WHAT IS A 70 VOLT SOUND SYSTEM?
[a.k.a. Distributed Sound or Constant Voltage]
Authored by
OK! .... So you're it, you're the one, you're
the guy or gal that's been given the job (voluntarily or otherwise) of
repairing, installing, or expanding that 70 volt P.A. system at work or
at church. Or maybe you've finally decided to install that home
background music system you've been thinking about for years, and you've
heard that a 70 volt sound system is the way to go, but you don't know
much about it.
Well here it is, the info you need to help you decide
if a 70 volt system is right for you, or to help you get through the
repair of, installation of, or addition to, a small scale 70 volt sound
system. We will be covering the following topics:
(NOTE: a proper explanation will require some overlap, therefore, these
topics will not necessarily be covered in separate sections)
■ What is a 70 volt sound system?
■ How is it different from a home stereo system?
■ What are its advantages and disadvantages?
■ What features are available?
■ How do I design a home or small business system?
■ How do I troubleshoot problems in a 70 volt system?
**** Disclaimer ****
Let's get this requisite legal stuff out of the way
first. Although a concerted effort has been made to be accurate
and complete with the following information, this overview is not
intended to be a substitute for a full education and apprenticeship in
the electrical and related trades. This article is for informational
purposes only. Orvac Electronics, it's owners, agents, assigns, and
employees are not responsible for any loss or damage resulting from the
use or misuse of this information. It is the readers responsibility to
shoulder all risks with regard to safety, building & fire codes,
permits, licensing requirements, proper use of tools and equipment,
suitability of parts, tools, equipment, information, and procedures for
any particular endeavor, and all other risks.
**** Let's define some boundaries ****
This article IS ABOUT the typical background music
system found in most small to medium sized businesses. It is a (usually)
mono amp driving from a few to many speakers to provide a steady flow of
music, and/or informational narrative, and/or occasional live paging, as
in "Attention owner of a red Chevy Malibu - your lights are on".
Or perhaps a lecture or sermon needs to be piped to ANOTHER ROOM for
monitoring or to handle overflow crowds.
This article IS NOT about sound reinforcement, as in
singers, preachers, musical instruments, or presenters that need to be
amplified so that the people IN THAT ROOM can hear them better.
There's a degree of similarity in equipment and purpose, but the rules
are quite different! With sound reinforcement, things like
phasing, choice and position of microphone, number and placement of
speakers, thickness and type of floor covering, drapes, ceiling tiles,
the shape of the room, and amplifier equalization all need to be taken
into very serious consideration, or a system that looks good on paper
can turn into a nightmare. Rampant feedback and unintelligible voices
can flummox all but the most experienced sound technicians. If you
find yourself in the latter situation, seek help. If you must do this on
your own, you will find many of the equipment manufacturers' web sites
helpful, along with the rest of this article. Try to apprentice with a
working professional for at least six months. Offering your
assistance on a volunteer basis may help you get a foothold in this very
restricted access occupation. If, however, you are in the former
situation, and need only a brief primer on 70 volt systems for
background music or paging or occasional announcements, then put on your
thinking cap, grab a cup o' joe, and read on!
**** OK, lets get started ****
The best way to describe a 70 volt sound system to
those unfamiliar with it is to back up a few steps and start with a home
stereo system and its limitations. On a normal home stereo
amplifier (sometimes called "receiver" or "amp") with connections for
just one left and one right speaker, you can attach one pair of speakers
and you're fine. You can add on an additional pair of speakers and
you're probably (?) ok. But somewhere around the second or third
set of speakers, you can bet that you'll be dealing with an over-heated
amp, blown fuses, or worse (a smoke filled room)! All this leaves
us with a burning question. How do places like Sears or Dodger Stadium
use an amplifier to drive dozens of speakers (or more)? Obviously
they do something differently.
**** The Basics ****
A good analogy is the following: Hoover Dam does not
send electricity to Los Angeles at a voltage level of 120 volts (the
voltage at your normal wall outlet). The electricity makes most of
the journey, about 300 miles, as a much higher voltage. Then, just
outside your house, (in Los Angeles, as elsewhere) there is a large
transformer on a pole that converts that high voltage down to the normal
120 volts that we use in our homes. This high voltage line from
Hoover Dam feeds one transformer, then goes on down the block to feed
the next transformer, then the next, and so on, in "daisy chain"
fashion. This high voltage line can feed dozens, even hundreds of these
transformers. Although not always the case, we will assume each
transformer feeds only one house.
