Professional power amplifiers

From a technical point of view, an audio amplifier is a device into which a low-power signal enters and a high-power signal exits. The function of amplifying as faithfully as possible, i.e. without distortion, a low-level electrical signal to make it suitable for driving loudspeakers, which obviously require more power, in no way contradicts that famous law of physics whereby 'nothing is created, nor destroyed, but transformed'. In fact, in order to perform the function described, the amplifier requires energy consumption, often considerable, from the mains.

Many manufacturers of professional amplifiers have been inspired by models for hi-fi use, with the primary goal of maximum sound quality, the result of careful design and to a large extent 'measurable' by technical parameters.

Final amplifiers, also known simply as "power amplifiers"Since they do not include a preamplifier stage, unlike integrated amplifiers, they can be classified into two categories, depending on their use: those from installation and those from touring. The former, as the term indicates, are intended for fixed installations, for example a theatre, a church, an entertainment venue, an airport or a ship. The touring ones, on the other hand, are designed for touring events, thus typically in show business: concerts, street events, shows of various kinds. In both cases, the applications are many and in a later chapter we will see what the differences between the two are.

Power stages and load

As we have said, amplifiers receive an input signal at line level and make it available, amplified, to their output connectors to drive a load, represented in our case by loudspeakers. For electrical purposes, the latter are seen as an impedance, whose nominal value is measured in ohms and varies according to frequency.

The output power of an amplifier is indicated in watts per channel (W/ch) over a very precise impedance, for example 8 ohms. In the technical specifications of a power amplifier we will therefore always find a power rating, for example 2 x 400 W into 8 ohms, in the case of a two-channel stereo amplifier. This power should also be expressed as a function of distortion, because you well understand that 400 W with 5% distortion is different from 400 W with 0.1% distortion, but this figure is not always given by the manufacturers.

Typical connection diagram of a 2-channel amplifier with a pair of loudspeakers

If we now connect two more identical speakers in parallel at the outputs, the load seen by the amplifier is halved, becoming 4 ohms. In correspondence, the power of an ideal amplifier should double, in reality it is always less than double for technical reasons related to the limits of the power supply.

Typical connection diagram of a 2-channel amplifier with 4 speakers (2 by 2 in parallel)

As the speakers continue to be added, the load impedance gets lower and lower, reaching values that can approach short-circuit, with the powers that correspondingly continue to increase (e.g. in the ideal case, 1600 W per channel into 2 ohms by connecting 4 speakers in parallel on one output). Under these conditions - they can be several thousand watts - the amplifier cannot continue to deliver power and reaches a point where the protections kick in. For this reason, if you plan to connect many loudspeakers to the outputs, it is advisable to choose high-impedance loudspeakers, e.g. 16 or 32 ohms, or trivially use multi-channel amplifiers.
For even more, there are the distributed sound systems, the 100 V ones for instance, which we will not discuss here, or solutions such as the one proposed by Zero Ohmof which Ligra DS is the exclusive distributor for the Italian market.
Power in itself is not a significant figure if you do not relate it to the efficiency of the acoustic loudspeakers used, i.e. the SPL sound pressure they are able to emit with 1 W at a distance of 1 m. More efficient loudspeakers require less power, and vice versa.

Class D power stage, the secret to high efficiency

In a nutshell, a Class D amplifier transforms an analogue input signal into a PWM (pulse width modulation) signal, i.e. a sequence of pulses corresponding to the amplitude and frequency of the input signal . The PWM signal is then amplified and, before being transferred to the loudspeakers, subjected to low-pass filtering to restore the original audio waveform and to eliminate spurious ultrasonic noise.

The greatest advantage of the class-D amplifier is its high efficiency, at around 90%, which is far higher than that of its main analogue rival, the class-AB amplifier, which has an efficiency between 50 and 70%. In practice, it manages to use almost all the energy from the mains to amplify the signal. For this reason, the class-D amplifier has found fertile ground in the professional sphere, as its high efficiency allows high power, easily thousands of watts per channel, with reduced physical dimensions and the possibility of eliminating, in some cases, heat sinks and even cooling fans. By now, all professional power amplifiers, with a few exceptions, make use of class-D circuits; and the amplifier modules that are already installed in many loudspeakers (called active) and subwoofers are also class-D.

