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Thomann's Cool Online Guides: PA-Amps

8. Working with your Amp

How loud can you go? In fact, in terms of the sound out front, you can keep your power amp turned up full, and regulate the actual sound level through the mixer - provided, of course, that your speakers are up to the job.

As we have seen, the key question is concerned with impedance and power ratings. An amp optimised for low impedance performance will not be able to deliver its maximum RMS power rating when connected to higher impedance speakers. Getting the match right also involves using the correct cables, because they too introduce resistance within the circuit - thick, low impedance cable kept as short as possible works best.

You should also consider how you are connecting your speakers. In parallel, that is each speaker receiving a direct signal from the amp, each additional speaker actually progressively lowers the impedance presented to the amp. A pair of 8 Ohm speakers connected in parallel for example, will represent 4 Ohms of resistance to the amplifier, not 16 Ohms as you might think.

However, if you connect your speakers in series, or one after the other in a single (electrical) line from the amp, each additional speaker adds to the total resistance. Three 8 Ohm speakers, for example, would give 24 Ohms of resistance. There are also various series/parallel configurations possible, for which the calculations are rather more complicated!

Placing the Power Amp

Distance equals resistance, as far as power amps are concerned. The further the amp is from the speakers, the more loss of signal there is, with consequent detrimental effects on the sound. You should use short cables with low impedance, and a power amp that sits as close to the speakers it is serving as possible - save your long cable runs for balanced line-level signals.

A rack is the best way to stow your amp, not least because it makes it easier to transport. Key points to remember when choosing your rack are:

  • The depth of your amp, and the requirement for space for any other rack units.

  • The weight of the amp - do you need a rear rack rail to help distribute the load?

  • The strength and practicality of the rack - will it take the weight, and is it easy to move? (Castors with brakes are essential!)

  • Cooling requirements - remember to leave some space around the amp - separating multiple amps with vented blanking panels is ideal.
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