C O N T R A C T O R A M P L I F I E R S Application Guide 29
Table of Contents Distributed line principles ................................................................................................... 3 Sidebar: Ohm’s Law ....................................................................................................................................... 4 Why 70 volts? .................................................................................................................................................
©Copyright 1996, 1999 QSC Audio Products, Inc. All rights reserved. QSControl is a trademark of QSC Audio Products, Inc. “QSC” and the QSC logo are registered with the U.S. Patent and Trademark Office.
CX Series Application Guide With 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. It starts with a tutorial on distributed (constant voltage) speaker systems.
as much as 28.3 volts, as determined by Ohm’s Law, while an amp that does 200 watts into 8 ohms can put out 40.0 volts. Ohm’s Law Nearly two centuries ago a German scientist named Georg Ohm This is where the true concept of “constant voltage” comes in; it helps simplify system design by converting one of the variables into a constant value. But you can’t just connect typical 8-ohm speakers across a 70volt line because they’ll want to draw about 625 watts each.
Transforming voltages and impedances Imagine driving a system of 100 8-ohm speakers at a low power (say, 8 watts each) with a single amplifier, like you might need to do in an office building’s paging system. How would you do it? Connect them all in parallel, perhaps with 00 AWG cable to wire them all together, and find a power amp that can do 800 watts into 0.
Designing the distributed sound system 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 In placing loudspeakers in a distributed system, the goal is to provide coverage effectively but economically.
Reverberation and RT60 to be understood: this is the “what did he/she say?” syndrome. In this example, DC is approxi- A common and useful measurement of a room’s reverberance is its reverberation mately 15 meters. time, or RT60. It is defined as the time it takes a sound in a space to decay 60 dB (or one millionth of the acoustic power). The more reverberant the room, the longer the Not all reverberation is detrimental. A controlled RT60.
One of the most common uses of distributed lines is to power ceiling speakers in office, retail, and commercial buildings. With ceilingmounted loudspeakers, one common rule of thumb is to make the center-tocenter distance between them no greater than twice the floor-to-ceiling distance.
Better, more uniform coverage will result from spacing the loudspeakers at 1.5 times the ceiling-to-ear distance. In the lunchroom example, this would require spacing the loudspeakers about 2.7 meters (9 ft) apart. Some manufacturers now offer ceiling loudspeakers with dispersion angles much wider than 90 degrees. These allow greater spacing between speakers, and consequently it takes fewer of them to cover the same area, although each one will require more power.
from the distributed line. Use the formula dB = 20×logD or the Inverse Square guide below to convert distance-related attenuation to dB; you’ll need to add this figure to the desired SPL and then subtract the sensitivity rating to determine how much more or less than 1 watt the loudspeaker requires.
97.2 dB 1.8 me ters 2.1 watts (3.2 dBW) in (sensitivity = 94 dB @ 1W, 1 m) EXIT 92 dB Ambient noise = 67 dBA EXAMPLE A loudspeaker (sensitivity: 94 dB @ 1W, 1 meter) in a busy office covers an area with an ambient noise level of 67 dBA, measured at a seated person’s ear position at the desks. The client wants superb intelligibility, so your goal is to provide an SPL of 92 dB (25 dB above ambient SPL) to the intended listeners, the office workers. The ceiling-mounted speaker is about 6 feet, or 1.
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. To compensate for lesser-quality trtansformers with insertion losses of 1.5 dB and 2 dB, add 40% and 58%, respectively, to their individual power tap figures.
Using components with different line 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, a 70-volt transformer could be used in a 25-volt system, although you would have to derate the power taps similarly. But never use a transformer with a higher voltage than what it is designed for; i.e., you couldn’t use that same transformer on a 100- or 140-volt line.
Sample applications & output configurations 70V subwoofers 75W 20W 20W 75W 20W 20W 75W 20W 20W 75W 20W 20W 20W 20W 20W 20W 70V full-range speakers subwoofer x-over Load Charge = 300W Last Carga Ch. 1 CX 602V Ch. 2 Load Charge = 240W Last Carga A CD megastore 70V system, with subwoofers Ch. 1 > 2Ω > 8Ω > 4Ω Ch.
8Ω 15W Center cluster 15W 20W 20W 25W 5W 10W 20W 25W 5W Mens room 8Ω 10W Dressing room A Rehearsal room 15W Ladies room 10W 20W 5W 10W 20W 10W 5W 10W 10W 20W 5W 20W 25W w/ L-pad house mix 20W Ch. 1 parallel input mode CX 1202V Theater Dressing room B manager’s office Ch.
Other design considerations +1 1 LF OFF 2 Speaker transformer saturation 3 0 dB -1 5 6 -2 7 Ch. 1 Ch. 2 4 8 LF OFF -3 9 10 Speaker transformers tend to be fairly small and can vary widely in quality. Many are thus prone to core saturation at low frequencies, which occurs when the magnetic -4 field induced in the transformer’s iron core by the audio signal waveform reaches the limit the core can handle.
Speaker wire loss A wire’s resistance is inversely proportional to the cross-sectional area of its conductor, but even the highestquality copper wire has some amount of resistance to electrical current flow. 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.
AC current consumption A 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. Actual current consumption will depend on the amp model, the power level it is operating at, and the load impedances. “Normal conditions” in power amplifier ratings means operating with a random program source (pink noise), at an average power level equal to one-eighth of maximum power.
