<b><font size="+4">Speakers of the House



Copyright 1991, Rick Chinn. All rights reserved.


This article was originally published in Audio/Video Interiors


Magazine.

Introduction

If you've only got two pairs of speakers in your home audio system, then the A-B switch on your receiver is probably more than adequate. But what if you've got 10 pairs? What about impedance matching? What about remote volume controls?

As you'll see, you can't just connect the 10 pairs of speakers up and have things work. They may, for a while, but sooner or later, something will go wrong (the resident teenager has a party and tries to listen at concert volume) and you'll need to send your amplifier or receiver to the hospital.

Even if you manage to make it work, there's the problem of volume control. Running back to system central to make an adjustment will get old quickly, even if you've got help from an accomplice at the remote location. The obvious thing is to install some sort of volume control at the other end. In days past, you went to the electronics store and bought an "L pad." Unfortunately, high-wattage L pads are difficult to find, and lower wattage units tend to burn out. Worse, they turn amplifier power into heat that you paid good money for, a function better reserved for your furnace. There are several common systems for distributing audio throughout a home or other building. We'll touch on four of the more common ones here: parallel distribution, multiple amplifier, constant voltage (70-volt), and low level. Wired remote systems, such as those made by Niles Audio and Audio Access are beyond the scope of this article. Suffice it to say that these systems use a combination of parallel distribution and multiple amplifiers. If you don't understand these terms yet, read on...by the end of the article, you will.

Why bother?

The most basic audio distribution system consists of two speakers connected to an amplifier. From here on, we'll only consider one of the stereo channels...the other channel is a duplicate of the first. In this system, the amplifier must be able to drive the parallel combination of the two speakers. If each speaker is 8 ohms, then the amplifier sees 4 ohms. Most amplifiers can drive a 4 ohm load without difficulty...but what if we decide to expand the system? Let's try four speakers on one amplifier.

Four 8-ohm speakers connected in parallel present a combined load of 2 ohms (calculate by dividing the number of speakers by the quantity) to the amplifier. With rare exception, most home amplifiers will not tolerate this much of a load without distress (read smoke). Every amplifier made has voltage and current limits at its output. Reaching the voltage limit is also known as clipping. If this were a perfect world, amplifiers would not have current limits. This would allow connecting an infinite number of speakers. As the load impedance drops, the amplifier's power output rises.

For example, an amplifier rated at 100 watts/8-ohms would deliver 400 watts into 2 ohms. Into a 1/2-ohm load, the same amplifier would deliver 1600 watts! Sadly, we live in an imperfect world, and amplifiers have current limits, which determines the minimum load impedance that may be connected to the output terminals. By now, you can probably see the need for a better way. Indeed, there is.

An Overview

Previously, I mentioned four basic systems: parallel distribution, multiple amplifiers, constant voltage, and low level. Before getting into the fine details of each, let's take a brief look at each one.

Parallel distribution systems are an extension of the A-B switch found on most receivers. Quite simply, every speaker in the system is connected in parallel and thence to the amplifier. As pointed out earlier, the major disadvantages are: the rather abnormal load presented to the power amplifier (there are ways of dealing with this), the wire size required to keep wire loses to a minimum, and volume changes as various zones/rooms are switched on and off. A major advantage is simplicity.

Multiple amplifier systems get around the abnormal load impedance problem very simply (and somewhat elegantly): give each load its own amplifier. This could take the form of a stack of integrated amplifiers located at system central, or a card cage (rack) of amplifier boards located in the garage. This is an effective solution with only a few drawbacks: the cost of the multiple amplifiers, the space required, wire cost, and somewhat higher system complexity. The major advantages are: the lack of interaction between different rooms or zones, and the ability to equalize each zone separately. In this case, equalization helps smooth out the overall frequency response curve. Since the acoustics in each zone are probably different, it makes sense that each zone will require separate equalization. In all fairness, the cost of the multiple amplifiers may be only slightly higher than the price of one large amplifier with sufficient power to drive the entire house.

Constant voltage systems operate much like the power company does in your home. At home, you plug appliances into a single circuit, until the circuit breaker or fuse blows, then you back up one appliance, right? In a constant voltage system, you think of each load in terms of the number of watts it consumes. The total of all loads (speakers) must not exceed the amplifier power. The disadvantages: a slight increase in complexity, a certain amount of thought is required, some golden-eared types might object to the presence of transformers in the signal path, and the extra cost of the transformers (one required per speaker). The advantages: lower wire cost, the ability to set maximum sound levels at each speaker location, the ability to connect a large number of speakers to a single amplifier, and a relative lack of interaction between the volume settings in different speaker locations.

