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 SPL of 97 dB (72 plus 25) to the intended listeners. The ceiling- mounted speaker is about 6 feet, or 1.8 meters, above the listeners’ ears. The dB figure from the 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 AMPLIFIER

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 than the 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. A lower- quality transformer may have a 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, it is 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 or two, 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.

INSTALLATION TIP:

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 a service call on the system, measure the impedance again and see how it compares to the recorded figure; it’s 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 a transformer tap, added or removed a loud- speaker, or made any other adjustment to anything on the distributed line.

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QSC Audio CX Series user manual Selecting the Amplifier, Example
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CX Series specifications

QSC Audio, a recognized leader in professional audio solutions, presents the CX Series of amplifiers, a fusion of power, efficiency, and cutting-edge technology designed to meet the demands of various sound reinforcement applications. Known for their reliability and outstanding performance, the CX Series amplifiers are perfect for installations in venues, theaters, and houses of worship, delivering high-quality audio reproduction regardless of the setting.

At the core of the CX Series is its robust Class D amplification. This technology not only ensures excellent audio fidelity but also enhances energy efficiency, resulting in lower operational costs and cooler running temperatures. The amplifiers utilize a linear power supply, which contributes to exceptional audio quality by providing stable and clean power to the components. With varying power ratings, including options ranging from 200 to 1200 watts per channel, the CX Series caters to different venue sizes and specific audio requirements.

The CX Series is engineered with a range of features that enhance usability and performance. Integrated DSP (Digital Signal Processing) allows users to access advanced sound processing algorithms, enabling fine-tuning of audio settings for optimal performance. The amplifiers include comprehensive protection circuitry to safeguard the equipment from thermal overload, short circuits, and signal clipping, ensuring longevity and reliability in demanding environments.

Connectivity is also a significant aspect of the CX Series. The amplifiers are equipped with a variety of input and output options, including XLR, 1/4" TRS, and barrier strip connectors, facilitating seamless integration with a wide range of audio sources and speakers. Moreover, the user-friendly interface includes LCD displays for easy monitoring, making setup and adjustments straightforward for audio engineers and technicians.

Another defining feature of the CX Series is its lightweight design. This portability makes installation and transport easy without compromising on performance. Additionally, QSC’s commitment to sustainability is evident in the design, as the amplifiers are constructed with environmentally friendly materials and have a reduced carbon footprint, aligning with modern practices in eco-conscious audio engineering.

In summary, the QSC Audio CX Series amplifiers blend professionalism with versatility, delivering exceptional performance that satisfies both audio quality and operational reliability. Their innovative technologies and thoughtful design make them a powerful ally for professionals in any sound reinforcement scenario.
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