with an inverted signal from | channel 1. Thus, when one channel “pushes,” the other “pulls,” providing twice the voltage swing of

I N asingle channel.

BRIDGED-MONO MODE CAUTION:

ALL MODELS-— Output voltages as high as 160 volts rms are available be- tween the CX amplifier's bridged terminals (200 volts at isolated outputs).

CXAT, CX6T and CX12T (140V and 200V opera- tion) Fully insulated CLASS ONE wiring must be used to connect the amplifier to the loud- speakers

Instructions for bridged operation

1. To engage the bridged mono mode, jumper the barrier strip inputs on the rear, in accordance with the diagram printed under the barrier strip inputs. Connect the amplifier input to channel 1 only, with the jumpers as shown to channel 2. Channel 1 and channel 2 gains must be matched. A 4, 8, or 16 ohm speaker load can be connected across the two red speaker terminals, using the red binding post of channel 1 for “+" or “hot.”

CX6, CX6T, CX12 and CX12T 1 Setthe switch to the “Bridge” position.

2. Connect the signal to channel L's input only. Do not connect an input signal to channel 2. However,

as à convenience for routing the same signal to multiple amplifiers without using Y connectors, the PARALLEL BRIDGE

mode switch connects the channel 1 and channel 2 inputs in parallel. Therefore, you can use channel 2's input terminals to pass the audio signal on to other amplifiers.

3. Useonlychannel l's gain control to setthe level. Both channels’ "SIGNAL" and "CLIP" indicators should flash identically when the amplifier is operating.

4. Connect the single speaker load across the +amplifier outputs as shown in the Outputs chapter. The positive speaker wire goes to channel 1, and the negative to channel 2.

MAXIMUM LONG-TERM OUTPUT POWER In a properly designed sound system, you can obtain the desired sound level without using the full output power of the amplifier; if the “CLIP” indicators flash once in a while, that is perfectly normal.

Ifthe amplifier is operated at extremely high power levels, it may overheat or may even damage the speakers. The amount of stress onthe amplifier depends largely on the load and how hardthe amplifier drives it. In general, the lower the load impedance, the more stressful the load is.

8-Ohm Loads and Lightly Loaded 25V, 7OV, or 100V Lines

The amplifier can operate at practically any power level without risk of overheating. However, ifitis pushed hard enough to continually light the “CLIP” indicator, the amplifier's average output power can reach 110 to 475 watts (depending on the model), which is more than many speakers can handle.

4-Ohm Loads and Heavily Loaded 25V, 7OV, or 100V Lines

Ifthe “CLIP” indicator flashes occasionally, the amplifier is approaching its maximum long-term power capacity. fitis lit about half the time, the amplifier channel will probably go into thermal protection within a few minutes.

Except for an occasional flash, keep the “CLIP” indicator dark to avoid overheating the amplifier channel.

AC CURRENT CONSUMPTION

À major objective in the design of the CX Series amplifiers-even the higher-powered models-is to permit their operation from readily available, standard AC power sources.

“Normal conditions” in power amplifier rating means operating with a random program source (pink noise), using pink noise as à source, at an average power level equal to one-eighth of maximum power. This is recognized by most of the world's safety agencies as the loudest you can play music through an amplifier and still keep the incidence of clipping to a reasonable minimum. An amplifier's peak current draw at full output power into 2 ohms is several times what the “normal” draw is, but its various protection circuits will prevent this condition from lasting more than a minute or two.

When you plan the AC power hookups for your amplifiers, use the table in the Specifications section to predict the current requirements per amplifier. You can use the one-eighth power figures to predict the normal continuous current draw, then add a safety margin to allow for occasional crescendos.

HEAT EMISSIONS Essentially, a power amplifier draws electrical energy from the AC mains, converts it to DC, and then converts itagain into an analog of the input signal to send out to the loudspeakers. Any AC power that enters the amplifier through the power cord and does not exit through the speaker outputs turns into heat, which the amplifier must rid itself of by exhausting it to the outside. In indoor use this may present a sizeable challenge to à building's air conditioning system. Use the table below to predict the heat that will be emitted by your amplifier.