Think of these transformers as power taps. As
part of the process of converting the voltage, they siphon off only a
portion of the available power from this high voltage feed line. This
line may be capable of delivering 200,000 watts of power and each
transformer may tap off about 10,000 watts. If you do the math,
you will see that this high voltage line can feed only about 20
transformers. If more transformers were to be added, then the feed
line would be overloaded, causing blackouts or other problems.
However, if each transformer were adjusted to tap off
only 1000 watts, then up to 200 transformers could be attached to this
one feed line. Keep in mind here that these "adjusted"
transformers would be delivering only 1000 watts to your home (barely
enough to run a toaster) not 10,000 watts, as before. You don't
get something for nothing! We are merely redistributing the available
power to a different number of homes. Yes, I know, in real life
commercial electric power makes several transformer step-downs before it
reaches your home. I've simplified things a bit here for the sake of
demonstration.
A 70 volt sound system, consisting of an amplifier
feeding many speakers, works much the same way. The sound is sent
out from the amplifier on a high voltage feed line at about 70 volts.
- (duh!) - The speaker voltage from your home stereo, for
comparison, rarely exceeds a dozen or so volts. This 70 volt line
arrives at the first speaker, where there is a transformer that taps off
only a small amount of the available power (watts) coming from the
amplifier. The 70 volt feed line can then "daisy chain" along to
all the additional speakers, each with its own transformer.
These transformers are nearly always adjustable so
the installer can set them to tap off a pre-determined amount of power
(watts), usually done during the installation process. This
adjustment is usually accomplished by connecting the feed line to
different pins or wires on the transformer, commonly called taps.
Less often it is done by selecting a switch position using a knob or a
screwdriver. The limiting factor here is the same as with the
electrical power line. That is, the combined wattage draw of all the
speakers (by speaker, I'm referring to each speaker/transformer set)
cannot exceed the wattage capability of the amplifier.
**** Review this important point: Combined power draw
of all the speakers (in watts) cannot exceed the power available from
the amplifier (in watts).
For the non tech savvy person it might be helpful to
explain that when talking about sound, "power" means how loud the volume
of sound is, and this volume (power) is measured in watts.
Why the high voltage? The short answer is: The
laws of physics show us that the transfer of electrical power over a
long distance is more efficient at a higher voltage. The long
answer is....... well, very long! Let's just say that many of the
advantages of the 70 volt system would not be possible without this high
voltage.
**** The Cool Stuff ****
And now for some really cool stuff. There is no
need for all the transformers to be adjusted to the same wattage
setting. For example, one 70 volt feed line could daisy chain
around to four speakers, two in a warehouse area, and two in an office
area. The transformers on the warehouse speakers could be set at
10 watts, while the transformers on the office speakers could be set at
2 watts. [see fig 1] This way the warehouse speakers would
play loud enough to be heard over the sound of machinery, while the
office speakers would play softer, so the office workers wouldn't be
jumping out of their socks every time an announcement is made. By the
way, these wattage settings are typical for a small 70 volt system.
**** Review this important point: The speakers
in different areas can have their transformers adjusted to provide power
as needed for that particular area. For example: INDOOR/OUTDOOR ...or...
ROOM#1/ROOM#2/HALLWAY.
NOTE: The 70 volt line does NOT have to go to
the higher powered speakers first. It can go to the speakers in
any order, usually determined by the logistics of laying out the speaker
wire.
NOTE: A "dead end" 70 volt line [no speaker at
the end] has no effect. There is no need to "terminate" the end of a 70
volt line, as there is with T.V. coax.
Now for some more cool stuff. That 70 volt feed
line can be split to supply sound to two or more different areas, or
zones, each zone with one, several, or even dozens of speakers.
Splitting the 70 volt feed line is electrically the same as a continuous
"daisy chain" with total wattage limitations being calculated the same
as before. But with a split feed line, each zone can now have its
own independent volume control, installed somewhere along the branch of
the line that feeds just that group of speakers.
For example, a restaurant manager would be able to
adjust the volume of music in each of three dining rooms independently,
with each room containing from one to several speakers [see fig 2].
These volume controls are usually mounted in the wall like a light
switch. These "in-wall" controls are not able to boost the volume
of sound. They only act as restrictors, allowing from zero% to
100% of the original sound to pass through. Don't confuse these
controls with the volume control on the amplifier, which is left at its
normal setting and remains unchanged.
**** Review this important point: Different zones can
have independent volume controls. The number of speakers in each
zone can vary.
NOTE: Volume controls for a 70 volt system are NOT
normal speaker rheostats. 70 volt systems require 70 volt volume
controls! And these volume controls have wattage ratings which
must be at least as high as the total of all the wattage settings of the
down-stream speakers that each one controls. Also note that some
flush mount and "hang-on-the-wall" 70 volt speakers are available with
built in volume controls. These controls adjust the volume at that
speaker only.
