QSC CX4T User Manual download pdf (Page 26)

X. How Many Speakers?

How many loudspeakers can you connect to the distributed line? The amplifier itself determines the amount of

audio power available to the line, and from that figure you must compensate for insertion loss in the speaker

transformers and for headroom when you determine how much power is available

for the speakers.

For example, let’s suppose you have an amplifier rated at 500 watts into a 70-volt

line, and your speaker transformers each have an insertion loss of 1 dB, which is

typical. The losses in the transformers mean that they will draw 25% more audio

power than they deliver to the speakers. For headroom, as mentioned before, you

add at least another 25% to the total amount of power drawn from the amplifier.

The additional 25% headroom is optional but highly recommended. It will allow

for future additions to the system and reduce the stress on the power amplifier.

1.25 x 1.25 = 1.5625

So you’ll need at least about 56% more power than the total sum of the transformer

taps. The reciprocal of 1.5625 is .64, so conversely you can figure on allowing a

maximum of 64% of the amplifier’s rated power for the loudspeakers. Therefore,

the highest combination of transformer taps the 500-watt amplifier can drive

satisfactorily is a sum of about 320 watts, e.g., 32 speakers each tapped at 10 watts,

or 64 speakers tapped at 5 watts, etc.

This table lists the recommended maximum load (sum of all speaker transformer

power taps on one line) for the CX6T and CX12T amplifiers. These figures assume

a typical transformer insertion loss of about 1 dB.

You can, of course, use different output configurations on

the two amp channels if it’s not operating in bridged mono

mode. For example, one channel of a CX12T could drive a

70-volt line loaded at 320 watts while the other channel

drives a 25-volt line, or even a direct loudspeaker.

For every watt put

into the speakers,

you need…

…approx. 1/4 watt

more to compensate

for transformer

insertion loss*

…+25% for

headroom.

†

*assuming a typical insertion loss of 1dB

†a good engineering practice

1 watt

1.25 watts

1.56 watts

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QSC CX4T User Manual Page 26