Proel ACDT90 - Manual (Page 10)

This unit can be used both with constant impedance speakers (4 Ω) and
constant voltage speakers (25V, 70V, and 100V).
CONSTANT IMPEDANCE LINE
To use this amplifier with a constant impedance line (4Ω), please
connect the COM terminal to the negative post of your speaker, and
connect the 4Ω terminal to the positive post COM (fig. 2 ref. 17).
• To obtain the best performances, total speakers impedance
connected to the line, should be equal to the amplifier output
impedance.
• The speakers power sum should not be lower than the amplifier
output power.
• We suggest you don’t use long cables. Anyhow, for longer
distances application use a thicker cable.