Full Power 1/3 Power 1/8Power Idle Model Channel Load BTU/hr kcal/hr BTU/hr kcal/hr BTU/hr kcal/hr BTU/hr kcal/hr CX4,CX4T 80 +8Q 645 163 615 155 510 128 85 22 4Q +4Q 1265 318 1060 267 830 210 85 22 2Q +2Q 2340 590 1700 429 1295 326 85 22 7OV +70V 1365 344 1050 264 800 202 85 22 CX6,CX6T 8Q +8Q 1406 354 1020 257 604 152 229 58 40 +4Q 2525 637 1617 408 921 232 229 58 2Q +2Q 4658 1174 2532 638 1290 325 229 58 TOV +70V 2010 507 1503 379 1141 288 229 58 CX12, CX12T 8Q +8Q 1791 452 1239 312 805 203 297 75 40 +4Q 3330 840 2081 525 1222 308 297 75 2Q +2Q 5698 1437 3146 193 1611 406 297 75 TOV +70V 3772 951 2338 589 1280 323 297 75

Heat emissions of CX amplifiers under various conditions

PROTECTION CIRCUITS The design goal in high-efficiency amplifiers such as the CX Series is to increase the amplifier's ability to deliver peak power without increasing its size, weight, and cost. However, the higher power flow through the output components makes effective, responsive protection circuitry absolutely vital. To this end, the design of the CX series takes a comprehensive approach to protection.

As in all QSC amplifiers, the inputs are resistively buffered for overload and RF protection. Chassis bypass capacitors at inputs and outputs further improve RF rejection.

CX amplifiers use the proven Ouput Averaging” short circuit protection system. This circuit permits full output current even into resistive or reactive 2-ohm loads, but reduces the current safely by about 75% if the output is shorted.

Turn-on/turn-off muting keeps transients-both from the amplifier itself and from upstream equipment-from reaching the speakers when the amplifier is turned on or off. The turn-on delay is approximately three seconds to allow the power supplies and circuitry to stabilize. Turn-off muting occurs almost immediately after power is shut off. M uting occurs whether power is turned on and off using the front panel power switch or externally atthe AC source.

Inside a CX6, CX6T, CX12, or CX12T amplifier, an NTC (negative temperature coefficient) thermistor in series withthe power switch andtransformer primary limits inrush current. Thethermistorinitially has ahighresistance, which then diminishes rapidly as it warms, to avoid power loss. Typically, the inrush current of an CX Series amplifier thus is equal to that of another amplifier of about 1/3 to 1/2 its power rating. The low inrush of the smaller CX4 and CX4T does not require an NTC thermistor.

When the DC fault protection circuitry of a CX6, CX6T, CX12, or CX12T senses à DC voltage on a channel output, it actuates à relay which shorts the output and load to ground. The amplifier's Output Averaging protection circuitry will limit current to further protect the output devices. If the DC problem persists, that channel will go into thermal protection mode and should be serviced by à qualified technician.

The output stage of a CX4 or CXAT channel has AC coupling to prevent it from passing DC.

Temperature sensors on the channel heatsinks and the power transformers are part of the thermal management circuitry. The temperature sensors govern whether the dual-speed fan should run at low or high speed. Below 552C, the fan runs at low speed; above, it runs at high speed. At or above 8522 C, the channel mutes to go into thermal protection.

Built-in second-order 50 Hz (CX4T) and third-order 45 Hz (CX6T and CX12T) Butterworth high-pass filtering helps prevent saturation in speaker transformers on distributed lines by sharply reducing the amount of ultra-low frequency energy the amplifier puts out. However, some lower-quality speaker transformers have rated low- frequency responses that do not even extend as low as these filters; if you have any of these in your distributed line system you should insert corresponding high-pass filtering in the audio path before the amplifier.

