SWR POWER 750 Bedienungsanleitung Seite 10

GROUND LIFT
A ground lift is provided for the Balanced Inputs. In the normal (out) position of the push switch, Pin 1 on the XLR connector is grounded. In the Lift
(in) position, the ground on Pin 1 is disconnected. When using the Balanced Inputs, always be aware of the position of the ground lift switch. This
function only affects the Balanced Inputs and does not affect the Unbalanced Inputs.
SLAVE OUTPUT
A 1/4" Slave Output (some call it a "patch-through") is provided in the event you want to "daisy chain" more than one power amp at a time. To use
the Slave Output to run an additional power amp, run a shielded patch cord from the Power 750 Slave Output to the input of the additional power
amp. Whatever signal is coming into your Power 750 will be available at the Slave Output, where the signal can be sent to the additional power
amp of your choice (preferably ANOTHER SWR Power 750!). The Volume controls on the Power 750 do not affect the level of the Slave Output.
The Slave Output can also be used as an unbalanced record out or a monitor send.
SPEAKER OUTPUT SECTION
The following section of the manual will deal with the proper connection of speaker cabinets to the power amplifiers in the Power 750. 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 cabinets are all crucial in determin-
ing how best to operate your new Power 750.
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 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 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 RATINGS
Here's what this means to the power amp in the Power 750. The Minimum Impedance Rating of the Power 750 is 4 ohms. This means that you
can connect:
a) one 4 ohm cabinet
b) two 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 (NOT 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:
10 • POWER 750