550x • 19
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 resis-
tance 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 and lots of power for about five minutes…and then over-
heat, short out and fail completely. In other words, THE LOWER THE OPERATING IMPEDANCE OF THE AMPLIFIER, THE HOTTER
IT WILL GET.
550x POWER AMPLIFIER MINIMUM IMPEDANCE RATINGS
Here's what this means to the power amp in the 550x. As mentioned previously, the 550x contains a mono power amp, which
makes things fairly simple. The Minimum Impedance Rating of the 550x is 4 ohms. This means that you can safely con-
nect:
- One 8 ohm speaker enclosure
- Two 8 ohm speaker enclosures
- Two 16 ohm speaker enclosures
- One 4 ohm speaker enclosure
The number one enemy of any power amp is heat. We have listed the minimum impedance of the 550x at 4 ohms so that the
extraordinary amount of power available in this two-rack-space configuration is not compromised by exposing the power amp
to the additional heat that operation at 2.6 ohms or 2 ohms would generate. (SWR recommends that, when rackmounting your
550x, leave at least one open rack space to allow for proper ventilation and cooling.) Consequently, damage to the power
amplifier section of the 550x 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 550x? Here's a quick key of the two most
common setups:
One 8 ohm enclosure + one 8 ohm enclosure = 4 ohms total impedance (OK)
One 8 ohm enclosure + one 4 ohm enclosure = 2.6 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 "Products", then click on "Technical 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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