Typical Class D amplifier module with associated mains power supply used, for example, inside loudspeakers, typically studio monitors, and subwoofers

At one time, class-D amplifiers were discriminated against because of their inferior sonic performance compared to amplifiers based on traditional schemes, typically class AB. Over the years, however, the latter have improved dramatically and today they have reached levels that do not make the other mentioned types regrettable.

The professional power amplifier

Having clarified these fundamental aspects concerning the functioning of amplifiers, let us try to understand how a professional power amplifier is made, intended as a stand-alone device and not as an amplification module. We have mentioned the number of channels: generally they range from 2 to 8, but they can also be paired two by two to operate in mono "bridged" mode according to a configuration that is called, precisely, bridged mode, which presupposes the doubling of power. The amplifiers are modular, so they replicate the same channel for all of them, except that amplification stages of different power ratings can be used as required. It is not uncommon to find 2 x 500 W + 2 x 100 W models, so they are basically two separate power amplifiers enclosed in the same chassis. Generally, the simplest and cheapest amplifiers have a basic front panel, with only the gain adjustment for the various channels and a series of LEDs to indicate, for example, overload or presence of signal.

4-channel Class D power amplifier, interior view
Powersoft DSP module

As they go up in price, more sophisticated devices increasingly integrate a DSP, i.e. a digital signal processor. The DSP allows all sorts of processing to be performed on the input audio signal: equalisation, compression, expansion, limitation, filtering, delay, etc. For this to be possible, upstream the input signal must be converted from analogue to digital by means of, precisely, an A/D converter.
Thanks to the DSP, speaker libraries can be stored in hundreds of internal memory locations so that for whatever system is connected to the amplifier, the correct preset can be set to optimise its operation with the utmost precision, without the need for
costly (in terms of time) ad-hoc calibrations. The advantages of using a DSP inside an amplifier are numerous: linearisation of the frequency response, increased dynamics, better signal-to-noise ratio, but above all the ability to benefit from more precise and wider settings, particularly when subwoofers and satellites need to be handled separately.

Setting the DSP parameters is done via software by connecting the power amp to the computer:

APEX Intelli-Ware Software

Installation and touring amplifiers, what are the differences?

Since the intended uses of installation amplifiers, as we have seen, are different from those of amplifiers for use in touring events, their construction characteristics are also, to some extent, different.
In general, an installation amplifier may be somewhat less robust in chassis and physical construction as it does not have to withstand the stresses of on-the-road use. On the other hand, fixed-installation amplifiers are generally less frequently moved and repaired. Functionally, one of their characteristics is their ability to drive distributed or multi-zone audio systems, which touring models generally do not.

Installation amplifier
Touring amplifier

The biggest differences, however, concern the connections. On the rear panel of amplifiers are both line-level signal input and output connectors. The input ones can range from unbalanced RCAs to the much more professional, and safe, female XLR Cannon or balanced XLR/jack hybrids, more frequently called combos. For the outputs, on the other hand, Euroblocks and SpeakOn are usually used. The former are used in installation amps, the latter for touring amps.
The Euroblock removable connectors, both signal and power, are used when space must be saved on the rear panel and individual connectors are not required. This solution gives the user as many options as possible if both signal and chassis ground terminals are available, allowing him to decide which type of wiring to use.

Typical rear panel of a 4-channel installation amplifier. Note, left, the four output terminals and Euroblocks for the four inputs A, B, C and D

A single SpeakOn power connector carries up to two channels, and having a bayonet fastening system is very practical when you need to proceed quickly and safely, think of major live events where perhaps dozens, if not hundreds, of amplifiers need to be connected to as many line array modules and subwoofers.

Typical rear panel of a 2-channel touring amplifier. Note, on the left, the two combo input connectors and, on the right, both Speakon and red/black screw connectors, very common in hi-fi

Increasingly, we find RJ45 network connectors on the rear panel, which are used for remote control in models with DSP or receive the input signal from the Dante® audio network.