Constant voltage (100 volt) type distribution systems.

Constant voltage (100 volt) type distribution systems.
Constant voltage (100 volt) type distribution systems.
Constant voltage distribution systems have been a source of confusion for many people for a long,
long time. It seems that the more technical the person, the more complicated he perceives a constant
voltage distribution system. Once learned, constant voltage systems are really not very complicated at all,
but rather just "simple solutions to a very complex problem." Most good solutions are just that...simple.
Constant voltage systems rely on the simplicity and easy usage of line matching transformers. Before we
proceed with this; however, let's create a little scenario...
Imagine a rather simple distributed "sound system" installation. The requirements are to install 10
speakers in the ceilings of a small office building for background music and paging. Although nothing is
formalized, there must be provisions for some of the speakers to "play softer" than others in parts of the
office building. An enterprising technician might just take on this "simple" project and set about to figure
the arrangement he could use to connect 10 speakers. Without a specific knowledge of constant voltage
distribution system techniques, he first takes ten 8 ohm speakers and mentally connects them all in
parallel..... "hmmmmm: 8 divided by 10 or 0.8 ohm load...what kind of an amplifier will I need to drive that
low an impedance?" Next, he connects them mentally in series...'"hmm: 8 times 10 or 80 ohms
load...what kind of an amplifier will drive that impedance effectively? How about some series/parallel
combination...we'll try 'series-ing' them in pairs and then connecting the combinations up in
parallel...hmmmm: 16 divided by 5 or 3.2 ohms load...a 4 ohm amp...great. Next question...just how much
power will this system need...well, I'll worry about that later. Now, how will I address the requirement for
changing levels at the various loudspeakers...(long time thinking)...use different impedance speakers?
Gosh...the math gets what...PUNT!!!!'
Enter the 70 volt distribution system...a knowledgeable installer has in hand a 70 volt, 10 watt line
matching transformer (Component Specialties - Model No. T-7010...$3.89 each...see Figure 1 for
transformer diagram). This particular unit has six primary wires available and three secondary wires. The
secondaries are common - 4 ohms and 8 ohms. The primary wires are common - 10 watts, 5 watts, 2.5
watts, 1.25 watts, & 0.625 watts. This transformer will be used as follows: One will be installed at each
individual speaker location with the correct secondary wire connected to an 8 or 4 ohm speaker; the
primary will be connected across a 70 volt distribution line (pair); and the particular wires used will be
ones that will deliver the desired power to the speaker. The system power amp must have a 70 volt
output, and its power rating must be equal to or greater than the "sum" of all the individual power levels
desired to be delivered to each speaker by each of the line matching transformers connected to the
power amp.
The only decision to be made in such a system is how much power is to be delivered at each speaker
location. Once this is determined, then the "tap" associated with that power level is selected, and you
simply add up all the individual "powers" to determine the power amp requirement. Superficially, that's all
there is to it, but in practice most installers "pad" the power requirement and use a larger power amp than
necessary, because they know that the transformer loading is greater than expected. Additionally, there
are power losses associated with the line matching transformers, so the delivered speaker power will be
less than expected. Nevertheless, no complicated impedance calculations or series/parallel
speakers...just simple addition. Neat, huh?...Now for some details...
As I mentioned earlier, the whole constant voltage distribution system relies on relatively "small and
inexpensive" line matching transformers, and as such is the 'secret' of the system. The concept of a
transformer for many of us goes back to usage as a kid with the "electric trains" or some other low voltage
application where the transformer was designed to plug into the wall socket and convert the AC mains
(line) to some lower value of AC. In audio applications, transformers are used to convert impedances from
one value to another. This is where they really "excel" and is the real "magic" that makes these
distribution systems work. Let's review some theory:
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Transformers only work on AC voltages...they are short circuits to DC. They raise or lower AC voltages in
direct relation to the turns ratio, and they convert impedances up or down in direct relation to the "turns
ratio" squared.
Now, back to the matching transformer diagram: It has secondary wires of common, 4 ohms and 8 ohms.
The primary wires include common, 10 watts (500 ohms), 5 watts (1 k ohms), 2.5 watts (2k ohms), 1.25
watts (4k ohms), & 0.625 watts (8k ohms). What this is saying: If you put an 8 ohm load (speaker) on the
8 ohm secondary tap (or 4 ohm load on the 4 ohm tap), this transformer, by its design, will convert the
load upward to a new indicate value (assume 500 ohms), dependent upon which primary tap you select
together with the common tap. Now, it just so happens that if you place a 500 ohm load on a constant 70
V RMS line, you will deliver exactly 10 W RMS to that 500 ohm load, and in this case you will "transform"
that 10 W RMS into the 8 or 4 ohm loudspeaker load.
