SWR Sound Bass Combo Amplifier - Manual (Page 14)

if someone tells you that they have a “4 ohm power amp,” their terminology and understanding of the con-
cept 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 impedance 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 HOTTER IT WILL GET.
BLACK BEAUTY POWER AMPLIFIER MINIMUM IMPEDANCE RATINGS
Here’s what this means to the power amp in the Black Beauty. Like most combos, the Black Beauty contains
a mono power amp, which makes things fairly simple. The operating impedance of the internal speaker
system in your Black Beauty is 4 ohms, so that’s your starting point. When you add an extension cabinet to
the Black Beauty, invariably the total operating impedance of the amplifier will get lower. The Minimum
Impedance Rating of the Black Beauty is 2 ohms. This means that you can safely connect to the extension
speaker jacks:
• Two 16 ohm speaker enclosures (2.6 ohms total impedance)
• One 8 ohm speaker enclosures (2.6 ohms total impedance)
• One 4 ohm speaker enclosure (2 ohms, the minimum total impedance allowed)
These three hookups will provide between 400 and 450 watts of power, but as previously mentioned, the
lower the operating impedance, the hotter the amplifier will run. (2 ohm operation in particular will run the
amp quite hot.) An amp that runs at or near its minimum impedance all the time may wear the life of its
components faster than normal. Furthermore, damage to the power amplifier section of the Black Beauty 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 Black Beauty? Here’s a
quick key of the most common setups:
One 4 ohm enclosure (internal) + one 8 ohm enclosure (extension) = 2.6 ohms total impedance
One 4 ohm enclosure (internal) + one 4 ohm enclosure (extension) = 2 ohms total impedance
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.)
BLACK BEAUTY POWER DELIVERY CAPABILITIES (POWER RATINGS)
After determining how the extension cabinet you wish to use affects the total operating impedance, you need
to take into account the power handling capabilities of that speaker cabinet as compared to what the Black
Beauty can deliver at that operating impedance. Those ratings are as follows:
450 Watts @ 2 ohms
400 Watts @ 2.6 ohms
So if you use an 4 ohm speaker cabinet, it will get up to 225 watts of power, and more during transient
peaks. An 8 ohm extension cabinet will allow the power amp to deliver 400 total watts of power (and again,
12 • BLACK BEAUTY OWNER’S MANUAL