Isolated outputs, watts per channel,

both channels driven TOV or 100V, 45 Hz-16 kHz, 0.25% THD** 175 250 500 25V, 45 Hz-16 kHz, 0.25% THD** 150 200 400

Direct Outputs, bridged mono

DISTORTION SMPTE-IM , less than 0.05%

FREQUENCY RESPONSE** CX4, CX6, CX12: 20 Hz-20 kHz, +0.2 dB CXAT direct outputs: -3 dB @ 50 Hz, -0.2 dB @ 20 kHz CX4T isolated outputs: 50 Hz-16 kHz, +, -3 dB CX6T, CXI2T direct outputs: -3 dB @ 45 Hz, -0.2 dB @ 20 kHz CX6T, CX12T isolated outputs: 45 Hz-16 kHz, +, -3 dB

##Low frequency response on “T” versions is limited by 18 dB/octave 45-Hz high-pass filter. Low frequency response on CXAT versions is limited by 12 dB/octave 50-Hz high-pass filter.

DAMPING FACTOR 200 (@ direct outputs)

INPUT IMPEDANCE 10K unbalanced, 20K balanced

FRONT PANEL/INDICATORS (per channel)

PROTECT: Red LED (except CX4, CXAT)

CLIP: Red LED SIGNAL: Yellow LED (except CX4, CXAT)

POWER: Green LED REAR PANEL/CONNECTORS (each channel)

Input: Euro-style detachable header Output: Covered barrierstrips COOLING

2-speed fan, with back-to-front air flow

AMPLIFIER PROTECTION Output Averaging" short circuit protection, open circuit, ultrasonic, RF, thermal muting. Stable into reactive or mismatched loads.

LOAD PROTECTION Turn-on/turn-off muting, DC-fault load grounding relay with internal fault fuses

OUTPUT CIRCUIT TYPE CX4, CXAT, CX6, CX6T: Class AB complementary linear stage CX12, CX12T: Class H complementary linear stage, with 2-step high-efficiency circuit

POWER REQUIREMENTS: 100, 120, 220-240 VAC, 50/60 Hz

Normal operation: 1/8 power @ 4Q per channel Worst case continuous program: 1/3 power @ 2Q per channel Maximum: full power @ 2Q per channel Multiply current times 0.5 for 220-240 VAC operation

10.9 A Faceplate Width Standard 19" (48.3 cm) Rack M ounting Chassis Depth 17.9" (45.5 cm) deep (to rear support ears)

Part2— CX Series Application Guide

ith the helpful advice and input from contractors and consultants around the world, engineers at QSC designed the CX Series amplifiers to be a versatile and reliable foundation for high-quality installed sound systems.

This applications guide will help you design your sound system properly and utilize your CX amplifier(s)effectively.

VIILDistributedlines

The best way to distribute audio power from à common amplifier among numerous relatively low-powered loudspeakers is to use a distributed line, whether allthe loudspeakers are meant to receive the same or different

À distributed line is usually referred to by its maximum voltage- 25-,70-, 100-, or 140-volt lines are most common. An amplifier's |

12W : 2.5W 16 power rating is determined by how much current it can put out : 5w 8Q 4,5V at a standard voltage level; a low-power 70-volt amp has less 10W sal |

current capabilitythan a high-powered 70-volt amp, even though they can both put out the same voltage. The high-pow ered amp can drive more speakers and at higher power levels than the lower-powered one can. This is somewhat analogous to AC electrical distribution, where you might have circuits rated for 15A,20A, 30A, etc., at 120 or 240 VAC, and you can connect any number of appliances to them as long as you don't exceed the circuit's current capacity.

| CcoM Example of à loadspeaker connected to a distributed line through à transformer

12W Loudspeakers on à distributed line use tapped

2.5W 16a transformers to match their power requirements sw se to the line. A loudspeaker with a 70-volt trans- 70V 10W 4a com A 0 COM 0.5W

former tapped at 2 watts, for example, will

receive that amount of power when the amplifier

puts out 70 volts on the distributed line. Trans-

former taps are usually set at 3 dB intervals, ie.

each successive tapis twicethe powerlevel ofits

predecessor. An example would be a transformer

1 = tapped at 0.5, 1, 2, and 4 watts. This allows you

a to select a power level that is most appropriate

SPKR «| for the situation while still retaining the versatil-

com ity of configuring other loudspeakers onthe same line for different power levels.

