Although the speakers in your vehicle may provide clear and crisp midrange and high frequencies, the sound is probably lacking any significant bass. Subwoofers counter low-frequency losses incurred from road and engine noise, using their larger cone diameter to move more air than small dash and door speakers can. Using the right amplifier, a new subwoofer adds depth and impact to your music, creating a richer sonic experience. When the amplifier and subwoofer combination is chosen after matching impedance and power considerations, wiring the speakers is a fairly straightforward matter.
Impedance is the measurement that describes how much work an amplifier must produce to get a speaker or subwoofer moving. In every case, a lower impedance means more power output, measured in watts. Although it sounds like a good idea to constantly run at the lowest impedance possible, amplifiers have limitations. Impedances below 2 ohms typically cause problems for amplifiers, such as overheating, entering protection mode, shutting down and failing. In most cases, an amplifier's "safe range" is between 2 and 8 ohms, with the most common impedance hovering around 4 ohms. Subwoofer impedance is shown in product manuals and on labels attached to the bottom of the subwoofer's magnet. You can also find the speaker's impedance by setting a multimeter's resistance to 200 ohms and then placing the probes on the positive and negative input terminals.
Subwoofers may be driven with mono, two-channel stereo or four-channel stereo amplifiers. Mono amplifiers are typically of Class-D design, a topology that runs cooler and more efficiently than traditional amplifier designs. Two- and four-channel amplifiers can drive one subwoofer per channel, or with channel pairs bridged to form a more-powerful single output channel. Four-channel amps offer increased versatility, with two channels bridged for a single subwoofer with the other channel pair running a pair of speakers in stereo. Despite this versatility, stereo amplifiers typically do not support low impedance operation below 2 ohms, a range that many mono amplifiers thrive in.
Single or Dual Voice Coil Subs
Single voice coil subwoofers come in 2-, 4- or 8-ohm flavors, wiring directly to an amplifier like any other speaker. Dual voice coil subs must have one of their two sets of voice coils jumped together to raise or lower the impedance. For example, jumping the voice coils one way on a dual 4-ohm subwoofer produces a 2-ohm mono load, while wiring the other way creates an 8-ohm load. Conversely, a stereo amplifier may be wired to directly power each voice coil, as though wired to a pair of speakers. Caution must be taken when using this approach however, since each of the amplifier channels must have their gains precisely matched to prevent premature subwoofer failure.
Amplifier Wiring Tips
Proper amplifier operation is fundamental when it comes to getting the expected performance from any subwoofer. This means using correct wiring techniques, preventing short circuits and ensuring proper amperage from the vehicle's charging system. Prior to wiring, disconnect the negative battery terminal to prevent accidental short circuits. Setting the vehicle's parking brake, especially when working on an incline, is also recommended. Always find the sum amperage total from your amplifier by either adding together fuse values or looking at the product manual. Select the power and ground wires accordingly, fusing the power lead no more than 18 inches from the battery. Similarly, ground lines must be bolted to a bare portion of the chassis as closely as possible to the amp. All wires must be placed away from foot traffic, preferably in the existing wire chase located under each doorsill. Failure to do these things causes system noise, product damage or -- in the worst case -- fire.
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