Having the power rating indicated, you really don't have to get involved in the fact that the "reflected" load
is 500 ohms if you don't want to, and most installers who don't understand don't need to. This is what
makes using the matching transformer so easy. All the information is right there. If you hook up this tap to
a constant voltage line you will get this many watts into your speaker. Now add up all your watts to come
up with a minimum amp closed. Practically speaking, the transfer of power between
primary and secondary is always accomplished at a typical 33% loss. Matching transformer spec sheets
show this as an INSERTION LOSS expressed in dB. Notice I said "minimum." Let's talk about the power
amp first...
The design of a constant voltage capability in a power amplifier is not a trivial exercise, although what is
created is simply 25/70/100 output V RMS. Such an amplifier will produce a constant voltage "cleanly"
(usually less than 1 % total harmonic distortion) as a maximum output rating, just before clipping.
Generally, this voltage level is created using (of all things) a specially designed OUTPUT
TRANSFORMER. In the old days of Stubs' type power amps, large, bulky output transformers were a way
of life, needed to convert the relatively high tube plate impedances down to 4, 8, & 16 ohm levels, and the
constant voltage tap(s) was just another winding on that transformer. In contemporary amplifiers, direct
coupled, solid state designs have virtually eliminated the need for the output transformer, since they can
drive the typical speaker load directly. For a 70 volt output, for example, the power amp design usually
includes an output transformer (or AUTO-FORMER), since it often cannot produce 70 V RMS directly and
therefore it must be "transformed." We call these systems "constant voltage" systems, when they really
are audio signals with a maximum of either 25, 70, or 100 V RMS when the power amplifier is at full rated
output. To understand this voltage concept better, let's review a power amplifier rating system that most
folks are familiar with.
Assume we have a power amplifier that is rated at 400 W RMS into 4 ohms. Such is the rating for one
channel of Peavey's CS-800(TM) power-amplifier (the actual rating is 400 W RMS per channel, with both
channels operating).
In this case, one channel of the CS-800 can produce 40 V RMS, or we may call the output a "40 volt line."
Some common levels are as follows:
•200 W RMS/4 ohms=28 V RMS
•100 W RMS/4 ohms=20 V RMS
•100 W RMS/8 ohms=28 V RMS
•50 W RMS/8 ohms=20 V RMS
Notice certain combinations of power & load will give the same voltage. Continuing, if you were to
connect a transformer to one channel of a CS-800, that would convert 40 V RMS up to 70.7 V RMS; then,
the CS-800 could successfully 'drive' a 70 V RMS line up to a 400 W RMS power level. Such a
transformer exists and is available through the Peavey Accessory Program. The unit is called an
AUTOMATCH, and it is actually an auto-former, which is a transformer that does not have a separate
primary and secondary, but rather just one winding with various 'taps' available. (It has the same common
terminal for both input and output.) It has been designed to "create" 70 volt lines from various standard
level power amplifiers including the CS-800 and other popular models. The AUTOMATCH will accept
voltage levels of 20, 28, 40, & 56 V RMS and transform these to both 70.7 & 100 V RMS at up to 400 W
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RMS levels. This unit is priced very reasonably and provides an effective means to drive 70 volt lines with
standard, direct coupled type power amplifiers. The unit can also be used to convert speaker (load)
impedances either up or down (8 ohm to 4 ohm, etc.) to effectively match amplifier ratings. (Figure 2 is
the power, impedance, and voltage chart on the AUTOMATCH. You should notice that the chart indicates
all the output voltage levels we just reviewed.)
Now, let's use our "creation" of a 70 volt amplifier with the CS-800 and the AUTOMATCH. Suppose we
take only one 70 volt line matching transformer, connect the secondary 8 ohm tap to a single 8 ohm
speaker, and connect the 10 watt primary tap together with the common tap of this single "system" across
the 70 volt line. How much power will the CS-800 deliver to the speaker? If you answered 10 watts, you're on. Notice the power amplifier can deliver a maximum of 400 W RMS, but with just one 10
watt/70 volt load connected (500 ohms), it will only deliver 10 W RMS. It can, however, drive a total of 40
such loads (40x10 = 400).
This is what makes constant voltage (70 volt) distribution systems so flexible and explains why previous
discussions have always said, "Add up the individual powers of the 70 volt system; this is the MINIMUM
70 VOLT SYSTEM. Although economically impractical, a 1000 watt 70 volt power amplifier could be used
on a 50 watt system with no problems, and the system performance would be exactly the same as if a 50
watt amplifier were used instead. As a practical matter, however, most experienced sound installers will
rarely use a 50 watt power amp on a 50 watt system. Let's review why.
Contrary to prevailing impressions, the "simple" line matching transformer is one of the most 'complex'
electronic components. It is completely understood by fewer engineers than almost any other part of the
system. However, there are several basic characteristics that need to be discussed that will assist you in
the system design and power amp selection.