NOTE: In the drawings I've shown the
transformers as separate pieces. In real life the transformer is almost
always built into, or attached directly on, its associated speaker.
Also, each line representing wiring is, of course, 2 wires.
NOTE: When a single feed line goes out from the
amp to speaker #1, then jumps over to speaker #2, then to speaker #3,
and so on, that is called DAISY-CHAIN wiring, as in [figure 1].
When each speaker or zone has its own separate line coming from the amp,
that is called HOME-RUN wiring, as in [figure 2 ]. Often these wiring
methods are mixed in the same system.
And even more cool stuff: Another nice facet of
the 70 volt system is that when it comes time to add one or several more
speakers, you can run a new line from the amp, OR tap into
the existing 70 volt feed line AT ANY POINT to get the sound for
your new speakers! Don't forget to confirm that the amplifier has
enough watts for all those additional speakers!
**** Amplifier Features ****
The amplifiers for these systems are available with a
wide range of output power and other features. Output power
ratings range from a low of about 5 watts (useful at a small fast food
drive-thru) to upwards of 500 watts (large warehouse with parking lot).
If more power is needed (Dodger Stadium) then several amps can be
stacked with each amp driving many speakers. The average small
business with up to 6 speakers (all indoors) should not need more than
35 watts. If up to 3 outdoor speakers, or 5 indoor speakers are
added, then double the power. For more than that, consult your
contractor or hardware salesperson.
Nearly all amps have provisions for at least one mic,
some have built in mixing controls for six or more mics. Some amps
come with a radio tuner built in so that a local radio station can be
used as background music. Many also have provisions for adding
your own music or other program material from an external source, such
as a CD player, i-Pod, satellite radio, etc. Often available too is a
special connection for the paging feature on your telephone system.
This last function is, in fact, why most businesses buy a P.A. system!
Most amps have an adjustment or a small switch that
can be set so that when an announcement is made, the background music is
muted (reduced in volume) temporarily. Some amps can be triggered
to play a pleasant musical tone preceding announcements, and some have a
siren type sound that can be played in case of fire or other emergency.
Some of the more expensive amps have multiple outputs that can be
engaged or disengaged separately and remotely, so that announcements can
be made only to selected speakers or zones. And some can even
override those remote "zone" volume controls so that announcements are
always played at full volume!
**** Acoustics ****
One more area we should touch on here is "acoustics"
(the study of sound waves and how they interact with their environment).
Even if all the numbers are right, if not enough speakers are used in a
given area, or a speaker is in a corner when it shouldn't be, or is not
in a corner when it should be, or there is too much bass, or not enough
treble, then the quality of sound can rapidly go down the tubes. Two
speakers on a wall could be aimed AT each other or AWAY
from each other. Which is best? These and related questions have
to do with acoustics.
Unfortunately, to add a full dissertation on this
subject would require another fifty pages or so, which is way beyond the
scope of this article. This ties in with why we are not including
"sound reinforcement" in this discussion. For now, let's just say
this: Place the speakers evenly with ceiling speakers not much
farther apart than the ceiling height. Wall speakers (generally)
should not face each other. Avoid placing mics and speakers too
close. Try not to skimp on the number of speakers, as too few
speakers can cause dead spots. (dead spots are not good!) Too few
speakers can also tempt the operator to crank up the volume, which can
worsen any tendency toward reverberation, distortion, echoes, and
feedback.
If the client indicates that rich, full, robust music
is a high priority, then DON'T choose the cheapest speaker
available. A two-way speaker (sometimes called coaxial) with a
larger magnet would be a good choice.
NOTE: During your adventures in 70 volt land, you may
encounter equipment labeled 25 volts, or 100 volts. These are
simply the little brother and big brother versions of what we have been
studying. Don't let these throw you. The rules and the math are
pretty much the same. The size of the system and local building
codes will often dictate one over the others, with the 70 volt version
being by far the most popular. Some amps have several or all of these
settings.
NOTE: Can regular home stereo speakers be used
in a 70 volt system? Yes. 70 volt transformers are available
separately and can be attached on the outside or inside of the speaker
cabinet, or any nearby surface, and wired according to the label on the
transformer. Presto, you have a 70 volt speaker!
While we are on the subject of transformers,
note that a 20 watt transformer tapped at 10 watts will give
better sound than a 10 watt transformer tapped at 10 watts. [ Little
Known Secret! ]
**** Putting it all together ****
If you are selecting equipment for a new system,
evaluate your needs carefully. It can be difficult and expensive
to change equipment and/or wiring in the middle of an installation job.
