Ohms & Impedences, How to Select the correct amplifier and load

Ohms & Impedences Explained


All audio equipment comes with an OHMS Ω rating. Some equipment, especially amps, have multiple OHMS ratings. But what does it all mean, and why is it all so important ?

OHMS are a way of measuring Resistance. Resistance is the measurement of an electrical device or circuits ability to flow electricity. Low resistance means the device is able to "pass" more electricity, high resistance means it "passes" less. Resistance is a Static measurement (it does not change). Speakers do not have a resistance, as whilst the cone moves the effect of the magnets in the motor causes the resistance to change. This dynamically changing resistance is called Impedence. It is still measured in OHMS, and is the Resistance of the Speakers Coil at rest (not connected). Impedence is also known as Speaker Load.

OK so now you understand what OHMS are and how they related to the speakers impedence, how does that help ? It helps you determine which speakers are going to be suitable for your equipment ! As we know, LOW Impedence means more power passes from the amplifier to the speaker making it louder, and vice versa.

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But first lets familiarise ourselves with different audio configurations:

Amp Bridging: Amplifiers which support Bridging allow you to "team up" two channels to increase the power. You connect the +ve from the speaker to the +ve on Channel 1, and the -ve from the speaker to the -ve on Channel 2. This doubles the power delivered to the speaker as now BOTH channels are delivering power to ONE speaker.
HOWEVER, this also changes the Load ! Placing a 4ohm speaker onto one channel  gives a Load of 4ohm. Bridging the same 4ohm speaker onto 2 channels makes each channel see HALF the load ! So now you have half the impedence on each channel which means more power (usually 1/2 the impedence = 2x the power). So 4ohm bridged gives 2 channels of power, and half the impedence on both, so you can easily gain 4x the power for "free" just by bridging your 4ohm speakers !

Series Wiring: When speakers, or Duel Voice Coil subwoofers, are wired in Series, it means one +ve & -ve are connected to each other, and the remaining 2 are connected to the amplifier. This configuration will double the impedence of the speaker/s. Example: Series wiring a DVC 4ohm subwoofer will give you an 8ohm load.
Being able to INCREASE impedence like this might be usefull if you've bought a subwoofer with impedences your amplifier cant handle. If you have a 2 channel amp that is 2ohm stable, a DVC 2ohm subwoofer, and wish to Bridge the amp, you could Series the coils for a 4ohm load, then bridge the amp for the correct 2ohm loads*.

Series wiring a subwoofer

Parallel Wiring: When speakers, or Duel Voice Coil subwoofers, are wired in Parallel, it means the two +ve terminals are joined together, and the two -ve joined together, then connected to the amplifier. Parallel connects HALVE the impedence of the speaker/s. Example: Parallel wiring a DVC 4ohm subwoofer give you a 2ohm load.
Being able to DECREASE impedence like this might be usefull if your amplifier supports lower impedences and would like to get maximum power.

Parallel wiring subwoofers

* when running subwoofers it is preferable to use a mono (single channel) configuration, especially with duel voice coils. Attempting to connect 1 coil to each channel, instead of bridging the amp, can result in burnt out coils as stereo signals may cause the coils to attempt to move in opposing directs causing damage. Always wire DVC subwoofers in series or parallel.


FINALLY some quick rules of thumb:
- Subwoofers should always be run Bridged, on a monoblock or off a single channel to prevent possible damage.
- Always use a multimeter to check the DC Resistance (impedence) of you speakers after wiring.
- Running a HIGHER impedence than rated is safe (8ohm on a 4ohm amp is safe).
- Running a LOWER impedence than rate is NOT safe (2ohm on a 4ohm stable amp may result in damage).


So how does all help me shop ? I hear you saying.

Headunit: If you are not going to run external amps, you must be mindful of the lowest load the headunit is rated for when buy speakers. Typically a headunit will support 4ohm at its lowest, so you should buy a 4ohm, or higher, rated speaker set.

Amplifier: Again, the lowest load the amplifier is rated for should be kept in mind when buying speakers or subwoofers. Some amplifiers (classic SoundStream for example) support extremely low impedences, like 0.5ohm. This means a speaker setup of 1/2 an ohm, or higher, impedence, will be safe to use. TYPICALLY 2ohm is the lowest load supported by an amp.

Speakers: Typically speakers are of a 4ohm impedence. Some are lower (2ohm), some are higher (8ohm). You should only use 2ohm speakers if your amplifier supports it.

Subwoofers SVC: Typical single voice coil woofers are 4ohm, so you can safely bridge most amplifiers to run them.

Subwoofer DVC: When choosing a DVC subwoofer, you need to select one which will allow you to maximise the power from your amplifier. Typically, a DVC subwoofer is teamed with monoblock, so you would select a DVC woofer you can Parallel wire to place the best load on your amp.