SWR Sound Amplifier Speaker Output: Understanding Impedance and Power Ratings

SPEAKER OUTPUT SECTION

This section of the manual will deal with the proper connection of speaker cabinets to the power amplifier in the Workingman’s 8004. Some of this ground has already been covered in the Getting Started— Connecting Your Speaker Cabinets section in the beginning of the manual. This is meant to supplement that section and provide information in greater detail, as power amplifiers, impedance and speaker cabi- nets are all crucial in determining how best to operate your new Workingman’s 8004.

How Impedance Affects Power Ratings

People often have questions about impedance. What is it? The root of the word “impedance” is the verb “impede,” which means to block or resist. That’s what impedance is—resistance to power.

Power amps do not have a pre-determined impedance. They deliver power at whatever impedance the speaker cabinet tells it to. That’s why you hear the term “slave amp”—amplifiers only do what they’re told. So if someone tells you that they have a “4 ohm power amp,” their terminology and understanding of the concept is way off the mark.

Unlike power amps, every speaker cabinet has a pre-determined impedance rating measured in “ohms.” In most cases this rating is either 4 or 8 ohms (though there may still be some old 2 ohm creakers out there). The higher the impedance of the speaker cabinet, the more resistance to power it will offer. The lower the resistance of the speaker cabinet, the less resistance to power it will offer. In other words,

HIGHER IMPEDANCE MEANS LESS POWER CAN ENTER THE SPEAKER CABINET. LOWER IMPEDANCE MEANS MORE POWER CAN ENTER THE SPEAKER CABINET.

You may be thinking that you’ve found the solution to the universe—just use speaker cabinets with really low impedances and you can get skull-crushing power out of your amplifier, right? Wrong. There’s a catch. Power amps have limits as to how low an impedance they can drive safely. This is what’s known as an amplifier’s “Minimum Impedance Rating.” If you try and operate a power amp below its minimum imped- ance rating, it will give you lots and lots of power for about five minutes… and then overheat, short out and fail completely. In other words, THE LOWER THE OPERATING IMPEDANCE OF THE AMPLIFIER, THE HOT-

TER IT WILL GET.

Power Amplifier Minimum Impedance Ratings

Here’s what this means to the power amp in the Workingman’s 8004. As mentioned previously, the 8004 contains a mono power amp, which makes things fairly simple. The Minimum Impedance Rating of the Workingman’s 8004 is 4 ohms. This means that you can safely connect:

•One 8 ohm speaker enclosure = 8 ohms total impedance

•Two 8 ohm speaker enclosures = 4 ohms total impedance

•Two 16 ohm speaker enclosures = 8 ohms total impedance

•One 4 ohm speaker enclosure = 4 ohms total impedance

The owner’s manual that came with your speaker cabinet should state its total impedance. On SWR speak- er enclosures, the total impedance is generally indicated on the speaker’s input panel.

(For an even more in-depth discussion of impedance and power rating issues, go to the SWR web site at www.swrsound.com, click on “Press,” then click on “Articles,” then click on “Plug and Play–Setup Tips for Amps and Speakers”—an article by SWR founder Steve Rabe that ran in the August ’92 issue of Bass Player Magazine.)

Power Delivery Capabilities (Power Ratings)

After determining how the number of cabinets you wish to run will affect the total operating impedance, you need to take into account the power handling capabilities of your speaker cabinets as compared to what the Workingman’s 8004 can deliver at that impedance. Those ratings are as follows:

750 Watts @ 4 ohms

450 Watts @ 8 ohms

So if you have two 8 ohm speaker cabinets, they will each get up to 375 watts of power, and more during transient peaks. A single 4 ohm cabinet will get 750 watts of power, and again, more during peaks. Make sure your speaker cabinet(s) can handle the power!

12 • WORKINGMAN’S 8004 T/O/P