Questions regarding any of the above, or how much power you need, number
and placement of speakers, what kind of wire, what type of mic, etc.
should be discussed with your contractor or hardware salesperson.
A knowledgeable salesperson can be worth their weight in gold, GET
THEIR PHONE NUMBER !
If you've been following along with us, and have a
rudimentary background in electrical wiring, you should now be able to
"rough sketch" a workable 70 volt sound system that will meet your
expectations. Some valuable hints here. Always select an amp
with way more watts than you need. We recommend a 30% safety
margin. I know it seems like a lot, but it is cheap insurance and
provides plenty of head room for future growth. You will see lots
of charts for speaker wire selection. Let's keep it simple.
For amps under 100 watts and wire runs under 100 feet, 16-2 wire will be
fine. To handle more power or longer runs, the slightly thicker 14-2 is
called for. (Yes, the smaller # is the bigger wire.) Be sure
to pay attention to polarity (plus and minus) when connecting the
speaker wire. And most important of all: Read and understand
all instruction manuals COMPLETELY before beginning.
**** Problems with existing systems ****
If you need to troubleshoot problems in an existing
system, designed by who knows and installed by who cares, you have my
sympathies. The system may be suffering from not only the symptom du
jour, but may also have been on the verge of a breakdown since day one!
Finding the bug in a system like this can be quite daunting. However,
there are some basic checks you can do before calling in the pros. In
the following text I will be using the term: "OBD". It means the
equipment in question will: Overheat, Blow fuses, or Destroy itself.
Take your pick!
■ If the combined wattage settings of all the speakers
exceeds the wattage capability of the amp .....OBD!
■ If someone adds more speakers to the system
without checking to determine if there is enough
wattage available from the amp .....OBD!
■ If there is a short in the 70 volt feed line .....OBD!
■ If someone turns the "bass" control up too far .....OBD!
Turning the bass control up causes the transformers
to draw much more power than they should,
over-loading the amp. After any installation or service
call that I'm involved in, I instruct the client to leave
the bass control all the way down. This results in pages
that are easier to understand and improved equipment
longevity. However, music will sound a bit "tinny". A
compromise may be in order here. Just be cautious.
■ If someone adds an extra speaker to the system without
a transformer .....OBD! (That's a very common one)
■ If someone throws a magazine or paper on top of the
amp, blocking the air vents .....OBD! (Also very common).
■ If someone rearranges the office and places the amp
where it can't get plenty of air .....OBD!
■ If someone installs or replaces a ZONE volume control
and wires it incorrectly .....OBD!
■ If someone adds a new speaker to the system that is
too close to the microphone .....OBD! Even if the
numbers look good, a speaker too close to the mic can
cause feedback
squeal, and feedback squeal can
cause..... you guessed it .....OBD!
If the background music is ok but the announcements
are distorted, weak, or missing, the mic or its wiring, or the
connectors at either end, could be damaged or loose. Don't forget to
check the mic's level control on the amp. (The term "volume
control" usually refers to the main or master control, often the largest
knob on the front of the amp. The terms "level control" and "gain
control" are usually interchangeable and refer to the knob that adjusts
THAT ONE INPUT ONLY.) Pepsi, kitchen spatter, lipstick,
saliva, coffee, sweat; these are all arch enemies of the microphone.
Keep the mic as free from these as possible. Soft foam mic covers
are cheap and work well. Use them and replace them often.
If the reverse is true (mic ok, music from radio bad)
possibly the antenna is loose, damaged, fallen down, or disconnected.
Or perhaps the dial has been tuned off frequency. Or the station
may have gone off the air. (I actually had a service call for that
once!) Don't forget to check the radio's gain control. If the
background music is from an in-house source, such as a CD changer or
i-Pod, first check the gain control for THAT input, then see if
that source plays ok through a different amp, or, if available, through
its own headphone jack.
If everything sounds good when first turned on in the
morning, but worse as the day progresses, the amp is probably going bad,
only a heartbeat away from ....OBD! Certain parts, like
transistors, when they fail, may still function properly when cold, but
go bad as they warm up to operating temperature. Check to see if
someone has covered the amp's air vents, or has turned up the bass.
If it can be done without too much difficulty, disconnect most of the
speakers from the system. If the symptoms clear up, or the volume
in the remaining speakers goes up noticeably, you almost certainly
have an over-load problem. This trick works much of the time, but
not always.