IX. Designingthe Distributed SoundS ystem

There are several main steps in designing a distributed sound system: + Determining loudspeaker coverage and placement + Determining power levels for each loudspeaker + Choosing the right amplifier

LOUDSPEAKER COVERAGE AND PLACEMENT The goal in placing the loudspeakers in a distributed system is to acoustically cover the required area effectively but economically. An effective coverage would be one where the sound from the system's loudspeakers is not only audible, but intelligible, over all of the area. An economical coverage would be one that achieves the goal with the fewest loudspeakers necessary.

A loudspeakerina partiallyorfullyenclosed area produces two soundfields:the directfield, whichis soundcoming directly “line-of-sight” from the loudspeaker; and the diffuse field, which is sound from the loudspeaker which hasreflected off of surfaces inthe area, suchasfloors, walls, tables, ceilings, etc. The direct sound field decreases with distance from the loudspeaker; every time you double the distance, the sound pressure level drops 6 dB. This is called the Inverse Square Law because the result of à 6 dB sound pressure level decrease is the same as 1/4 ofthe sound intensity (i.e., the sound power per square centimeter), which varies inversely with the square of the distance.

Eventhoughittoo is subjectto the Inverse Square Law, the intensity ofthe diffuse field is more difficultto predict, especially with multiple acoustically reflective surfaces. The problem is that moving away from one surface may also be moving closer Lwattin

=92 dB, to another. Controlling the diffuse field often involves Sens 92 08, uatgIm

architectural planning as well as acoustical treatment with absorptive surface materials. 1 meter

To maintain intelligibility, maximize the

ratio of direct field to diffuse field; 92 dB SPL this generally involves placing

close to the listeners

as is both physically

and economically prac-

86 dB SPL Sometimes, a distributed line 825 dB SPL

system can help à coverage

area withdifficultacoustical char- acteristics. For example, à very re- verberant room with all large hard,

reflective surfaces— wood floors and ceiling, concrete and plaster walls, etc., the type of room that tends to make speech unintelligible— might benefit from using many low-pow ered loudspeakers placed throughout the room and close to the listeners, instead of using a couple high-powered, centrally-located loudspeakers.

With ceiling-mounted loudspeakers, one general rule of thumb is to make the center-to-center distance between them no greater than twice the floor-to-ceiling distance. Used with loudspeakers that have a 90° angle of coverage, this is often suitable for background music systems, but for paging and public address applications it leaves some erratic coverage at listening positions higher than floor level.

MN A better technique is to space the ceiling speakers at twice the distance from the listeners’ ears to the ceiling. This requires more loudspeakers spaced closer together than the previous scheme does, but provides good, uniform coverage at realistic listening positions. In a room where people are standing, you'll need closer spacing than if people tend to be sitting. For example, if a lunchroom has a ceiling height of 2.9 meters (9.5 feet), and the height of an average listener's ear, when seated, is about 1.1 meter (3.5 feet) above the floor (and therefore about 1.8 meter or 6 feet from the ceiling), then you should space the loudspeakers no more than 3.6 meters, or about 12 feet, apart.

Even better, more uniform coverage will result from spacing the loudspeakers at 1.5 times the ceiling-to-ear distance. In the above example, this would require spacing the loudspeakers about 2.7 meters (9 feet) apart.

