zapco ST-2X SQ Owner's Manual page 14

It is possible to run the 2Ch amps in "3-Channel" mode by running a pair of
speakers for the mids and highs on left and right channels, and at the same time
run a woofer bridged between the L+ and R- terminals as shown. However, since
each channel will see 1/2 the impedance of the woofer you must use a woofer of
no less than 4Ω. The amplifier sees impedance by frequency, so you can have two
2Ω loads but you must use a passive crossover on each speaker in the three chan-
nel mode. With the crossovers in the line, the amplifier will always see a minimum
load of 2Ω on each channel at all frequencies.
• Main speakers can be 2Ω~4Ω. Woofer can be 4Ω~8Ω but cannot be less than
4Ω (as in any bridged situation)
• The active amp crossovers are not used in this system
A 3-Way hookup requires a coil on the
woofer and capacitors on the highs to
act as a crossover and maintain correct
impedance. Consult the speaker
manufacturer for correct cap and coil
values.
12
3Ch Mode
2Ω Min
4Ω Min