QSC Audio ISA 800T 사용자 설명서

The low frequency filtering is another difference in the characteristics of the “non-T” and the “T” amplifiers. Because transformers and
other magnetic devices are prone to low frequency “misbehavior” (called saturation) , the autotransformer equipped “T” models are
low frequency limited. The default low frequency roll-off is at 50 hertz; 75 hertz is the other available selection. 

filtering is always active on the “T” models. This low frequency roll-off is required primarily to protect the smaller matching
transformers on the speakers of constant voltage distributed audio systems, but also aids in minimizing low frequency distortion of the
system in general.

The varying degree of quality of coupling transformers may require the roll-off frequency to be set to the higher 75 hertz selection. The
change from 50 to 75 hertz may seem minor, but in terms of low frequency drive, it reduces the required “magnetic flux” of the
coupling transformers by about 30%. If the application requires driving speaker loads at or near their rated power, or with program
material that’s heavy on the bass, a marked performance increase may be realized with the 75 hertz low frequency roll-off. A careful
system listening evaluation will aid in the best selection for any application.

The low frequency filter is not a “brick wall” and if the input level is increased beyond 100% it is possible to “over-ride” the filter to
some extent. Normally, such signal levels will cause the amplifier to clip, which is the first sign of potential danger.  Midband clipping
puts no unusual strain on the transformers, but full-range material with heavy clipping will probably overdrive the 70-100V transform-
ers  to satuartion, even with the 50 Hz filter. This will cause an additional layer of “hash” in the clipping, and will also result in audible
clicking noises from the transformers inside the amplifier. These symptoms indicate abnormal stress and should be avoided for
maximum service life. If unpredictable clipping is possible, use the Clip Limiter to prevent long-term overloads and use the 75 Hz filter
position for extra protection. The clip limiter will not effect performance where no clipping is present, and therefore is recommended
as the default setting. The 50 Hz filter is suitable for program material which remains within the amplifier’s normal headroom and will
provide slightly better bass on wide-band speakers.

If a system with sub-woofers is desired, we recommend the use of the low-impedance ISA 280, ISA 450 or ISA 750 with low-imped-
ance sub-woofer drivers. The low frequency performance of the directly-coupled system is superior to the magnetically-coupled
systems. Remember that all of the outputs on the “T” models are low frequency limited to 50 hertz by default.  For all applications
requiring audio performance below 50 hertz, use the low impedance, “non-T” models with low impedance speakers.

The illustration to the right shows a typical
two-zone 25 volt system. When using 25 volt
speaker loads, make sure that connections to
the amplifier are made to the 25 volt output
terminals.

If using the ISA 300T, 300 watts per channel
(zone) can be delivered. Make sure that the
total speaker power load on each channel
does not exceed 300 watts.

If using the ISA 500T or ISA 800T, the per-
channel power limit is 550 and 800 watts,
respectively.