Low level systems are an extension of the multiple amplifier system with one big difference: each zone has its own amplifier and that amplifier is located at the zone (instead of system central). Advantages: lower wire cost since the wiring to the zones is low-cost shielded cable and easier zone programming since the zone may have its own local sources. Disadvantages: the extra cost of multiple amplifiers (which again may be only slightly higher than the one monster amp needed to run the whole house).

Fine Details

Now that you have an idea of what each system is, we'll get down to nuts and bolts and describe the pros and cons in more detail.

Parallel Distribution

Without a doubt, parallel distribution is the simplest system to install. Within its limits, this system works well, doesn't require much thought on the part of the user, and is relatively inexpensive. The limitations are: maximum number of speakers that may be connected at once, maximum volume level at each speaker, variations in volume when individual speakers are turned on or off, and higher wire expense than other systems.

The impedance problem

As discussed earlier, connecting speakers in parallel reduces the overall load impedance presented to the amplifier. Unfortunately, this reduction in impedance means that the amplifier must work much harder to drive the load. Connecting a load such as this directly to an amplifier is an invitation to disaster (unless the amplifier was specifically designed for it).

If you ever took electricity in school, then you know about series and parallel circuits. If not, then think back to the last time that you fiddled with a string of Christmas lights. Have you ever had a string that wouldn't light? After searching for the bad bulb, you replace it and voila, the entire string comes to life. This is a series string of lights. If the circuit is broken, anywhere, the entire string goes out.

You can connect loudspeakers in series, which beneficially raises the impedance presented to the amplifier. You can also use various combinations of series and parallel (series-parallel). Sorry, there's no free lunch here. The series connection makes each loudspeaker interact with its electrical neighbors. The largest audible change is in the bass, which may become boomy and/or tubby. If the speakers aren't the same, then the ones with the lowest impedance will be the loudest. A series-parallel connection may be suitable for speakers mounted in a hallway which just need to be audible, but you only want to count them as a single speaker. Four speakers can be connected in series-parallel and will have a total impedance that is the same as one of them.

So you ask, "How do the commercially made switchers such as the ones made by Adcom and AudioControl get away with this?" Easy. They insert a resistor in series with the parallel connected remote speakers that increases the total impedance to one that the amplifier can safely drive. The price you pay for this is a certain portion of your amplifier power being used to heat this resistor. This isn't quite as onerous as it may seem. This is a very viable solution to the problem and with amplifier power being relatively inexpensive these days, the cost of heating the protection resistor is relatively low.

The interaction problem

Another problem faced by parallel distribution systems is that of interaction. This means that the volume of the remote locations will change as individual zones are switched in or out. The worst case is going from one zone on to all zones on. With 8 ohm speakers and a typical six- zone switcher, this can mean an overall level change of 8.5 dB which is easily audible. If you keep a minimum of two zones on at any given time, then the level change reduces to 6 dB which is probably tolerable. Certainly the listeners at the remote locations can reduce the settings of their zone volume controls.

Wire size considerations

One decided disadvantage of parallel systems is wire size. A reasonable rule of thumb is 1 dB of loss due to wire losses. Remember that wire has resistance, and this resistance is in the path between your amplifier and your speaker(s). Since the wire's resistance increases with decreasing wire size (increasing gage number), minimizing wire loss can be a significant cost factor (larger wire means larger cost).

For a typical 50 foot run, with 8-ohm or 4-ohm speakers, here are some sample figures:

wire gage awg	wire resistance (ohms/1000 ft)	wire resistance (ohms/50ft)	loss (dB) 8-ohm load	loss (dB) 4-ohm load
24	26.17	2.62	2.46	4.38
18	6.51	0.68	0.68	1.31
16	4.09	0.41	0.43	0.85
14	2.58	0.26	0.28	0.55
12	1.62	0.16	0.17	0.34

Note: The wire resistance for the 50 foot run is double what it may seem that it should be because there is actually 100 feet of wire involved...50 going, and 50 coming.

