Subwoofers are speakers that are specially designed to handle extremely low bass frequencies. They are large and heavy. Some subwoofers are active, meaning they have built-in power. Others are passive, meaning they require power from an external source. Subwoofer power amplifiers function like typical power amplifiers, the only difference is that they drive more power to the speakers to generate sufficient movement to reproduce low frequencies at high volumes. Building your own subwoofer amplifier is a rewarding way of saving money on custom audio equipment.
Source your parts. There are two approaches to acquiring your parts: Use a self-assembly amplifier kit or salvage and purchase parts individually. The advantage of the former is that all of the parts are made to measure and compatible. The advantage of the latter is that you have more flexibility with specifications. The kit approach is suitable if you are unfamiliar with amplifier construction. For example, the Amps Lab website sells a basic one-channel subwoofer amplifier kit. It doesn't come with a power transformer.
Source a schematic. This document denotes the layout, values and relationship of the board-mounted and chassis-mounted parts. It's essential reading for any DIY audio project. Self-assembly amplifier kits come with a schematic. If you don't have a schematic, you can use generic power amplifier schematics for reference.
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Load the circuit board. Typical board-mounted components include: Resistors, transistors, diodes, fuses, op-amp chips and RCA jacks. Load each part into the vacant turret as per the wiring schematic. Push the component in so that the connectors protrude through the bass of the board. Once fully loaded, flip the board over and solder the connectors to the metal strip on the base.
Solder the circuit board inside the chassis.
Load the chassis-mount components. The power transformer, IEC power supply and potentiometers are located on the chassis. Slot the power transformer into the large square pre-drilled hole on the chassis. Line up the solder lugs so that they are level with each other. Solder the primary winding to the relevant point on the circuit board. Load the power supply into the square hole on the back, solder the red wire to the positive eyelet on the board and the black wire to the negative eyelet. Fit the potentiometers to the pre-drilled holes in the front of the chassis.
Cut a piece of power wire and a piece of ground wire for each potentiometer. Strip a half-inch of insulation from both ends of each wire. Solder one end of the power wire to the output terminal of the potentiometer and solder the other end to the correct eyelet on the circuit board.
Fit the dials to the potentiometers. Line up the dials so that the zero is level with the notch on the potentiometer pole.