This is ultimately easy, but before explaining the process I feel it is necessary to provide a warning. Multiple sets of speakers usually can not be hooked directly to a standard audio amplifier without some sort of impedance matching device. This is in reference to those persons whom might want to run speakers in several rooms at the same time (distributed audio). If several sets of speakers are run from one set of speaker terminals the amplifier will usually overheat and shut down, and may blow the output stage (see footnote 1). These remarks do not apply to PA style amplifiers with 25 or 70 volt outputs, which require special speakers with transformers.
The correct solution is to use either an impedance matching speaker selector with the protection enabled, or use impedance matching in wall volume controls. Notice the underline in the sentence above. This is because most speaker selectors are made with a dangerous feature: a button, right in front, to disable the protection. If the switch was in back to prevent accidental deactivation of the speaker protection it would be much better. If the protection is accidentally switched off while running multiple pairs of speakers the amplifier will shut down, may blow output fuses, and very well may damage the output stage of the amplifier. There are really only 2 reasons to turn this switch off, the most relevant being that impedance matching volume controls are being used on ALL pairs of speakers. The other reason would be if only one pair of speakers are being run, making impedance matching unnecessary. In this event, though, leaving the protection switched in will make only a very small difference to the sound, so why not leave it on?
Remember it this way: only put one speaker per pair of terminals (usually red and black) on the amplifier. Do not try to use a surround amp to feed several rooms with one room on the center, one room on the rear surrounds etc. This is due to the way a surround receiver distributes the sound as you may end up with only the voice in one room and only the music in another! The correct hookup for a surround receiver puts surround sound in the main room and sound from the left and right main speakers is distributed. My recommendation for hooking up a surround receiver is as follows. Run the speaker selector from the front left and front right outputs on the amplifier. Hook your front left & right speakers to the first speaker switch on the speaker selector. You will need to re-balance your surround system by running the pink noise test as the speaker selector will decrease the output to the left and right speakers by a small amount. This allows running the main speakers & the other speakers connected to the speaker selector without one set being louder than the others. If your speaker selector has volume controls, you need to make sure when you use your surround system for movies the volume control is at the same setting it was when doing the pink noise test. You may hook the speaker selector to the 'b' speaker switch on the amplifier if speaker volume balance between your main left & right speakers and the rest of the speakers is not an issue.
Another variation is amplifiers with a direct speaker output for zone 2, 3, etc. These are set up to drive 1 pair of speakers, and must be used with impedance matching if more pairs are to be used. The zone outputs allow a second (or third etc) source, for example CD in one room and radio in another.
An impedance matching speaker selector provides multiple outputs from one input, and protects your amplifier from damage. Speaker selectors come with 4-12 outputs. As long as your amp has enough power, you can push as many sets of speakers as you want. Simply connect the speaker selector to your 'A' (or 'B') outputs and the rest of your speakers on the speaker selector. You can purchase speaker selectors with volume controls for each individual speaker. Another option is in wall impedance matching volume controls, which require no speaker selector. Most of these are set with jumpers at install time, providing the correct matching. If you want to run more pairs of speakers than the speaker selectors or volume controls are made for (usually 12 pairs max. depending on the hardware) you probably want a second amplifier to run the second set of volume controls (or speaker selector) from.
So, what is impedance and impedance matching? (Warning: semi technical material ahead)
The music signal to your speakers is called alternating current (or AC), because it varies polarity and voltage. This is in comparison to a battery, for example which produces a steady, or direct current. You may picture current as the amount of water flowing in a pipe (the wire) and voltage as the water pressure. Alternating current can be imagined as a flow that reverses direction and direct current as a steady flow in one direction. The analogy is not exact but is close enough to get a picture of what is happening. Standard house current in the US reverses direction (polarity) at an interval (or frequency) of 60 times per second, measures as 60 Hz (Hertz). If you visit our site you can see this article with explanatory diagrams included.
