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Power amplifiers for larger-scale live sound reinforcement applications

While powered loudspeakers have risen in popularity, it’s important to keep in mind one of the big reasons why: the tremendous strides that have been made in power amplifier technology over the past 15 years or so that make them more easily integrated into loudspeaker cabinets.

At the same time, rack-mounted “outboard” amplifiers continue to see healthy use in a wide range of sound reinforcement applications, and it’s these units that are the focus here.

Amplifiers are far lighter and smaller than their predecessors (most long-timers can likely recall when they were referred to as “boat anchors” due to their prodigious weight and size), and they’re able to generate tremendous audio power from that reduced physical footprint.

The primary advancement is efficiency. Greater output efficiency means less heat, and therefore, less weight (reduction and/or elimination of heavy heat sinks), as well as more audio power making it to the loudspeakers.

The most common amplifier topology, by far, is class D (and variations), which uses an on-off switching method for its transistors called Pulse Width Modulation. Because its output devices are either on or off, the efficiency of the amplifier is greatly increased, and this is done without jeopardizing the integrity of the audio waveform by switching positive and negative output transistors on and off many times per waveform cycle.

The method is analog but similar in theory to digital sampling where a 44.1 kHz sampling rate is used to accurately capture a 20 kHz signal. This rapid switching creates a square wave that is then low-pass filtered to recreate the audio waveform.

Class G amplifier designs take a post-AB Class signal and switch it between two power supplies – one for softer output levels and other for louder. Efficiency improves as the power amplifier draws full power from the AC source when higher a level of amplification is needed.

Class H also works on the two-power supply output, but instead of simply switching between lower and higher voltage power supplies, the second power supply’s voltage level is controlled via the audio input signal. If the signal increases, so does the power supply’s voltage.

Class I, developed and patented by Crown Audio, is an advancement of the Class D switch-mode design. It helps eliminate a drawback of Class D – the potential for distortion in the moment that one transistor turns off and another turns on.

The addition of DSP into the package is a further enhancement, offering convenience, space and cost savings, operating efficiencies, performance advantages and more. Many modern amplifiers also benefit from networked control protocols, making them easier than ever to configure and monitor during a show.

In the early 1990s, Yamaha introduced the D2040 digital loudspeaker processor, allowing users to program crossover presets, including gain, delay, parametric EQ and limiting, to optimize multi-way loudspeaker systems beyond just manually turning analog pots to choose the crossover slopes.

At about the same time, Crown put DSP into its amplifiers with P.I.P. (programmable input processor) DSP cards, employing a Motorola 56002 microprocessor chip, to provide limiting, delay, crossover and EQ filters. This allowed the amplifier to be self-processed for bi-amplifying, optimizing specific makes of 2-way loudspeakers and eliminating the need for external digital signal processing.

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Integration of DSP into amplifiers has led to standardization of amp racks, such as the L-Acoustics LA-RAK with amplified controllers for L-Acoustics loudspeakers, Crown VRACK with I-Tech HD amplifiers for JBL VTX line arrays, NEXAMP TD Controllers for NEXO loudspeakers, and the expanding D Series amplification/DSP protocol for d&b audiotechnik loudspeakers.

The standardization of loudspeaker-amplifier-DSP combinations is the “Holy Grail” for large-scale tours, permitting extreme consistency around the world. Touring engineers can specify a particular loudspeaker configuration that will have predictable results on every continent. Further, “universal racks” can be deployed with different models of loudspeaker inventory by simply recalling the correct preset.

And for years, amplifiers with multiple channels (generally defined as more than two) were for either multiple headphones in studios or low-power-per-channel commercial applications. Now they’re a norm for mainstream professional sound reinforcement. Again, credit the advent of switching power supplies in class D output topologies, making design limitations a thing of the past.

The weight, size and dollar-per-watt ratio of multichannel amps has dropped dramatically. Cost savings is another advantage—multichannel amps cost less to build. Further cost savings can result from less rack space used, thus less racks needed, and for mobile use, less truck space occupied, faster load-ins, load-outs, etc.

Enjoy this Real World Gear look at the latest in modern power amplifiers for larger-scale sound reinforcement applications.