The simplicity factor

As discussed before, parallel distribution has one strong suit: simplicity. This is by far the simplest and easiest system to wire. If your house is pre-wired, then anyone who can connect a pair of speakers to a receiver should be able to install a switcher and connect it to their receiver or amplifier.

Multiple Amplifiers

If you like the appearance of a mountain of equipment, then this is probably the system for you. In a nutshell, the multiple amplifier distribution method overcomes the loading problems and impedance protection losses of the parallel distribution method by using a separate amplifier for each zone. Usually, this takes the form of a medium sized (30-50 Watts/channel) integrated amplifier (an integrated amplifier combines a preamp and a power amplifier, but no tuner). Ten speakers or ten zones means ten integrated amps.

Selecting inputs may or may not be a problem. In simple systems, it is probably sufficient to tap from the main system at its tape output jacks, which are unaffected by the main system's volume and tone controls. A reasonable alternative might be a separate preamp or preamp/tuner. This way, the distribution system in the house can receive one program while the main system has something entirely different. By selecting the input having the tape output from the main system, the distribution system now mimics whatever is being played at system central.

An impedance problem of a different sort

In the parallel distribution system, the problem that had to be overcome was that of the combined impedance of all the speakers being much lower than most amplifiers are comfortable with. Multiple amplifier systems have a similar problem, except that it is the load created by all the paralleled amplifier inputs. This could create a load that most preamps are uncomfortable with driving. The solution is simple: a distribution amplifier, which has one input and many outputs.

Wire size considerations

This distribution method has the same limitations as the parallel distribution method. The distribution wiring is at speaker level and impedance and subject to the same size limitations.

Flexibility options

Now, there's no reason why you couldn't combine a multiple amplifier system and a parallel distribution system and have the best of both worlds. For instance, have separate amplifiers for the master bedroom, main listening room, and den. Now use a larger amplifier and an impedance-protected switcher to handle the two remaining bedrooms, central hall areas, kitchen, patio, and front entry.

Constant Voltage

How would you like to be able to connect your speakers in parallel to your amplifier, be able to switch them on and off in any combination, be able to preset the maximum volume level at any location, and never worry about impedance problems (once the system planning is finished)? If your answer was yes, then read on...a constant-voltage system may be in your future.

What is it

Basically, a constant-voltage system takes the tactic used by the power company with the 110V wiring within your house. When you plug a lamp into the wall, do you worry about the impedance being presented to Grand Coulee? No way! What you do worry about is whether you'll pop the breaker because of the 1800 watt metal-halide lamp in the greenhouse that happens to be on the same circuit. You add wattages, and if they are less than about 2400 watts (120 volts times 20 amps), then you're golden. Constant-voltage audio distribution systems do the same thing with loudspeakers. Add the wattages, and if they don't exceed the rating of the power amplifier, then you're golden again.

How does it work (ok, where's the magic??)

Constant voltage systems get their name because they operate at a constant line voltage. In the United States, the most common voltage is 70 volts, but 25 volts is another common voltage. What this means is that the output voltage of the amplifier is 70 volts at full output. What this also means is that the impedance that the amplifier will drive goes down for more powerful amplifiers and up for less powerful ones. A couple of short examples should help here:

A 30 watt amplifier delivers 70 volts into its load at full output. What is the impedance of that load? Using Ohms law, R = E2 / P. 70V X 70V / 30W = 163 ohms. Stated another way, a 163 ohm resistor with 70 volts across it dissipates 30 watts.
A 612 watt amplifier delivers 70 volts into its load at full output. What is the impedance of the load? R = E2 / P. 70V X 70V / 612W = 8 ohms.

The important fact here is that if you hold the voltage constant, then you must change the impedance to change the power. This is the key to the whole thing. Since 70 volt systems are most common, we'll use the term "70 volt" instead of constant voltage. Remember that this applies to all constant voltage systems, 70 volt or not. Lest you think that this is something new, it's not. 70 volt systems have been in common use for over 40 years! In a 70 volt system, you have an amplifier(s) at system central, whatever switching is needed, and a speaker equipped with a 70 volt line transformer at each location. The switching system is just on/off switching. The line transformers are marked in watts; you select the connections that deliver as many watts as you wish to the speaker. Small, under 5 watt transformers are quite inexpensive while larger ones capable of handling more power are more expensive. Still, 10 watts is probably about right for most in-wall speakers (if the speaker has a sensitivity of 90 dB/1 watt/1 meter, then 10 watts is 100 dB / 1 meter, or 90 dB at about 9 feet). During the system-planning phase, you distribute your amplifier power according to the needs of the location: 10 watts to the bedrooms(4), kitchen, den, and living room, 2 watts each to the hallway speakers, 15 watts to the patio, 5 watts to the front walkway, and 5 watts to the entry speakers. If you have 5 stereo pairs of hallway speakers and one stereo pair per room elsewhere, then you're going to need a 105 watt amplifier to make things run. Only got 100 watts? Simple...reduce the patio to 10 watts, or reduce the front walkway and entry speakers to 2 watts each. Remember, it doesn't matter if you come out under your amplifier's rated wattage.

