SWR Amplifier POWER 750, Owner'S Manual

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POWER 750 • 7
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 determining
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 under-
standing 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 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 speak-
er 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 enc one 8 ohm enclosure = 4 ohms total impedance (OK)
One 8 ohm enc one 4 ohms enclosure = 2.6 ohms total impedance (OK)
One 4 ohm enc one 4 ohm enclosure = 2 ohms total impedance (NOT OK!)