2 X height above ears 2 X height above ours hoight oïgnt| above above ears when ars when

DETERMINING POWER LEVELS After you've determined where to place the loudspeakers, you need to calculate the power each one requires.

Ifthe installation is in an existing facility already in use, use an SPL meter, set for slow response, to measure

the A weighted ambient noise atthe listener's ear position. Try

to take this measurement at the noisiest time— in à factory, 20.0 when the machines are running; in a restaurant, when it's full 18.01 dB(reflwatt) of patrons, etc. Background music will require an SPL at least 16.0 10 dB above the ambient noise. For good paging intelligibility, 14.0: you'll need an SPL approximately 15 dB higher than the 12.0 ambient noise; 25 dB above ambient will yield excellent 10.0 intelligibility. 8.0 Next, use the distance from the loudspeaker to the listener's 6.0 ears, along with the loudspeaker's sensitivity rating (typically 4.0 expressed as “n dB @ 1 watt, 1 meter,” which means n dB of 2.0 Power(Watts) SPL with 1 watt input, measured at a distance of 1 meter) to oI 0.2 0.5 0.0 h 2 5 T0 20 50 Tdo determine how much powerthe loudspeakerneeds to get from 2-0 the distributed line. Use the inverse square law guide below 4.0 to convert distance to dB; you'll need to add this figure to the 6.0 Decibels desired SPL and then subtract the sensitivity rating to deter- 8.0 referenced to mine how much more or less than 1 watt the loudspeaker 10.0 Lwatt requires. Distance from speaker 2 3 4 5 6 7 8 10 15 20 30 40 50 60 feet | EE En | bed TIM) INIIINTN l Cr] | Li LL LIT) |

-5 0 5 10 15 20 25 dB Power increase in dB; referenced to loudspeaker sensitivity rating (1 watt @ 1 meter)

(ref: 1 meter, or 3.28 feet)

EXAMPLE A loudspeaker (sensitivity: 94 dB @ 1W, 1 meter)in a busy office covers an area with an ambient noise of 72 dBA, measured at a seated person's ear position at the desks. The client wants superb intelligibility, so your goal is to provide an SPLof 97 dB (72 plus 25)tothe intended listeners. The ceiling- mounted speakeris about 6 feet, or 1.8 meters, above the listeners’ ears. The dB figure fromthe inverse square law conversion guide for that distance is 5.2 dB; this means that the loudspeaker would have to produce 102.2 dB (97 plus 5.2)at a distance of 1 meter to achieve 97 dB at a distance of 1.8 meters.

The loudspeaker will require a power level 8.2 dB (102.2 minus 94) greater than 1 watt; this works out to 6.6 watts. The speaker transformer taps are 0.5, 1, 2, 4, and 8 watts; the closest choice would be the 8-watt tap, which turns out to be only 0.8 dB higher than the design ideal.

SELECTING THE AM PLIFIER After you've determined the power taps for each loudspeaker transformer, add them up. The sum will be what you determine your amplifier requirements from. If you have 16 loudspeakers tapped at 2 watts, seven at 1 watt, and eight at 10 watts, the total audio power the loudspeakers want is 119 watts.

Thanks to a transformer phenomenon called insertion loss, though, your amplifier needs to provide more power thanthe loudspeakers will draw. High-quality speaker transformers typically have an insertion loss of about 1 dB, meaning that it takes about 1.25 watt going into the transformer to put 1 watt into the loudspeaker. À lower- quality transformer may have à loss of 2 dB, which requires approximately 1.6 watts for every watt that the loudspeaker receives. Poor-quality transformers may have even higher losses, but they will probably degrade the system's audio performance severely even if there is adequate amplifier power to overcome the losses.

To compensate for the insertion loss, add a corresponding percentage to the sum of the transformer power taps. For transformers with a 1 dB loss, add about 25%; in the example above, that would increase 119 watts up to 149 watts.

