As you build your car audio system, you may notice that bass is lacking. The fast way to solve this common problem is to add a subwoofer. These drivers are dedicated to supplying the low-frequency information in your media, robbed by small speakers, road noise and engine hum. Adding a subwoofer is fairly straightforward when you understand the relationship between the amplifier's wattage output and the subwoofer's rated impedance.
The first step when selecting an amplifier and subwoofer is to determine the impedance load the amplifier can sustain for any length of time. Impedance governs the amount of work the amplifier must do to get the sub to move. Low impedance loads create more wattage from the amplifier but at the cost of high heat and lower overall stability. Too high an impedance causes lower bass output, due to the amplifier supplying less power than it's capable of producing. Check the impedance by looking at the subwoofer's literature or at the bottom of the magnet. Alternately, set your multimeter to "0-200 ohms" and touch the positive and negative sub terminals with the probes. Compare these findings with the stated impedance capabilities from the amplifier's manufacturer.
Voice Coil Wiring Implications
It's easy to understand the impedance impact by a single voice coil subwoofer to an amplifier. The subwoofer wires directly to the amplifier the same as any other speaker, so the impedance stated by the manufacturer should be the load presented to the amp. Dual voice coil subwoofers however wire in multiple different ways, presenting drastically different loads to the amp. You will notice that DVC subwoofers have two sets of terminals, one pair per coil. To the amplifier, this is the same as a pair of speakers and must be wired to present the intended load to the amp. For example, a DVC 4-ohm woofer may be wired for 8 or 2 ohms, depending on the way the terminals are jumped together. Safe range for most car audio amplifiers is at or between 2 and 8 ohms, although depending on the amplifier, the former may be too tough a load when the music gets intense.
Mono amplifiers are generally intended to run subwoofers. Class-D mono amps in particular run cool, are efficient with their power output and are geared toward low-impedance operation. Stereo amplifiers can be wired in stereo or bridged mono, depending on the system design. For example, a four-channel stereo amplifier may drive four 2-ohm subwoofers or two 4-ohm bridged, with the load appearing equivalent to the amplifier. You evaluate which path to take based on the amount of space in your car, volume goals and wattage and impedance capabilities of the amplifier itself. When bridging, the left positive and right negative amplifier terminals are used, whereas with a mono amplifier there is only one positive and negative terminal.
Series and Parallel Wiring
Series and parallel wiring produce two different results using the same pair of subwoofers. Series wiring increases the impedance presented to the amplifier, while parallel does exactly the opposite. For example, two 4-ohm subwoofers wired in series present an 8-ohm load to the amp, while parallel wiring goes the opposite direction, showing the amplifier a 2-ohm load. The difference in wiring between series and parallel is whether the positive terminal on the first subwoofer jumps to the negative or the positive on the second. Pay close attention to the relationship between the different sets of voice coils to ensure you're extracting all the power you can from the amplifier while keeping the amp and subs safe.
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