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DIY Audio Speaker / Amplifier Wiring Guide / FAQ

What is a 8 ohm load, 4 ohm load?

This is the resistance (impedance) presented by the speakers that is seen by the amplifier. This can also include any crossovers and circuits connected to the speakers. When the load decreases, the amplifier's output increases. There is less resistance to the current, and the speakers can draw more power from the amp. Drawing more power than the amplifier was designed for will damage the amp.

Every amplifier is designed to handle a certain load. For home amplifiers this number usually starts with 8 ohms. With car amplifiers it is usually 4 ohms. All amplifiers can handle a higher resistance (load), but they will produce less output. Most quality amplifiers can also handle a lower resistance. Most car amps can handle a 2 ohm load, while some can go as low as 1/2 ohms.

How to Bridge an Amplifier
How to bridge an amp?

First, make sure that the amplifier you have is bridgeable. If you try to bridge an amp that is not designed to be bridged, you can damage the amp. Basically, when you bridge an amp you use the + terminal from one channel and the - terminal from the other. The instructions that came with the amp should describe which terminals to use. If not, look for some indication next to the speaker terminals on the amp. Sometimes, there will be lines connecting the two terminals you should use to bridge the amp, as shown on the right.

What happens when you bridge an amp?

Theoretically, the output should be 4 times the power of a single channel on the amp. A 50Wx2 amp bridged would act as a 200Wx1 amp. Unfortunately, you only see about 2/3 to 3/4 of this power. This number is usually the same as the combined power of the 2 channels at half the load. Take the following example of a 2 channel car stereo amplifier:

50x2stereoat 4 ohms
75x2stereoat 2 ohms
150x1bridgedat 4 ohms

Unless stated otherwise, this amp would be unstable below 2 ohms stereo or 4 ohms mono bridged. Attempting to use an amplifier at an unstable load will damage the amp.

Note that some high end car audio vendors intentionally underrate their amplifiers so that a 50x2 Watt amplifier @ 4 ohms really does produce 200x1 @ 4 ohms. This is done so that car stereo systems using their equipment can enter car audio competitions in a lower Wattage category, giving them an edge over the competition. Meanwhile, many low end manufacturers overstate the power of their amplifiers.

How do you run speakers in parallel?

Wiring speakers in parallel is simple. Connect the + terminal on the amp to the + terminal on each speaker. Then do the same for the - terminals. An example of this is shown below.

Wiring Speakers in Parallel

For two 4 ohm speakers, the total impedance would be 2 ohms. To find the total impedance of speakers in parallel, use the formula below.

Parallel Wiring Formula
How do you run speakers in series?

It is not suggested that you run speakers in series. No two speakers will be exactly identical, even if they are the same model from the same manufacturer. This means that they will act slightly different from each other when presented with the same input signal. When wired in series, these differences will cause distortion in the form of back EMF.

You can however run both coils of a DVC (Dual Voice Coil) speaker in series. The cone movement for both coils will be identical, and will not cause distortion problems. This is the formula to calculate the impedance of speakers in series.

Series Wiring Formula
What is a dual voice coil (DVC) speaker?

A DVC speaker has 2 coils instead of one as in a Single Voice Coil (SVC) speaker. Both coils have the same resistance, and both coils MUST be hooked up to the same channel of the same amp. You cannot run both coils off different channels of the same amp with the same signal, and you must use both coils.

DVC speakers offer a lot of flexibility. Given 2 DVC speakers, there are 4 different wiring methods. These 4 methods are shown below for DVC drivers with 4 ohm coils.

Wiring 4 DVC Speakers in Series
Wiring 4 DVC Speakers in Parallel
Wiring 4 DVC Speakers in Split Series
Wiring 4 DVC Speakers in Split Parallel

There are even more possibilities when more than 2 DVC drivers are used in a series/parallel configuration. For example, 3 DVC JL Audio 12W6 woofers at 6 ohms per coil can be wired into a single 4 ohm load.

What is bi-amping?

Bi-Amping refers to using different amplifiers (or different channels of the same amp) for the low and high frequencies in the same speaker. On a 3-way speaker, usually the mid and tweeter are driven by one amp, and the woofer is driven by more powerful amp. This allows you to purchase a high quality low power amp for the highs, and a more powerful amp for the lows. With the proper pre-amp you can also have more control over the bass output. On a 2-way speaker, the mid and tweeter are driven by different channels on an amp. This is usually done so that you can use an active crossover before the amplifier.

In DIY audio, bi-amping has even more advantages. Low pass crossovers for woofers require very large inductors. These inductors are basically very long coils of copper wire which can have a very high resistance. Using an active crossover before the amplifier removes the need for these inductors. Bi-amping also removes the need for any circuits to fix problems caused by different sensitivities or impedances between drivers.

If a speaker is capable of bi-amping, then the plate on the back of the speaker will have 4 binding posts: 2 + terminals and 2 - terminals. Both + terminals and both - will have a piece of metal connecting them together. To bi-amp the speaker, remove the metal piece. Then, use the top 2 terminals for the high frequency amp, and the bottom two terminals for the bass amp.

Speaker Binding Terminals

Note: some professional audio equipment has 4 binding posts on the back. This is for ease of running multiple speakers in parallel. It is not for bi-amping, and the terminals should not be connected.

What size speaker wire should I use?

There is only a difference of .1db between 18 gauge and 12 gauge. Usually, 18 gauge wire is sufficient, except for high power (subwoofer) applications. Different wires will have different line capacitances, which can cause the wire to act as a low pass filter. This is one of the selling points of high end cables.

You may be temped to use typical electrical wire instead of speaker wire to save money. The copper in electrical wire is of less quality than speaker wire, and will be prone to rust. Also, the insulation isn't of the same quality as speaker wire. These points are especially important when dealing with car audio. Still, it can be done. It would be best to solder terminals to the ends of the wire to prevent the rust problem.

What size power wire should be used when wiring amps for a car?

Use the following table to determine what size power wire you should use. Use the same thickness for the ground wire. Keep the ground wire length to 1.5 feet or less. Any more could introduce noise in the system.

CurrentLength of copper wire in feet
0-20A 1412121010888
20-35A 12108 86664
35-50A 108 8 66444
50-65A 8 8 6 44442
65-85A 6 6 4 42220
85-105A 6 6 4 22220
105-125A 4 4 4 22000
125-150A 2 2 2 200000