IMPEDANCE: A GENERAL OVERVIEW
The following terms will be helpful in understanding the information in this section:
Impedance: The resistance of a device to the flow of alternating current. Often used to rate the resistance of a speaker's voice coil.
Ohm: A unit of electrical resistance equal to that of a conductor in which a current of one ampere is produced by a potential of one volt across its terminals.
Parallel Operation: The connection of two or more power sources of the same output volt- age to obtain a higher output current.
There are three questions you should ask yourself prior to connecting multiple speaker enclosures to your amplifier:
1.What is the impedance of each enclosure?
2.What will the total combined impedance be?
3.Is the total combined impedance a safe load for your amplifier?
When multiple speakers are connected to an amplification system, they are generally connected in a parallel configuration. This is the case when you use the speaker output jacks on any SWR amplifier, or the in/out jacks on the input panel of your SWR enclosure. When you add speakers in parallel, the total impedance the amplifier “sees” becomes less.
Note: As parallel operation is most common, the following information will focus on this type of configuration. “Series” operation will not be discussed.
To figure out the total impedance of two or more cabinets of equal value connected in parallel, divide the impedance of one enclosure by the number of enclosures:
impedance of one enclosure / number of enclosures = total impedance
Let’s say for instance that you want to connect two 8 ohm SWR enclosures to one SWR amplifier configured for mono operation. The formula is: 8 divided by 2 = 4 (ohms), so the total impedance will be 4 ohms. Likewise, if you have four 8 ohm enclosures, the total impedance will be 2 ohms (8 divided by 4 = 2).
If you were to connect one 8 ohm enclosure and one 4 ohm enclosure in parallel, you can simply think of the 4 ohm enclosure as two 8 ohm enclosures (we know this is true from the first example), so you now have, in effect, three 8 ohm enclosures. The formula would be: 8 divided by 3 = 2.67 (ohms).
The owner’s manual that came with your amplifier should state the lowest (or minimum) imped- ance your amplifier is designed to drive. This may also be indicated next to your amplifier’s speak- er output jacks. If the total impedance of the cabinets you want to use is 4 ohms, your amp must have a minimum load rating of 4 ohms or less.
Before purchasing a second enclosure to add to your system, you should make a list of all the items pertinent to your additional enclosure, including: impedance,
Workingman’s Series Bass Speaker Enclosures • 11
