Subwoofers add the low octaves in a car stereo system that conventional dash, door and deck speakers cannot play effectively or at all. Subwoofers require amplification, with the relationship between subwoofer impedance and amplifier key in how well bass is reproduced. Wiring a subwoofer to a car stereo is fairly straightforward, but must take into account a few factors to achieve the performance you expect.
With multiple voice coils, amplifier channels and subwoofers often found in car audio systems, impedance is the first thing you need to have a grasp on prior to wiring up subwoofers. Impedance in practical terms means the amount of work the amplifier does to move a speaker cone in and out. The lower the speaker's impedance, the harder the amp must work. Just like an athlete, the more work that is involved, the more energy expended. This produces more wattage from the amplifier as impedance drops, at the expense of heat and lowered system efficiency. Too low, and amplifiers simply quit working, either temporarily as a protection circuit resets or permanently until it is repaired. Most car amplifiers "expect" to operate within two to eight ohms. Conversely, operating at a high impedance above eight ohms in a car environment reduces bass output to very low levels.
Series vs. Parallel Wiring
Series wiring is the process of raising the system impedance when wiring multiple speakers together. This is desirable if you are running two or three two-ohm subwoofers off of a single amplifier channel. In this environment, these woofers would present either a four- or six-ohm load, respectively. Parallel wiring does the exact opposite, lowering system impedance to increase amplifier power output. For example, wiring two eight-ohm subwoofers in parallel presents a four-ohm load to the amplifier, generating more output and keeping the system impedance within a safe range. You should always use caution when wiring in parallel to prevent the load from dropping below two ohms, since most amplifiers generate excessive heat or become unstable at this impedance.
Single and Dual Voice Coil Subwoofers
The two choices you have when shopping for subwoofers are single and dual voice coil models. The version of woofer you choose directly affects how you wire it to the amplifier, and potentially other subwoofers in a series or parallel chain. Single voice coil subs wire to an amplifier in the same way as any other speaker, with positive and negative amplifier and speaker terminals directly wired to one another. Dual voice coil subwoofers, depending on how they are wired, either raise or lower system impedance. This is important, since many amplifiers cannot handle impedances lower than two ohms, which a dual two-ohm subwoofer could present if wired incorrectly. DVC subwoofers offer two sets of binding posts which jump together, in addition to jumping to other subwoofers and the amplifier itself.
Mono or Stereo?
Mono amplifiers are single-channel affairs, often designed specifically to run subwoofers. Often Class-D designs noted for their cool operating temperature and efficiency, mono amplifiers operate best at lower impedances in the two-ohm range. Stereo amplifiers are optimized for subwoofer use at two or four ohms, attainable through bridging or wiring multiple woofers in series or parallel to stay within this range. Bridging is the act of wiring the left positive and right negative amp channels together to form a much more powerful single channel, ideal for addressing the high current required in most subwoofer applications. You can do this with a two or four-channel amplifier, making these models a little more flexible as your system grows and evolves.
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