Wiring diagrams — power, signal and impedance

1. Power distribution

Fuse at the pack positive, switch the main feed, and never run the charger through the amp's input terminals. The DC-DC converter is optional — it powers a 5 V USB outlet, a DSP board or a voltmeter.

Fuse the pack, not the amp. A fuse at the amp protects the amp. A fuse at the battery positive protects the wire that runs the length of the case, which is the part that actually shorts when a latch spring or a screwdriver finds it.

2. Signal and outputs

A 2.1 board with Bluetooth and a sub channel. Keep signal runs away from speaker runs and from the pack; twisted pair for the inputs if the run is longer than a few inches.

3. Impedance: how many drivers per channel

Two 4 Ω drivers in parallel present 2 Ω to the channel. Most class-D boards rate 2 Ω minimum on the satellites and 4 Ω on a bridged sub. Going under the minimum is the fastest way to a dead board.

Series does not make it louder. Two drivers in series each get half the voltage, so the pair is no louder than one — it just survives a board that cannot take a low load. Parallel is louder only if the amp can supply the current.

Wire and connector notes

16 AWG is plenty for speaker runs inside a case. 14 AWG if you are bridging a sub at high current. Keep runs short and twisted.
XT60 for the pack connection, XT30 for charge leads, spade terminals at the drivers. Solder, then heat-shrink, then strain-relieve.
Label both ends before the amp goes in. You will not remember which black wire is left.
Pre-charge a big amp's input capacitance with a 10 Ω resistor across the switch for a second, or you will pit the switch contacts every time you turn it on.