How to Determine Total Speaker Load

Knowing the total speaker load (or impedance) that a set of cabinets, or speakers in a single cabinet, represent is critical. An impedance mismatch (when an amplifier is not rated for the total speaker load connected to it) can damage both the speakers and amplifiers. Tube amplifiers are particularly vulnerable when connected to an improper speaker load.

Instructions

Difficulty: Moderate

Things You’ll Need:

• Calculator

1. Determine if the speakers are connected in series. Most cabinets make series connections, but this should be confirmed in the manual. In a series connection, the negative wire comes from the amplifier to one speaker's negative post. A wire runs from that speaker's positive post to the next speaker's negative post. This continues until the final speaker's positive post connects with the positive wire from the amplifier.
2. Identify parallel speaker connections. When the negative posts of speakers are connected together and the positive posts are also connected, they are in parallel. The positive side of the parallel speakers then connects with the positive wire from the amplifier, and the negative side connects with the negative amplifier lead.
3. Read the impedances marked on the speakers. Remember that series connections increase the total load, but parallel connections decrease the load. Add the impedances of speakers in series together for the total speaker load. For parallel connections, add the inverses of the impedances for the inverse of the total impedance. For example, to figure the load caused by 2 parallel connected 4 ohm speakers, add 1/4 plus 1/4 to make 2/4, which equals 1/2, and the load is its inverse which is 2.
4. Simplify large groups of connected speakers into the smallest connected groups because one cabinet may have series and parallel connections. A cabinet holding 4, 4-ohm speakers could be all in parallel (1/4+1/4+1/4+1/4=4/4=1/1 or 1 ohm total), all in series (4+4+4+4 equals 16 ohms total), 2 groups of parallel speakers in series with each other (in the example above, 2 parallel 4-ohm speakers create 2-ohm load and then the 2, 2-ohm loads in series would equal 4 ohms total load) or 2 pairs of speaker in series (4+4 equals 8 ohms) that are then parallel connected (1/8+1/8=2/8 or 1/4 which is the inverse of 4, so 4 ohms total).
5. Remember that the most common speaker loads amplifiers are configured for are 2 ohms, 4 ohms, 8 ohms and 16 ohms. Total cabinet impedances should always fall into one of these numbers.
6. Use a calculator to figure parallel connections if necessary. Enter a speaker's impedance rating, then press the "1/X key." Add the next speaker by entering its impedance and pressing the "1/X" key until all speakers are entered. Press the "=" key, and then inverse the answer into the ohm rating by pressing "1/X" a final time.

Tips & Warnings

• When all speakers have the same impedance rating, use the following tricks. When in series, multiply the single speaker impedance rating by the total number of speakers for total load. When in parallel, divide the single speaker impedance by the total number of speakers for total load.

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