QSC Audio ISA 500T User Manual

1- Set switch #3 and #8 to the on or off position as desired.

2- Select the desired roll-off frequency  at switch #2 and #9.

3- If filter off is selected, the frequency selection has no effect.

If operating in BRIDGE MODE-
1- When in bridge mode, channel 2’s low frequency filter and
clip limiter should be switched off.

2- Channel 1’s filter settings (and clip limiter) determine amplifier
behavior. Set channel 1’s low frequency filter on-off switch,
frequency selection switch and clip limiter as desired.

Generally, match the frequency selection to your speaker’s low frequency capability.  This improves speaker performance by limiting
subaudio cone movement, making more power available for the speaker’s rated frequency range.  

Unless you already have filtering in

the signal path preceeding the amplifier, use the low frequency filter  to protect your speakers from cone over-excursion caused by
frequencies below the speaker’s limits. The speaker’s documentation will specify the low frequency limit.

ISA 280, ISA 450 and ISA 750‘s have defeatable low frequency
filters with selectable roll-off frequencies of 30 and 70 hertz.
Each channel has its own low frequency filter which can be
enabled or defeated using the filter on-off switch. The frequency
is determined by  the frequency selection DIP switch. There is a
5 hertz roll-off in the filter off position to protect the speaker
loads from sub-audio or DC signals. The off position should be
used only for subwoofer systems with rated frequency response
below 30 hertz or if low frequency filtering is provided by other
devices.

 ISA 300T, ISA 500T and ISA 800T’s

have selectable roll-off

frequencies of 50 and 75 hertz. The frequency is determined by
the frequency selection DIP switch. 

is always active and is not defeatable. This is required by
the presence of magnetic devices (coupling transformers) in
constant voltage distributed audio systems. The low impedance
outputs of these models have the same low frequency roll-off as
the autotransformer coupled high impedance outputs.

If operating in STEREO or PARALLEL MODE-
1-  Select the desired roll-off frequency (50 or 75 hertz) at switch
#2 and #9. Each channel can be set independently.

If operating in BRIDGE MODE-
1- Select the desired roll-off frequency at switch #2. Setting of
switch #9 is ignored.

To Prevent Output Transformer Saturation-
The 50 Hz filter setting prevents transformer saturation for
normal, unclipped program levels. If heavy clipping is possible,
use the 75 Hz setting for extra protection. See “Low Frequency
Filtering for 70-100V Systems”, page 28.