Next, itis a good engineering practice to add another 25% to the figure. This allows a margin both for dynamic audio headroom and for some future adjustments to the system— an added speaker ortwo, a few transformer tap changes, etc. That would bring the total power requirement for the example system up to 186 watts. This would be the minimum power rating for the amplifier you choose.

Once the sound system is installed and operational, turn the amplifier off and take an impedance measurement across the distributed line at the amplifier output, using an audio impedance meter {not an ohmmeter), Record the measurement for later use. If you ever have to make à service call on the system, measure the impedance again and see how it compares to the recorded figure; its a quick and easy way to see if anything in the distributed line system has been changed. Likewise, measure the impedance anytime you've changed à transformer tap, added or removed à loud- speaker, or made any other adjustment to anything on the distributed line.

USING COMPONENTS WITH DIFFERENT VOLTAGES Sometimes it may be practical to use a transformer or loudspeaker/transformer combination with a different voltage system from what it was originally intended. For example, à 70-volt transformer could be used in a 25- volt system, although you would have to derate the power taps similarly. Never use à transformer with a higher voltage than what it is designed for, though: i.e., you couldn't use that same transformer on a 100- or 140-volt line.

To properly derate the transformer tap power levels, locate the column with the voltage the transformer is rated for in the table below, then locate the row with the system voltage you want to use it in. The table will indicate the dB reduction to use with the derating chart below it.

Voltage Rating of Transformer

-6dB 9dB -12dB -15dB Voltage of -3 dB -6 dB -9 dB System -3dB -6dB -8 dB

01 1 10 100 Original Value of Power Tap, in watts EXAMPLE

2.5, and 5 watts. What power points will

You have a 70-volt transformer with taps labeled 0.6, 1.2 those taps provide on a 50-volt line?

The table shows that 50 volts is 3 dB less than 70 volts. The chart confirms that the taps will provide 0.3, 0.6, 1.25, and 2.5 watts, respectively.

-18 dB -12 dB -9 dB -6 dB

-3 dB X.How M any Speakers?

How many loudspeakers can you connect to the distributed line? The amplifier itself determines the amount of audio power available to the line, and from that figure you must compensate for insertion loss in the speaker

—— 1.56 watts . +25% for headroomi approx. 1/4 watt | 1.25 watis more to compensate for transformer insertion loss* 1 watt

For every watt put into the speakers, you need...

*assuming a typical insertion loss of 1dB

+a good engineering practice

CX/T Recommended Maximum Power

transformers and for headroom when you determine how much poweris available for the speakers.

For example, let's suppose you have an amplifier rated at 500 watts into à 70-volt line, and your speaker transformers each have an insertion loss of 1 dB, which is typical. The losses in the transformers mean that they will draw 25% more audio power than they deliver to the speakers. For headroom, as mentioned before, you add at least another 25% to the total amount of power drawn from the amplifier. The additional 25% headroom is optional but highly recommended. It will allow for future additions to the system and reduce the stress on the power amplifier.

So you'llneed at least about 56% more powerthanthe total sum of the transformer taps. The reciprocal of 1.5625 is .64, so conversely you can figure on allowing a maximum of 64% of the amplifier's rated power for the loudspeakers. Therefore, the highest combination of transformer taps the 500-watt amplifier can drive satisfactorilyis asumof about 320 watts, e.g., 32 speakers eachtappedat 10 watts, or 64 speakers tapped at 5 watts, etc.

This table lists the recommended maximum load (sum of all speaker transformer power taps on one line) for the CX6T and CX12T amplifiers. These figures assume à typical transformer insertion loss of about 1 dB.