Wire size considerations

In a 70 volt system, the speaker impedance is magnified by the line transformer (the real secret to 70 volt systems is calculated, pre- meditated impedance mis-matching!). Thus, it takes much more wire resistance to cause 1 dB line loss. Translation: you can use smaller wire.

Zone level tailoring

If you guessed from the lead-in to this section, one of the beauties of this method is the ability to preset volume levels at each speaker location. Furthermore, if you want a local volume control, install one, but after the 70 volt line transformer. Even if you use an L-pad, the worst that will happen is the L-pad soaking up as many watts as the line transformer is connected for.

Amplifier Considerations

Not all amplifiers are equipped for 70 volt operation. In fact, most consumer-grade amplifiers aren't. Aside from buying a commercial/rock-n- roll grade amplifier that has an integral 70 volt output, you can do either of the following and use a consumer-type amplifier:

Use a step-up transformer connected to the amplifier's output to increase its output voltage to 70 volts at full output. Many manufacturers make these available for their amplifiers (although not too many consumer manufacturers do). You must get a model with sufficient power capability to handle the full output of your amplifier as well as the proper turns ratio to ensure the proper voltage step up.
Operate the amplifier in mono-bridge mode. If the amplifier can deliver about 612 watts to an eight-ohm load, then its output voltage is around 70 volts. No transformer is needed at the amplifier end (although you still need the transformers at the remote speaker end).

A plea for sonic sanity

You may have noticed a certain 11-letter word throughout the preceding paragraphs: transformer. You know, 2 coils of wire wound together on an iron core? Transformers have been blamed for many audio ails and some of them are deserved. Yes, you get what you pay for, and generally speaking, cheap transformers are that way because the manufacturer skimped on the core material (iron), and that means substandard bass response. Any dyed- in-the-wool purists may be retching by now and that's fine...for them. I'm not advocating that you put the Martin-Logan's in your main listening room into a 70 volt distribution system. What I am advocating is a bit of objectivity and practicality in putting the less critical parts of your system into a 70 volt system. Again, remember that you don't have to put the ENTIRE house into the system. Just remember that for some locations (hallways, entry-ways, walkways, bathrooms, etc.) a 70 volt system may be just what the doctor ordered since you don't need enormous amounts of power at any time, and the system isn't likely to be critically listened to. This plea for sonic sanity also applies to wire. If you believe in pedigreed wire, fine...save it for the Martin-Logan's. If you want to use it in the walls for the 70 volt system, fine...it's your money...but plain- vanilla twisted-pair stranded wire will probably work every bit as well. Are you really going to hear the improvement in soundstage when you walk down the hall?

Low Level Distribution

Low level distribution is really just a twist on the multiple amplifier method. Instead of making a mountain of amplifiers at system central, you put an amplifier into each room/zone. The local amplifier doesn't need to be huge...30 to 50 watts/channel should be more than adequate. If you buy small receivers instead of integrated amplifiers, then the local user has the option of a tuner (set to their favorite station) or whatever is on the main system. The wiring to each room is just shielded cable, usually shielded twisted pair, and each location just taps off of the cable. Really elaborate systems may have several cables run through the house so that the remote locations have the pick of several programs. With 6 cables run, each location could have the pick of two CD changers (one with classical music, the other with pop), or whatever is running on the main system.

What are the pitfalls?

Not many, really. The biggest one is driving the cable. You need a distribution amplifier to do this. The tape output of your preamp won't do this well at all, and you really need isolation between the main system and the distribution system. Another pitfall is ground loops. If you're careful, you can run the distribution system unbalanced and get away with it. At a minimum, its a good idea to have a transformer coupling into the distribution line. This way the grounding at the remote locations is much less likely to affect the main system. If the grounding becomes a problem, then you'll probably have to treat the distribution line as a balanced line and perform a balanced- to-unbalanced conversion at each remote location.

