Parallel Speakers and Speaker Cabinets
This article discusses the use of two speakers or two speaker cabinets with dissimilar impedance ratings, wired in parallel. Guitar amplifiers and speaker cabinets are sometimes equipped with two speaker jacks. Most often, these speaker jacks are wired in parallel (check the Owner's Manual to be sure).
When two speakers or two speaker cabinets with equal impedance ratings, are wired in parallel, their combined impedance is half of their individual impedance rating. For example: two 16 ohm speaker cabinets wired in parallel will have a combined impedance of 8 ohm.
In this example, you would normally want to set your amplifier's output impedance switch to 8 ohm, so that it matches the combined impedance of the two 16 ohm speaker cabinets running in parallel.
However, what is the combined impedance of two speakers or two speaker cabinets with dissimilar impedance ratings, when wired in parallel? The chart below provides this information.
Combined Cabinet Impedances (wired in parallel)
2 ohm & 2 ohm = 1.0 ohm
2 ohm & 4 ohm = 1.3 ohm
2 ohm & 8 ohm = 1.6 ohm
2 ohm & 16 ohm = 1.8 ohm
4 ohm & 2 ohm = 1.3 ohm
4 ohm & 4 ohm = 2.0 ohm
4 ohm & 8 ohm = 2.7 ohm
4 ohm & 16 ohm = 3.2 ohm
8 ohm & 2 ohm = 1.6 ohm
8 ohm & 4 ohm = 2.7 ohm
8 ohm & 8 ohm = 4.0 ohm
8 ohm & 16 ohm = 5.3 ohm
16 ohm & 2 ohm = 1.8 ohm
16 ohm & 4 ohm = 3.2 ohm
16 ohm & 8 ohm = 5.3 ohm
16 ohm & 16 ohm = 8.0 ohm
When the two cabinets are of equal impedances, each cabinet will receive 50% of the amplifier's output power. If the cabinets have dissimilar impedances, the cabinet with the lower impedance rating, will receive more power than the cabinet with the higher impedance rating.
How do you know the impedance value of a speaker or speaker cabinet if the value is not marked on the product? You can use a Multi-meter, set to the Resistance mode (range set to read 100 ohm and lower) and measure the DC resistance of the speaker or cabinet. In order to measure a speaker, you can place the meter's leads on the "+" and "-" terminals. For the speaker cabinet, you can place a speaker cord into the cabinet's jack and on the other end of the speaker cable, you can place the meter's leads on "tip" of the plug and the "ground" of the plug.
The DC resistance reading will normally be a bit lower (20%) than the speaker's or speaker cabinet's impedance rating. For example a speaker rated at 16 ohm, can have a DC resistance of 13.x ohm.
What is the difference between Impedance and DC resistance?
The impedance value is frequency dependent, meaning the resistance is measured at a specific input frequency. The resistance value will change depending on the input frequency. This can be seen on the Impedance Graphs provided by speaker manufacturers.
The DC resistance is the resistance measured with no frequency input (0Hz). It is the value measured with a Multi-meter, set to the Resistance mode.
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