HOW IMPEDANCE AFFECTS POWER RATINGSHOW 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 off the mark.
Unlike power amps, every speaker cabinet has a pre-determined impedance rating measured in "ohms." In most cases this rat-
ing is either 4 or 8 ohms (though there may still be some older 2 ohm models 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 impedance rating, it will give you lots of power for about five minutes, then overheat, short out
and fail completely. In other words, the lower the operating impedance of the amplifier, the hotter
it will get.
POWER 750 POWER AMPLIFIER MINIMUM IMPEDANCE RATINGSPOWER 750 POWER AMPLIFIER MINIMUM IMPEDANCE RATINGS
Here's what this means to the power amp in the Power 750. The Minimum Impedance Rating of the Power 750 is 2.6 ohms.
This means that you can connect:
a) two 8 ohm cabinets
b) one 8 ohm and one 4 ohm cabinet
c) three 8 ohm cabinets
Damage to the power amplifier in your Power 750 may occur if speaker enclosures with total
impedances less than the minimum loads listed above are connected to the speaker output section.
The owner's manual that came with your speaker cabinet should state its total impedance. On SWR speaker enclosures, the total
impedance is generally indicated on the speaker's input panel.
So how do you determine the total impedance of two cabinets hooked up to your Power 750?
Here's a quick key of the most common setups:
One 8 ohm enclosure + one 8 ohm enclosure = 4 ohms total impedance (OK)
One 8 ohm enclosure + one 4 ohms enclosure = 2.6 ohms total impedance (OK)
One 4 ohm enclosure + one 4 ohm enclosure = 2 ohms total impedance (NOT OK!)
Here's another formula: To figure out the total impedance of two or more cabinets of equal value hooked up in parallel, divide the
impedance of one cabinet by the number of cabinets:
Impedance of one cabinet / number of cabinets = total impedance
(For an even more in-depth discussion of impedance and power rating issues, go to the SWR Website 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.)
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