Volume Controls and Switchers

No discussion of remote audio distribution systems would be complete without a discussion of volume adjusting devices.

L-Pads

An L-pad is nothing more than a special type of adjustable resistor. It differs from the common potentiometer because it presents a constant, unchanging load to its source in spite of its setting. A potentiometer presents a changing load to its source whose value depends on the shaft position. L-pads are usually found in loudspeaker crossover networks as balance controls (like the tweeter level control in a 2-way system). Since they present a constant impedance to the crossover regardless of setting, the crossover's characteristics are less likely to be altered by the setting of the control. Nevertheless, both devices convert the unused portion of the audio signal into heat. In a simple parallel distribution system, an L-pad is a poor choice for a zone volume control because it must be able to withstand the amplifier's full output power. If the amplifier was rated at 100 watts, then you'd need a 100 watt L-pad! Such units are difficult to find, and expensive. In a 70 volt system, the L-pad is a good choice for a zone volume control because it only dissipates the wattage assigned to its zone. It also has the advantage of being almost infinitely variable (no click- steps).

Auto Transformers

An auto transformer is a variation on the more common two winding transformer. In this case, both the primary and secondary are part of the same winding. Auto transformers are popular as zone volume controls because they don't convert the unused audio signal into heat. When used as a volume control, intermediate taps on the winding are brought out to a rotary switch, which selects one of the taps for connection to the loudspeaker. The closer the tap point is to the input terminal, the lower the loss through the auto transformer (translate: louder). Auto transformers suffer from many of the same problems as transformers. Not enough iron results in poor low-frequency response, or distortion at high levels. On the other hand, actual units measured in a lab setting revealed a lesser dependence on physical size (iron content) for reasonable performance. I wouldn't hesitate using one of these units as a zone volume control.

Switchers and Impedance Protection schemes

A seemingly simple part of most in-home systems is a speaker switcher. From the outside, it doesn't seem like anything could be much simpler. If only it was... For speaker level distribution systems, the primary difficulty is one of impedance. When all speakers are switched on, the resulting load is too great (impedance value too low) for most amplifiers to drive. Trying to do so to an amplifier that wasn't specifically designed for such loads results in distortion, overheating, and/or smoke. Several manufacturers make simple switchers that incorporate an "impedance protection" feature. In most cases, this takes the form of a large power resistor. The resistor connects in series with the speakers and prevents the total impedance from falling below what is safe for most amplifiers. The advantage of this scheme is utter simplicity. The disadvantage is power loss. This is discussed more fully in the previous section on the parallel distribution method. If your system is a 70 volt, or constant-voltage system, then you can use a simple switcher with no impedance protection resistor. This is one of the big advantages of constant-voltage distribution.

Conclusion

As you can see, there are more than a few ways to wire your home for sound. To sum things up, the following chart relates all of the systems described to each other. In the chart, each plus counts as one point, each minus counts as -1 point. A superlative in a category counts as 2 points (i.e. lowest, cheapest, best ...). The "score" is the sum of all the pluses and minuses.

Method	Pluses	Minuses	Score
parallel	simplicity # of speakers do-it-yourself lowest cost	wire size amplifier limitations	2
para. /w switcher	simplicity do-it-yourself # of speakers low cost	power loss reduced damping factor wire size needed interaction	0
multiple amplifiers	moutain of gear flexibility expandability different progams in each zone possible	mountain of gear cost wire size needed distribution amp needed	0
low level	best flexibility exapandability relative simplicity low wire cost highest fidelity	probably highest cost possibility of ground loop trouble	2
constant voltage	simplest switching zone level tailoring good use of amp power no interaction lower wire cost	transformer fidelity marginally higher complexity	3

As you can see, there is no clear-cut winner. Every system has its strengths and weaknesses. Every system has something to say for itself. Your choice of a system should be driven by cost, needs, and simplicity considerations. For most systems, especially those that you install yourself, the parallel system using an impedance protected switcher is probably the winner. Larger systems, especially those with many zones are good candidates for a constant voltage system. Cost-no-object systems are probably best served by low level systems with locally located amplifiers.