Specifications

To compensate for the insertion loss, add a corresponding percentage to the sum of the transformer power taps.

For transformers with a 1 dB loss, add about 25%; in the example above, that would increase 119 watts up to

149 watts. To compensate for lesser-quality trtansformers with insertion losses of 1.5 dB and 2 dB, add 40%

and 58%, respectively, to their individual power tap figures. Therefore, even with medium-quality speaker

transformers, it could easily take up to 188 watts to provide 119 watts to the speakers! The sum of the power

taps, corrected for insertion loss, is the distributed line’s total power demand.

Fitting amplifier power

Choose an amplifier model whose power rating meets or exceeds the distributed line’s total power demand.

If it is a 70-volt line, use the per-channel power ratings of the CX-V direct 70-volt models (CX 302V, CX 602V,

or CX 1202V).

Hint:

It is a good engineering practice to add up to 25% to the line power demand figure, to allow

a margin both for dynamic audio headroom and for some future adjustments to the system—an added speaker

or two, a few transformer tap changes, etc.

Use the CX302 with the IT-42 isolation transformer accessory for these applications:

• for 25-volt lines up to 300 watts

• for 70-volt lines up to 400 watts that require isolation

• for 100-volt lines up to 400 watts

• for a 140-volt line up to 800 watts (with the amp in bridged mono mode)

• for a 200-volt line up to 800 watts (with the amp in bridged mono mode)

Question:

Like other QSC amps, the CX models carry two types of power ratings, EIA and FTC. The EIA ratings

are specified at 1 kHz, at no more than 1% THD. The FTC ratings are more stringent, measured over a wide

specified frequency range with THD not exceeding 0.05%. Which should you use?

Answer:

Use the FTC spec for sound systems with a high duty cycle, i.e., operating at or near full power for

extended periods of time. Some foreground music systems fall into this category. Use the EIA 1 kHz spec for

systems with a light to moderate duty cycle, such as paging and announcing systems or combination background

music/paging systems.