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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