zapco Z-150.4 II Owner's Manual page 25

When you strap two amplifiers together, you are using one amp for the positive
side of the signal and one amp for the negative side. Internally this is accomplished
by reversing the polarity (or phase) of the input to the slave amp.
After strapping, each amplifier will see 1/2 of the load. Since the minimum load
of each amp is 1Ω, the minimum load for the PAIR of amplifiers is 2Ω. You must
not strap 2 amps to a 1 Ω load.
2Ω Min
The positive of the master amp goes to the + Speaker terminal and the positive
of the slave amp goes to the - speaker terminal. The amps are connected together
by connecting a negative terminal from the master amp to a negative terminal of
the slave amp
Strapping two amps to one speaker voice coil
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