You can, of course, use different output configurations on the two amp channels ifit's not operating in bridged mono mode. For example, one channel of a CX12T could drive a

by distributed line application : : 1000 Y PP 1000 70-volt line loaded at 320 watts while the other channel CXAaT CX6T CX12T drives a 25-volt line, or even a direct loudspeaker. 800 El , 600 £ 500 [nn | 500 È 400 . 400 400 || Il 280 | El | 200 EE | Hu | !| 0 90 180 |[112 [11224 [[1128 | [256 | [160 | 320 |f]256 |11512 [11320 640 SR SES ES Sas PSS PS ES ES Os ES Os ES Si \ à N° *Bridged mono

M fac pour of ample I] Avance for approx, 1 dB transformer insertion Less

I Recommended maximum sum of pouer aps, including 25% allowance for headroom and austment

XL. OtherDesign Considerations

SPEAKER TRANSFORMER SATURATION Speaker transformers tend to be fairly small, and some vary widely in quality. They tend, therefore, to be prone to core saturation at low frequencies. Saturation occurs when the magnetic field induced in the transformer's iron core by the audio signal waveform has built up as high as the core can handle. Even if the signal voltage increases, the magnetic flux cannot increase any further, so it “clips,” in a way. High frequencies are less likely to cause saturation because the signal waveforms, rapidly alternating in polarity, reverse the direction of the magnetic flux before saturation can occur.

Saturation causes audible distortion. Inextreme cases it can damage the pow eramplifier drivingthe line, because as the transformers go out of saturation, their magnetic fields collapse and induce à voltage spike across the line. That voltage spike travels along the line back to the amplifier, which has to absorb it. Interestingly, this phenomenon is much more likely to happen on a lightly loaded line than on à heavily loaded one.

Aneffective wayto eliminate the saturation problemis to filter out the frequencies most likelyto causeit, without adversely affecting the audio quality. The CX6T and CX12T both have built-in 45 Hz high-pass filters with an 18 dB per octave bass roll-off, while the CX4T has a 50 Hz high-pass filter with a 12 dB per octave roll-off. This is adequate prevention for virtually all good-quality speaker transformers. However, if any speaker transformers onthe line do not have low-frequency responses that extend down to 45 Hz, you must insert corresponding high- pass filtering in the audio signal path before the amplifier.

CONNECTING BOTH A DISTRIBUTED LINE AND A DIRECT LOUDSPEAKER ON ONE CHANNEL

re te cl TheT versions of the CX amplifiers have simultaneous direct outputs (for loudspeak- Fe... ers) and 25-, 70-, and 100-volt isolated outputs (for distributed lines). This adds à sa Loue » Le . , rang: 2 200 (EXED measure of versatility that few other amplifiers can match: being able to drive both EC Î Éctri j i i CONNECT à loudspeaker and a distributed line on a single channel at the same time. Count DISTRIBUTED LINE [up to 40 watts (CXAT), Setting up such an arrangement is simple, but it requires some precautions: ToLow AMINAS 60 watts (CXET), or 125 watts (CX12T)] “Use an 8-ohm speaker for the direct connection. Make sure it has an TO APPROPRIATE TERMINALS

adequate power rating (150 watts for use with the CX4T, 200 watts for use with the CX6T; 400 watts for the CX12T). Connect it across the low impedance outputs.

+ Thedistributed line must not be loaded any higher (sum of transformertaps plus allowance for insertion loss plus allowance for headroom) than 40 F Elec Lounsrenen

watts for the CXAT, 60 watts for the CXGT, or 125 watts for the CX12T. Î “ ÿ FES Connect it across the appropriate transformer terminals. CONNECT CONNECT DISTRIBUTED LINE LOUDSPEAKER The directly connected speaker will constitute most of the load on the amplifier 10 nat (NET) ve ACROSS 2 channel, so it will receive considerably more power than the distributed line. If you 400 watts (CX127)]

adjust the amplifier gain controls to vary the sound level from the directly connected TERMINALS AWG sizes © 5AWG

© 10 AWG © 12AWG © 14AWG © 16AWG 9 18 AWG © 20 AWG

© 2 AWG Metric sizes ® 6 mm° © 4 mm © 2.5 mm° © 15mm ® 1 mm © 0.8 mm © 0.6 mm 0.4 mm°

speaker, it will affect the sound level from the distributed line's speakers also, possibly altering them from your intended levels.

