A Re-Volting Idea for Speakers
Steve Lampen
In the Aug.
15 issue, we began a discussion on 70-volt distributed loudspeaker
systems.
This approach uses transformers inside the power
amp, and on each speaker, to change the impedance of the system
so the resistance of the wire has much less effect.
In background music systems, the use of transformers
is valuable because you have different "zones" where you
will want speakers at different sound levels. These 70-volt transformers
on the speakers have "taps" for different power outputs.
So the lobby of a business could be set "low,"
and the warehouse set at "high." Then you adjust the overall
level with the controls of the amp, but the speakers will be at
the same relative level.
The downside is the added cost of the transformers.
In addition, you have to buy an amp with a 70-volt output.
In the past, I have used a regular amp and wired
the 8-ohm output into a very large 70-volt transformer, turned around.
Now they are hard to find and pretty expensive, but a 50-watt or
100-watt 70-volt transformer can still be cheaper than buying an
amp with the appropriate output. If you are using a stereo amp,
you need a transformer for each output channel.
The secret is to add up the little wattages of
the transformers on each of the speakers and make sure that the
total is no larger than the transformers on the output of the amp.
In the background
While you can run stereo, especially if you use
a stereo amplifier as suggested above, this is rarely done. Most
of the time, these systems are run monaural, with identical program
material running into both channels.
After all, we’re talking background music
here, not high-end audio. Of course, that doesn’t mean you
couldn’t have higher quality audio, and run things in stereo.
I have heard some amazing performance from 70-volt
systems, with the right speakers, of course. Naturally, the quality
of the reproduction is related directly to the quality not only
of the speakers, but of the transformers as well.
It is pretty hard to find 70-volt transformers
that rise above the "background music" quality. In addition,
if you do run stereo, the placement of speakers can be difficult,
or impossible, in most office, studio or similar commercial installations,
especially if you are using ceiling-mounted speakers. This would
require a listener to be standing in exactly the right location
to even get the stereo effect. Stick with monaural.
You can easily tell that background music speakers,
such as ceiling mounted speakers, are intended for 70-volt systems
because they have a mounting plate on the speaker perfect for the
little transformer to feed it. The better speakers also have a back
baffle or "can," to give a bit of acoustic loading to
the speaker and improve bass response.
And once you have your amplifiers, speakers and
transformers, you’re ready to wire it up. And here’s the
real joy of 70-volt systems.
First, you just run all the speaker lines in parallel.
And 24 AWG wire (i.e., telephone wire) will get you 500 feet with
almost no loss (0.5 dB, 11 percent).
To see how far you can go with a 70-volt system
and different gage wire sizes, check out www.belden.com/products/catalog/help/techspeaker.htm.
Unless you plan to go miles, you can use just
about any wire you want, with a couple of cautions.
Considerations
It’s a sure bet that most of your speakers
will be ceiling-mounted. That drop ceiling probably is a plenum
area. Then it’s a question whether your fire inspector or planning
board subscribes to the National Electrical Code.
If they do, then plenum cables will be required
in that plenum space. Of course, you could use the cheapest plenum
cable, even 24 AWG plenum telephone cable, and your 70-volt system
would work just fine.
The second consideration is the voltage rating
of the cable you are using.
If 70 volts exceeds the rating of your cable,
then you can change the entire system to a 25-volt distributed system.
This means an amp with a 25-volt output, and 25-volt transformers
on every speaker.
However, a 25-volt system cannot go a far as a
70-volt system, and the transformers are much harder to find.
If 70-volt won’t get you far enough, 100-volt
distributed loudspeaker systems also are available. This requires
much higher voltage rating in the cable, but can go very far.
Unless you are wiring up a factory that is many
blocks long, it is doubtful if you would ever need this option.
Transformers for this application are much harder to find and are
expensive.
This is the final word, at least from me, on resistance
and speaker cable.
So let’s examine the next parameter from
the list a couple of columns ago, capacitance.
Capacitance
Any time you have two metal conductors separated
by an insulator, you have a capacitor.
As you probably know, a capacitor can hold an
electrical charge. The one really bad thing about capacitance is
that it is frequency-based – that is, the effect of capacitance
gets worse as the frequency rises.
This effect is called "capacitive reactance,"
and you might recall the formula from your electronics classes in
school.
Capacitance runs in parallel, "across"
the cable. So if you know the capacitance per foot, you simply multiply
that number by the footage. This is yet another reason why short
speaker cables can outperform long cables: lower capacitance.
If you pull out any wire and cable catalog and
go to the speaker cable section, one of the things you will note,
which you may not have noticed before, is that capacitance is rarely
listed.
Why? Because it is probably very low. Even low-grade
zip cord runs around 20 pF per foot. And the larger the wires, the
more insulation is extruded over them, so the capacitance never
gets much larger.
At even the highest audio frequency (20 kHz),
the reactance is pretty small. In fact, we could go almost 17,000
feet before we were only 1 dB down due to capacitance. So most catalogs,
and most installers, just ignore the capacitance.
Now, if you really must know, you can always call
the cable manufacturer and ask them what the capacitance of a specific
cable is. I’m sure they know. They just didn’t think you
cared.
In our next column, we’ll talk about inductance,
impedance, skin effect and copper purity.
Steve Lampen is technology specialist, multimedia
products for Belden Electronics Division in San Francisco. His book
"Wire, Cable, and Fiber Optics for Video and Audio Engineers"
is published by McGraw-Hill. Reach him at shlampen@aol.com.
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