How to distribute multiroom sound
Dec 1, 1997 12:00 PM, Bob Heil
There are several methods of providing audio in various rooms of the house. Some get expensive and complicated, but for most applications a simple, high-quality audio feed is all that is necessary.
An audio distribution system is a low-level background music system originating from a single source and feeding loudspeakers in various parts of the house. These are usually kitchen, garage, utility room, bathroom, den, library and dining room. An audio distribution system is not of the type suitable for Madison Square Garden, nor is it a critical-listening system for audiophile listening. There's really no need for super high-fidelity stereo sound in these areas. Your client may not want remote loudspeakers for bedrooms, but rather, completely independent systems. This will be especially true of teenagers' bedrooms.
Many of the audiovisual receivers produced today have what their manufacturers call multiroom/multisource functions. This usually allows the two rear-channel amplifiers, usually 10 W or 15 W each, to be switched out of Pro-Logic mode and used to drive the rest of the house. Not every reader may agree, but I believe strongly installers should not use these functions. For one reason, consider this. One member of the family is watching a full-bore theater presentation when another pops in a compact disk. The theater sound system loses its rear channels suddenly, a very discombobulating experience. Another reason these channels seldom work satisfactorily is that 10 W or 15 W is insufficient for most purposes. How long a wire can such power drive? How loudly will the remote loudspeakers play? What impedance problems will you have powering multiple remote loudspeakers?
This last point is worth a further word if you don't want a Fourth of July fireworks festival in your customer's living room, or if you want to avoid driving over there and playing smoke doctor at 11 p.m. Power amplifiers are generally designed to play into 8e or 4e, seldom less. Because you may want six or eight areas of the house to have distributed audio, all those loudspeakers in parallel across the amplifier output may present a load of 1e or less. Trying to drive all that with the audiovisual receiver's rear channels is a lost cause, no matter how much it cost or who built it.
One of the most cost-effective methods of distributing audio to multiple loudspeakers, and doing so over long wires, dates from the 1940s. Is Bob Heil prescribing grandfather's technology? One thing I've always tried to do in my electronic career is look at things from a price-vs.-function standpoint. Is there something available that works well and is affordable? Is there something anyone can operate? These are important questions for all electronic designers to take into consideration.
This audio distribution system is known as the constant-voltage system. The amplifier output and the loudspeakers all have their impedances raised to a much higher value, and thus audio is distributed throughout the house at higher voltages. These are known as 70 V systems or 25 V systems. No, we don't actually feed 70 V or 25 V down the line. Through the use of impedance-matching transformers we are able to allow the amplifier to look into aproper load. Because the power is transmitted at higher voltages, it takes less current for the same power, so losses in the wire are greatly reduced.
Let's say we want to use a 100 W amplifier to drive 10 loudspeakers at 10 W each. Here's a typical setup:
* Amplifier. Select an amplifier with a 100 W, 70 V output. Many amplifier companies that make amplifiers for residential installation also make amplifiers for commercial installation, and you'll probably be able to buy one of those units if you need to. The amplifier may have a 70 V output only, with no transformer. That's the least costly way to go. Other units may have built-in output transformers, allowing you to select 4 e, 8 e or 70 V. You can turn any amplifier with a 4 e or 8 e output into a 70 V amplifier by buying an additional, outboard matching transformer, 8 e, 100 W to 70 V, 100 W.
* Loudspeaker. Mount and connect a small matching transformer to each loudspeaker, 70 V, 10 W at the primary and 8 e, 10 W at the secondary. Some loudspeaker manufacturers offer matching transformers attached and wired at the factory as an option.
* Wiring up. Run the 70 V line from the amplifier past each loudspeaker and simply connect the loudspeaker-transformer assemblies across this line.
The beautiful part of this distribution system is that you can have as many loudspeakers as you desire, so long as you don't exceed the output power rating of the power amplifier-transformer combination. For example, you can have 10 10 W loudspeakers or five 20 W loudspeakers driven from the same 100 W source. Be careful to select efficient loudspeakers, those that produce a lot of acoustic power for not much electrical power. You'll be able to provide a terrific amount of sound pressure level throughout the house. You can provide volume controls in each room without fear of damaging the power amplifier when they are all turned off or on.
Yes, there are some down sides to this type of distribution system. Transformers, like everything else, can be made cheap but poor or good but expensive. Only the biggest and most expensive transformers will be able to pass much power below 70 Hz, and some will have trouble above 12 kHz. For a background music system this is not a big problem. If you want to push the low end a bit, find a closet as close to the center of the house as possible and install a self-powered subwoofer, something like the Pioneer SW 1000 or those made by Velodyne or Altec Lansing. Because bass frequencies are omnidirectional and easily transmitted by the structure of the house, you will be amazed at how low frequencies will be felt in almost every room of the house. It certainly fills out any background music system and with the simplicity, low cost and expandability of a constant-voltage audio system, it should be a must for every new home being built.
There's an altogether new way to attack this problem. In recent years several companies, led by Xantech, have developed an automatching volume control that allows up to 7 pairs of loudspeakers to be driven by an ordinary amplifier. The proper impedance matching is automatically done right within the volume control. This is the answer for most residential audio distribution systems, period! There just isn't any need anymore for the four-button speaker control boxes filled with transformers. We are headed for the year 2000!
Returning to those audiovisual receivers, I highly recommend that 4 e loudspeaker systems of high efficiency be used and that you connect just one to each channel of the receiver. If you need more sound-pressure level, you must do three things:
1. Purchase more high-quality loudspeaker systems.
2. Use additional amplifiers driven from the pre-amplifier outputs of the receiver.
3. Try to find a very rich aunt to pay for all of this.
When you need multiple, remote loudspeakers, use a separate amplifier just for that and use constant-voltage transformers or else the automatching type of volume control. That's how to keep unintended pyrotechnics and smoke effects out of your installation.
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