First, it must be realized that associated with the transfer of current and voltage between the primary and
secondary windings there are losses. These losses are typically due to resistance in the windings of both
the primary and secondary, where the current itself causes internal heating. For example, a 10-watt unit
with a 1.5 dB "insertion loss" would deliver only 7 watts to the speaker with 10 watts of input power at the
primary. If 7 watts is not enough, you must select a higher powered transformer.
Here the speaker will simply operate at 1.5 dB lower SPL than expected, and the 70 volt system will still
only be "loaded" with a 10 watt system. To make matters more confusing, some manufacturers actually
adjust the transformer windings internally to compensate for insertion loss by reflecting a lower than
normal impedance at the primary winding, so the power amp will deliver the extra power necessary to
overcome the insertion losses. In this case, a "compensated" 10-watt unit with 1.5 dB less would require
14 watts of power at the primary to deliver 10 watts to the speaker.
With this transformer, then, the speaker will operate at the expected SPL but the 70 volt system will
actually be "loaded" with a 14 watt loads. Thus, one must ascertain: (1) the true amount of insertion
losses as opposed to the manufacturer's claimed losses, and (2) whether the windings are compensated
or uncompensated. Obviously, compensated matching transformer units have higher power amp
requirements than uncompensated ones.
Secondly, the majority of small line transformers use "reactive" at both the lower & higher frequencies.
This means the impedance that the unit presents to the 70 volt line will drop, open dramatically, as the
system program material moves away from either side of the 1000 Hz reference frequency "conveniently"
used by most manufacturers in the specifications and ratings. The real problem is dealing with the lower,
not the higher, frequencies. Suppose the impedance of transformer drops from 500 ohms to 250 ohms at
some low frequency value. Then the input power requirements have doubled! Hence, it is imperative that
we know AND ACTUALLY CONTROL the low (and high) frequency operating limits of the system, and at
those limits know what the actual transformer loading is on the 70 volt distribution line. The "inductive"
and "capacitive" reactances of matching transformers do significantly effect the frequency response of the
loudspeaker. This is particularly noticeable in the bass frequency range. If a typical music distribution
system is found to be lacking in lows, the only alternative is to "buy more iron" by purchasing a larger
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(more expensive) transformer and/or to use a smaller wattage rating tap. Great caution should be
exercised with equalization on systems with poor, low frequency response. For these and many other
reasons it is a good idea to evaluate the line transformers before installation. By far the best way is to
actually connect the unit to a 70 volt power amp and then measure the various critical parameters.
In our little office design we talked about previously, the target power level for each of the 10 speakers
was 10 watts, then the minimum power amplifier requirement would be 100 watts. However, a prudent
installer might use a 200 watt amp instead, which would allow for the system loading, and will also allow
for some future system expansion. If the budget was tight, he might just use a 150 watt amp (using the
target 150% value), or he might rewire each transformer to 5 watts and use a 75 watt power amp
accepting 3 dB less system level. This is what makes the design of a 70 volt system so neat. The
combinations are limitless. And notice with the 70 watt system, you don't necessarily have to wire
transformers the same or deliver the same power levels to all speakers.
Again, in our little office, the entry area could have 3 speakers at 5 W RMS; 6 separate offices - 1 speaker
each office at 2.5 W RMS each; and a break area - 2 speakers at 10 W RMS each. Calculating the total
load: (3 x 5) + (6 x 2.5) +(2x10)-15+15+20=50WRMS (USE 75 W AMP) Notice, this exercise was just
choosing taps, and yet the installer had great flexibility in setting levels throughout the office complex to
meet the sound level needs of all involved. To install such a system without using these constant voltage
techniques would be just about next to impossible.
At this point you might be asking, "Why 70 volts; who choses this level?" The original intention was to
have 100 volts peak on the line, and since the peak to RMS conversion for a sine wave is 0.707, a 100
volt peak line was 70.7 volts RMS. The technically correct value is 70.7 volt RMS, but 70 volts is the
common term. Various other voltages have been tried including 25 V, 35 V, 50 V, 70 V, 100 V, 140 V, and
200 V, but the 25 V, 70 V, or 100 V systems have become the most widespread. By the late 40's, several
standards had evolved to regulate 70 V specifications for amplifiers and transformers, and in the 1950's
we found the usage of 70 V systems very well established in the western hemisphere (100 V elsewhere ).