Thermal losses (heat emissions) Essentially, a power amplifier draws electrical energy from the AC mains, converts it to DC, and then converts it again into an analog of the input signal to power the loudspeakers. Power that enters the amplifier through the AC cord, less that which exits through the speaker outputs, is lost and turns into heat, called thermal loss. The amplifier must remove the heat to the outside surrounding space to prevent overheating.
Thermal losses (heat emissions) Essentially, a power amplifier draws electrical energy from the AC mains, converts it to DC, and then converts it again into an analog of the input signal to power the loudspeakers. Power that enters the amplifier through the AC cord, less that which exits through the speaker outputs, is lost and turns into heat, called thermal loss. The amplifier must remove the heat to the outside surrounding space to prevent overheating.
System design with CX Series amplifiers CX302 CX302 Left: a CX amplifier without its security cover installed Right: a CX amplifier with its security cover installed Model Power, 8Ω/ch Power, 4Ω/ch Power, 2Ω/ch 20 Hz–20 kHz, 0.03% THD 20 Hz–20 kHz, 0.05% THD 1 kHz, 1% THD CX 302 200 W 325 W 600 W CX 502 300 W 500 W 800 W CX 702 425 W 700 W 1200 W CX 902 550 W 900 W 1500 W CX 1102 700 W 1100 W 1700 W Power @ 70V Power @ 70V 20 Hz–20 kHz, 0.
Features • Barrier strip output connectors • Direct transformerless 70-volt outputs (“V” models) • Zero inrush current—won’t trip circuit breakers at turn-on and avoids need for sequential power-up • DataPort for use with QSControl and amplifier accessories • Independent, user-defeatable clip limiters • Fully selectable low-frequency filtering; choice of 33 or 75 Hz roll-off or 50 or 75 Hz roll-off (“V” models) • Stereo (dual-channel), parallel-input, or bridged mono operating modes • Balanced inputs: XLR a
Balanced In Filters The output circuitry is actively clamped during clipping for smooth and very fast recovery. The clamp also feeds a proportional clip limiter, which actually Gain senses the depth of clipping and responds accordingly. The balanced inputs use premium 0.1% precision resistors for very high noise rejection. The precision components used in the input filters and all other circuitry ensure accurate performance.
Accessories IT-42 isolation transformer pack For applications requiring isolated 25-, 70-, or 100-volt outputs, the IT-42 (pictured at right), a unique transformer “backpack” accessory, allows the CX 302 to deliver up to 400 watts per channel or zone (300 watts on 25-volt lines). In bridged mono mode, it can be used to drive a single 140- or 200volt line loaded at up to 800 watts.
Front & rear panels 8 1 3 2 5 4 CX302 7 6 2 Front panel 7 Rear panel 1. Power switch 1. Terminal block inputs, Channels 1 and 2 2. Cooling vents 2. DataPort 3. Gain control (Channel 1) 3. XLR inputs, Channels 1 and 2 4. CLIP, -10 dB, -20 dB and SIGNAL indicator LEDs, both channels 4. Configuration switch 5. Configuration switch chart 5. Gain control (Channel 2) 6. Barrier strip outputs, Channels 1 and 2 6. POWER, BRIDGE, and PARALLEL indicator LEDs 7. Cooling air inlet vents 7.
Specifications CX 302 CX 502 CX 702 CX 902 CX 1102 OUTPUT POWER in watts 20 Hz–20 kHz @ 0.03% THD 8Ω per channel 200 300 425 550 700 20 Hz–20 kHz @ 0.05% THD 4Ω per channel 325 500 700 900 1100 EIA: 1 kHz @ 1% THD 8Ω per channel 4Ω per channel 2Ω per channel 215 375 600 325 550 800 475 825 1200 625 1050 1500 1700 Bridge Mono: 16Ω, 20 Hz–20 kHz, 0.1% THD 8Ω, 20 Hz–20 kHz, 0.1% THD 4Ω, 1 kHz, 1% THD 400 700 1200 600 1000 1600 850 1500 2400 1100 2000 3000 1400 2200 3400 < 0.
Specifications CX 302V CX 602V CX 1202V 200 400 550 800 700 1100 EIA: 1 kHz @ 0.05% THD @ 70V 250 440 1000 EIA: 1 kHz @ 0.1% THD @ 70V 300 600 1200 400 600 800 1200 1200 850 2400 1400 2300 OUTPUT POWER in watts 20 Hz–20 kHz @ 0.05% THD @ 70V @ 8Ω per channel @ 4Ω per channel Bridge Mono: 140V, 20 Hz–20 kHz, 0.1% THD 140V, 1 kHz, 0.1% THD 16Ω, 20 Hz–20 kHz, 0.1% THD 8Ω, 20 Hz–20 kHz, 0.1% THD DYNAMIC HEADROOM 2 dB @ 4Ω DISTORTION SMPTE-IM < 0.
Address & telephone information Address: QSC Audio Products, Inc. 1675 MacArthur Boulevard Costa Mesa, CA 92626-1468 USA Telephone Numbers: Main Number (714) 754-6175 Sales Direct Line (714) 957-7100 Sales & Marketing (800) 854-4079 (toll-free in U.S.A. only) Technical Services (714) 957-7150 (800) 772-2834 (toll-free in U.S.A.
www.qscaudio.com 1675 MacArthur Boulevard Costa Mesa, California 92626 USA • PH: (714) 754-6175 FAX: (714) 754-6174 “CX” and “PowerWave” are trademarks of QSC Audio Products, Inc. “QSC” and the QSC logo are registered with the U.S. Patent and Trademark Office ©1996, 1999 QSC Audio Products, Inc.