Analternate method is to attach an 8-ohm speaker across the 25-volt outputs. This provides up to approximately 80 watts to the speaker and makes more power available to the 70V or 100V connections: up to 75 watts for the CXAT, up to 150 watts for the CX6T; up to 400 watts for the CX12T.

WIRE LOSS Even the highest-quality copper wire has a small amount of resistance to electrical current flow, and the wire's resistance is inversely proportional to the cross-sectional area of its conductor. Therefore, to minimize the power lost to speaker cable resistance, you should use the largest stranded (always stranded) copper wire that is practical for the job. This is especially important with direct low-impedance speaker connections; e.g., a half- ohm wire resistance would not affect a lightly-loaded 100-volt line noticeably, but it would reduce the amount of power going to a 2-ohm load by 36%, a 1.9 dB drop. It would also reduce the damping factor to no better than 4.

If an amplifier could drive à speaker load through theoretical zero-resistance wire, no power would be lost in the speaker cables. In the charts below we’ll compare the power delivered through real-world speaker cables with the theoretical zero-resistance ideal and express it as a ratio called the power transfer coefficient. It is determined by the formula

POWER TRANSFER COEFFICIENT =[R,,,,/(R Loan! uvre

Let's say you have an 8-ohm speaker load. With that imaginary zero-resistance wire, all the power would be delivered to the load, so the power transfer coefficient would be 1. If you then substituted wire with 0.2 ohm of resistance, the load would only get 95.2% of the power it got with the zero-ohm wire, so the power transfer coefficient would be 0.952 (a loss of 0.2 dB, by the way).

Power Transfer into 8€ Load

Speaker Cable Length (m) 30

Power Transfer into 4Q Load

100 Speaker Cable Length (ft)

Power Transfer into 20 Load

Speaker Cable Length (m) 30

100 Speaker Cable Length (ft)

© 1oawG @ 12AWG © 14AWG © 16 AWG O 18AWG © 20 AWG

© 22 AWG Metric sizes © 6mm @ 4 mm° ® 25m © 1.5 mm° © 1 mm © 0.8 mm ® 0.6 mm ® 04 mm

CX4, CX4T CX6, CX6T CX12, CX12T CX12, CX12T AC CURRENT CONSUMPTION

À major objective in the design of the CX Series amplifiers— even the higher-powered models— is to permit their operation from readily available, standard AC power sources.

“Normal conditions” in power amplifier rating means operating with a random program source (pink noise), using pink noise as à source, at an average power level equal to one-eighth of maximum power. This is recognized by most of the world's safety agencies as the loudest you can play music through an amplifier and still keep the incidence of clipping to a reasonable minimum. An amplifier's peak current draw at full output power into 2 ohms is several times what the “normal” draw is, but its various protection circuits will prevent this condition from lasting more than a minute or two.

When you plan the AC power hookups for your amplifiers, use the following table to predict the current requirements per ampli- AC Current Consumption fier, You can use the one-

4Q or Distributed Line eighth power figures to

FULL 13 18 FULL 13 18 FULL 13 1/8 il inu- POWER POWER POWER | POWER POWER POWER | POWER POWER POWER predictthe normal continu

ous current draw, then add à safety margin to allow foroccasional crescendos.

15A 8.6A 5.1A 24A 12A 6.5A 33A 17A 8.5A HEAT EMISSIONS Essentially, a power amplifier draws electrical energy from the AC mains, converts it to DC, and then converts itagain into an analog of the input signal to send out to the loudspeakers. Any AC power that enters the amplifier through the power cord and does not exit through the speaker outputs turns into heat, which the amplifier must rid itself of by exhausting it to the outside. In indoor use, multiple amplifiers may present a sizeable challenge to à building's air conditioning system. Use the table below to predict the heat that will be emitted by your amplifier. Use one-eighth power for normal, continuous usage.

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