If the trouble can be cured by replacing a blown
fuse, only to have it go out again in a few days or so, then you have
either an amp suffering from borderline over-load, or an intermittent
case of one of the above OBD scenarios. Intermittent problems can
be a nightmare. Check for loose connections, frayed wires, nails or
staples through wires (more common than you might think), wires rubbing
on sharp metal edges, wires rubbing on other wires, and burned or melted
wires in addition to all the OBD items mentioned above. On the
back of the amp, loose strands of wire touching something they shouldn't
(like other wires), is a very common cause of trouble. You may
need a magnifying glass here. I'm talking about one or two of
those tiny hair-like strands of wire that may come loose from the main
bundle of strands that make up the core of the wire.
Asking if there have been any recent changes or
additions to the system will often reveal important clues.
EXAMPLES: Just one or two speakers added to the system may
have pushed the amp past its breaking point, OR recent remodeling
or other construction may have pinched or severed some of the wiring,
OR you may discover that reports of suddenly diminished audio volume
coincide exactly with a recent mic replacement. In the latter case, the
mic could be defective (yes, the new mic!) or the wrong type, or wired
incorrectly)
Also don't overlook the possibility that the amp is
acting up on its own, and the rest of the system is actually ok,
especially likely if the amp has been in service for many years.
Remember, correct the cause of the problem BEFORE
installing that new amplifier, or you could have..... you guessed it
.....OBD. (Again!)
Note: Although somewhat rare, a few 70 volt systems
will appear to have the feed line going to the speakers directly,
WITHOUT a transformer. These speakers are specially made for
this application, and have an ohms reading of 50 ohms, give or take,
(while a regular speaker is less than 8 ohms). Don't forget to
disconnect the speaker from the rest of the system to get an ohms
reading. Although some installers frown on it, substituting or
mixing these speakers with regular speaker-transformer sets should not
be a problem.
When the speakers in an already existing system are
impossible to get to, how do you tell what wattage they are set at and
if the system is over-loaded? This is tough. Without certain
technical information and some rather pricey test equipment, you could
be in a pickle here. If you strongly suspect that the system is
over-loaded, and you have not been able to identify the specific cause,
you may need to call in a sound system professional, as the test
equipment to do this is rarely available on a rental basis.
The cure for each of the above defects should be self
evident. That is if you can find the defect! Remember, even
experts don't always find the bug in the system with their first guess.
The following is probably the most important point I can make: stay
calm, use common sense, think it through. Check the easy stuff first:
Switches, Volume/gain controls, Loose wires, Faulty mics, Dead tuners,
Blown fuses. Is everything turned on? Are all the power cords plugged
in?
Keep in mind that a brief over-load or short in the
wiring, even after those faults are corrected, may leave the amp with
permanent damage, resulting in the system still not functioning
properly. Also keep in mind that an amp can go bad without first
giving off smoke or overheating. If checking all the above
mentioned items doesn't reveal any good leads, then it might be a good
idea to remove the amp from the system and have it checked out
independently. Any qualified repair shop should be able to do this
for a nominal charge. If you feel that you are in over your head,
call a professional. Remember, the smartest people on the planet
are the ones who know how much they don't know!
**** Stereo ? ****
The 70 volt sound systems we have been talking about
here (so far, all in MONO) work great in the home as well as the
office. However, these systems are rarely available in STEREO.
So if you want stereo sound as background music, you should consult your
contractor or salesperson regarding a product that's rapidly gaining
popularity. It's called an "8 ohm impedance matching stereo volume
control". These controls are a little tricky to set up and
install, but if done correctly, they will give you many of the benefits
of a 70 volt system, (up to 16 pairs of speakers, sometimes more, each
pair with its own volume control, while allowing you to use a standard
home stereo receiver as the source.) This type of system is
usually limited to residential use because of the lack of a microphone
input. But we'll have more about these "non 70 volt systems" in a
upcoming addition to the Orvac web site, expected around Sept of '09.
**** Time and Space ****
Time and space constraints have forced us to cram a
lot of information into a very small article. If you've had
questions about 70 volt sound systems, I hope we've been able to give
you some useful answers. If you are still puzzled by some aspect
of this subject, please feel free to forward your question to our web
site. We'll do our best to get you the information you need.
I must hasten to add, however, that trying to troubleshoot a
specific set of symptoms over the phone or over the internet rarely gets
the results you hoped for. If your problem was a simple one, and this
article has helped you solve it, then great! If things get tough,
remember: Sometimes there is no substitute for a qualified technician,
on site, with meter in hand!
**** And Finally ****
Well, there it is. The official "Orvac Short Course In 70 Volt Sound Systems." We sincerely hope it has been entertaining as well as informative. Please let us know what you think!