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Why You Should Not Use Your Home Theater Amp For Whole-House Audio


by Jeff Fisher

OK, we get this all the time. Nearly every customer that is putting in a single-source multi-zone whole-house audio system expects to use their main home theater amplifier to drive the speakers in other rooms.

First let me say that I'm not writing this to sell more amplifiers; we sell very few amplifiers anyway...and they are a very low margin item. The truth is that, on installs that I've been involved with, and feedback from customers that have completed their install and are living with it, whenever the home theater amplifier is used do drive speakers in the rest of the house, it just doesn't work out very well.

Consider the following diagram.

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Zone 1 is the home theater. All the home theater speakers are connected to the amp in the normal way. (5 to 7 channels plus 1 or 2 subwoofers.) Now, most amps have a "B" or "Zone 2" output with two speaker terminals. Why not simply connect speakers from the other rooms to this output? Different amplifiers drive this "B" or "Zone 2" output differently. First, you have to determine which way your amp drives these spare outputs.

How does your amp drive the spare outputs?

In Parallel With Front Channel Speakers

The amplifier probably has button(s) to select "A", "B", and "A+B" speaker outputs. Here are some of the problems you can have with this setup:

  1. Impedance Matching When the A + B mode is selected, the amplifier connects the home theater speakers in parallel with the whole-house speakers. The impedance of a properly configured whole-house speaker system will be between 4 and 8 ohms. But since this is connected 'in parallel'' with the front (A) channel speakers (presumably 8 ohm speakers) the resulting impedance could be anywhere between 2.7 to 4 ohms. (Use the handy speaker impedance calculator I found here.) Most amplifiers cannot handle less than a four ohm load. I've seen many amplifiers set up like this go into "protect" mode when their output stage overheated. (And I've seen more than one amp go permanently off-line.) So, what if my amplifier is "2 ohm safe"? Then the impedance might not be a problem, but problem #2 below will be even worse. OK, you say, "I'll just never select the A+B mode." Really. Even by accident? And you'll never want to listen to anything in the home theater room when you have the whole-house speakers on?
  2. Channel "A" Volume When the A+B mode is selected, you have a master volume control (on the amp) for the whole-house audio speakers, and (depending on your setup) volume controls on the speakers in other parts of the house. You do not now have a volume control for just the home theater speakers. Because the audio power is split many times to drive the whole-house speakers, the power setting on the master volume control will have to be relatively high. The home-theater speakers, without such energy depleting power splits, will receive the full glory of the amplifier output and will be intolerably loud. If you turn down the master volume control until the home theater speakers are at a reasonably level, the whole-house speakers will be barely audible.

Using Zone 2 Outputs

Does your amplifier have a "Zone 2" or whole-house output of some kind? These outputs have become a common feature on newer amplifiers. But the feature can be implemented in many way, most of which are useless. It is up to you to determine how your "Zone 2" is implemented and how it would work to drive a whole-house audio system. Here are some questions to ask about your "Zone 2" support:

  1. Can the source be selected separately? This is the cheapest feature to implement. Together with shared outputs (number 3 below), it costs almost nothing to implement and lets the manufacturer list this cool "feature". If you cannot select the source separately on Zone 2, the same material will play on the whole-house speakers that is playing on the home theater. Imagine the whole-house having to listen to Dumbo just because the kids are watching it in the home theater.
  2. Can the volume be controlled separately? Does the main volume affect the Zone 2 volume? The answers to these questions will determine how useful the Zone 2 feature is. A separate volume control will let you set the "master" level for the whole-house speakers. But if the main volume affects it, it will screw up the whole-house setting.<br/>
  3. Does Zone 2 steal two amplifier channels from the home theater? Many amplifiers, when Zone 2 is turned on, disable two channels of the home theater. Essentially, it disables either the side surround or rear surround speakers (turning a 7.x system into a 5.x system and a 5.x system into a 3.x). This lets the manufacturer provide a Zone 2 feature without actually building in two more amplifier channels. Obviously, if this is the case, you need to realize that you're loosing two speakers in your home theater. And you probably can't switch back and forth, there might only be one set of speaker terminals. When setting up and wiring the system, you have decide if you're going to use those two channels for surround sound or for Zone 2.
  4. Do IR signals have to come in from a port in the back? I've seen systems that require and infrared pickup, connected to a jack in the rear of the amplifier, to control the Zone 2 features. The features cannot be controlled from a remote in front of the amplifier. When this is the case, the IR codes that change sources and volume are the same as Zone 1. So using a remote through an IR distribution system would not work as intended. Nor would an automation system be able to control Zone 2 without some extra hardware.
  5. What front panel controls exist to work all these features? Usually, none. In order to change sources or volume on Zone 2, you'll have to use the remote.
  6. How easy is it to access Zone 2 features from the remote? You wouldn't believe how difficult some remotes make it.
  7. Does Zone 2 have enough power for your whole-house audio system? A rule of thumb is to add up the average wattage rating of all the whole-house audio system speakers (only count one channel) and divide by 2. Does each channel of Zone 2 have at least that high of a continuous wattage rating?
  8. Remember also that standalone audio-only amplifiers have steadily decreased in price over the years. Are you convinced? See this article for information on setting up a single-source whole-house audio system.


Jeff


   

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