Your speakers have a certain amount of resistance to current. Imagine the resistance as a constriction in the pipe, limiting the flow. They have a DC resistance, termed the voice coil resistance, and resistance to AC is called impedance. Resistance and impedance values are measured in Ohms. Impedance is a complex sum of dc resistances, plus the resistance to various AC frequencies caused by capacitance and inductance (normal properties of electrical and electronic devices). It is usually specified for speakers as nominal impedance, and is referenced to particular frequencies . However, Just think of it as resistance to AC for practical purposes. This is usually rated at either 8 or 4 Ohms. Most home amplifiers prefer an 8 ohm impedance. Each time another speaker is added in parallel the impedance is reduced. Visualize several pipes connected together to the same pump, obviously the flow from the pump increases (up to the limit of the pumps capability). The amplifier is the pump. Two 8 ohm speakers reduce the impedance to 4 ohms, four 8 ohm speakers reduce the impedance to 2 ohms, and so forth.
An amplifier expects (most require) a certain amount of resistance to current flow. The lower the impedance, the more current flows through the output stage of a typical amplifier. This usually flows directly through a transistor (or other amplifying device) and damages the transistor or protective resistors in the output stage. If you get lucky it only blows an output stage fuse. The moral of the story is always use an impedance matching speaker selector, (or volume control) and your amplifier will always see a safe impedance load.
1 (footnote)
Why does the amplifier output stage blow? This is due to the nature of a typical amplifier. It must first be clear that an amplifier does not actually make the input signal bigger. What it does is recreate a larger (higher current and voltage) copy of the original signal, and puts that out to the speakers. It makes this copy from the power supply voltage, which is derived from the AC from the wall outlet. This voltage must be first converted to DC. If not done efficiently the amplifier will hum (because it doesn't know the words). The amplifier controls the power supply voltage by a sort of valve that controls the output voltage and current. These valves (transistors are usual, but may be integrated circuits, vacuum tubes, or other devices) are controlled by the input signal. In response to the input signal, the amplifying device allows more or less current (and/or voltage) flow through it to the output in an exact (hopefully) replica of the input signal. In order to have the majority of speakers work well with an amplifier, it needs a low output impedance (see the text above for a discussion of impedance). If the output impedance is too high, the frequency response will vary with the impedance of the speakers. This means that some sounds will be accented and some diminished from the levels they should be, and this effect will be different with every different set of speakers. In practical terms this means that, with the most commonly used circuit design, when the transistor (or other device) is fully on, it needs to pass almost all the power supply current available through it. This is the tricky part. The current flow is determined largely by the impedance of the speakers connected. Most amplifiers are designed to work with 8 ohm speakers, and may work into 4 ohms reasonably well. Some so called high current amplifiers may work well into 1 ohm. In other words, the amplifiers are designed that when the output transistors are passing the full current and voltage they can take, they are working into an expected value of impedance. When the impedance (ac resistance) of the connected speakers get too low, more current is allowed to pass through the amplifying device than it can take. It either burns up, or blows a fuse, or burns up emitter resistors, or some other form of damage occurs. Fusing often does not react fast enough to save the circuit. So why not design all systems to work into low impedances? Without going too far into it, this can result in huge cost increases. For example, for theoretically ideal performance into most of todays speaker systems, an amp should double it's power for every halving of impedance. So for 100 watts into 8 ohms, it must put out 200 into 4 ohms, 400 into 2 ohms, and 800 into one ohm(Remember,this is theoretically ideal, and almost no amplifiers can accomplish an actual doubling of power for a halving of load impedance). Most decent amplifiers today have significantly less than .5 ohms output impedance, so as to perform well with speakers that may drop to low impedances at some frequencies. Our theoretical amp, if made to operate into a .5 ohm load would need to have the capability to put out 1600 watts per channel without damaging anything (in the amplifier!). As you can imagine the cost of a good quality amplifier with this kind of capability is substantial. Now think about this: The average user of this massive amp, with 8 ohm speakers, will only see an average output wattage of 100 watts RMS (RMS stands for Root Mean Square, and is basically an average power measurement which is approximately 70 percent of peak power). For most users this would price the amp out of reach. Some amps are built to this type of standard, for the reason that there are speakers that drop as low as one ohm that sound much better if driven by a very high current amplifier. One such example is the Krell Evolution One, which is a monoblock. This means you need two for stereo. They will only set you back about ,000 dollars each. For those who might think I am making this up, use google to find prices on Krell monoblocks.
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