Audio power amplifiers are most commonly encountered in one of six formats, which exist to meet real requirements. In order of generally increasing complexity, these are:

1 A monoblock or single channel amplifier. Users are mostly audiophiles who require physical independence as well as implicit electrical isolation (cf.3); or else musicians needing clean, ‘mono’ instrument amplification.

2a A stereo or two channel unit. This is the almost universal configuration. In domestic, recording studio ‘nearfield’ and home studio monitoring use, the application is stereo. For professional studios, and for PA, the two channels may be handling different frequency bands, or the same bands for other speakers, but usually it is the same ‘stereo channel’, as amplifiers are normally behind, over or underneath the L, R or centre speaker cabs they are driving.

2b Dual monoblock – as 2 but the two channels are electrically separated and isolated from each other – the intention being so they can handle vastly different signals without risk of mutual interference. However, being in proximity in a single enclosure and possibly employing a common mains cable, together with having unbalanced inputs, inevitably allows some form of crosstalk through voltage-drop superimposition; and magnetic and/or electrostatic coupling and interaction, between wiring.

3 Multi-channel – most often 3,4 or 6 channels. Originally for professional touring use, for compactness, eventually working within the constraints of the 19" wide ‘rack-mount’ casing system, the de-facto amplifier casing standard for pro audio gear worldwide. Three and six channel mono and stereo ‘Tri-amp’ units have been made so the three frequency bands needed to drive many activelyconfigured PA speakers, can come from a single amplifier box. Multichannel power amps are also applicable to home cinema and home or other installed Ambisonic (higher-dimensional) systems.

4 Integrated power-amp + preamp. Not to be confused with monolithic integrated circuits (ICs), this is the familiar, conventional, budget domestic Hi-Fi ‘amp’. The control functions are built in, saving the cost of a separate pre-amplifier in another box.

But sensitive circuitry (such as high gain disc and tape inputs) may not sit comfortably alongside the stronger AC magnetic fields commonly radiated by power amplifiers’ transformers and supply and output wiring. Careful design is needed to reap cost savings without ending up with irreducible hum and degraded sound quality.

In practice, most integrated amplifiers are built because of a tight budget, and so amplifier performance is traded off in any event. But some high grade examples exist and the trend is increasing at the time of writing.

5 ‘Powered’. The power amp(s) is/are built into the speaker cabinet, to form a ‘Powered’ or ‘Active cabinet’. This approach has been slow to catch on. It has seen some niche use in the past 20 years in smaller installations, and in the home, usually in conjunction with an ‘on-board’ active crossover.

Having one or more amplifiers potentially within inches of the loudspeaker parts they are driving has the clear advantage that the losses, errors and weight in speaker cables are brought down towards the minimum. This is most helpful in large systems where speaker cables are most often at their longest.

Since an amplifier in a speaker cab does not need its own casing, there can be savings in cost, and the total system weight (of amps + speakers) can also be reduced. In the home, the need to live with the conventional amplifier’s bulky metal box is avoided.

One downside, at least for touring, is that even if there is an overall weight reduction, the speaker cabinets assume added weight, which may cause flying (hanging) restrictions. There’s the need to runs mains cables as well as signal cables to each speaker cabinet. This is more of a nuisance in large systems.

For touring sound, health and safety legislation is also unwelcoming to powered cabs, particularly when flown, on several counts. Also, if flown, maintenance can be onerous and adjustment impossible without remote control. Although beyond the remit of this book, it is worth noting that musician’s ‘combo’ amplifiers are an older, simpler and far more widespread variant of the powered cab.

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