There's another reason why the 70 volt system passed the test of time above all the other voltage
systems. The answer is simply one of "shock hazard." Years ago, Underwriters Laboratory determined
through extensive research that 70 V RMS was the highest AC voltage it would approve on the exposed
output terminals of an amplifier. The only restrictions that U. L. places on 70 volt outputs that the user
could possibly be "exposed to" is to provide an ADEQUATE VERBAL WARNING. Armed with these
WARNINGS, the constant 70 volt distribution system is still very much alive and kicking, even after almost
50 years of usage. Peavey has completed designs on its new Architectural Acoustics products, all of
which have constant voltage output capability. These units use the screw type barrier strips for all the
output connections, including the 70 volt line. U. L. will put its "blessing" on any such "exposed" strips
provided the manufacturer provides adequate notifications of voltage output and prints a disclaimer
Although the 70 volt system is the most popular and the most widely used in the U.S.A., there are many
STATE CODES which consider 70 V RMS a potential problem. For example, some states require 70 volt
cables to be double insulated, and/or single insulated cables must be placed in conduits. Still, others
impose severe application limitations on 70 volt systems. Some states won't permit 70 V system usage in
schools, churches, and public places. In cases such as these, installers will choose a lower voltage
system...a 25 volt system, which most codes allow. Like the 70 volt system, the 25 volt system also has a
key ingredient, the line matching transformer. In this case, the matching transformer must be a unique 25
volt version.
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All the "rules of the road" are the same as before. You simply replace 70 with 25 in all the calculations.
(Component Specialities offer many - Model No. T-2510...$3.89 each...see Figure 3 for transformer
diagram.) Like the 7010, it also has six primary wires and three secondary wires. Again, the secondaries
are common, 4 ohms & 8 ohms; the primary wires are common, 10 watts (62.5 ohms), 5 watts (125
ohms), 2.5 watts (250 ohms),1.25 watts (500 ohms), & 0.625 watts (1 k ohms). This transformer is used
exactly as the 70 volt count counterpart, the secondary wired to each speaker. The primary will be
connected across a 25 volt distribution line (pair), and the particular wires used will be ones that will deliver the desired power to the speaker. The system power amp must have a 25 volt output and its
power rating must be equal to or greater than the sum of all the individual power levels desired to be
delivered to each speaker by each of the distribution transformers (exactly the same as before ) and here,
too, you still should pad the power amplifier requirement by 150%.
Again, like the 70 volt version, the 25 volt transformer is saying, "If you put a 4 ohm load (speaker) on the
4 ohm secondary tap (or 8 ohm load on the 8 ohm tap), this transformer, by its design, will convert the
load upward to a new indicated value (assume 62.5 ohms), dependent upon which primary tap you select
together with the common tap." Now, it just so happens that if you place a 62.5 ohm load on a constant
25 VAC line, you will deliver exactly 10 W RMS to that 62.5 ohm load and in this case you will "transform"
that 10 W RMS into the 8 or 4 ohm loadspeaker load.
As mentioned, the 25 volt system must employ a power amp that delivers 25 V RMS at full power output.
Although not as common as 70 volts, the 25 volt output capability is still "around and kicking," and all the
new Peavey distribution type products will offer both 25 & 70 volt outputs. Although the 25 volt system
was intended to be exactly 25 V RMS, it will work quite well on 28 V RMS, which just happens to be the
voltage level that 200 W/4 ohm and 100 W/8 ohm direct coupled prover amplifiers supply. This "overage"
in voltage usually doesn't cause any performance problems and thus most distribution products will "call"
28 V RMS a 25 volt line. This voltage level is also available on Peavey's AUTOMATCH transformer, so it
can be used where necessary to convert any power amp to 25 V RMS.
Often installers are asked to upgrade existing systems in churches and schools. The most common
mistakes made are by those who proceed to add to or change an existing system without knowledge of
what type of system really exists: 25 volts, 70 volts, or "0 volts".
I make reference to a system called "0 volts." It is simply a system where vast numbers of 8 ohm or so
speakers are PARALLELED on forever, creating almost a "zero" load value, which some unfortunate
power amp has to drive. Such systems are (regrettably) very common. Where problems occur is when a 0
volt "add-on" system is connected to an existing 70 volt system. I have seen 70 volt parts added to an
already overloaded 70 volt system, and even worse, watched folks "hang" 70 volt parts on a 25 volt
system and wonder why the system just doesn't sound right. I make the following suggestions to those
who would attempt to mix 25 & 70 volt pans in the same system: Unless you really have your constant
voltage fundamentals mastered, don't attempt this. Wire 25 & 70 volt pans separately, and then use
separate amplifiers/systems.
There is another technique that allows direct coupled stereo amplifiers to drive 70 volt lines directly
without using a conversion output transformer. This technique is called BRIDGE MODE. When a twochannel amplifier is operated in BRIDGE MODE, it is converted to a single channel amp with a power
rating and a load rating TWICE that of the single channel ratings. For example, the CS-800 bridge rating
is 800 W RMS into 8 ohms (the single channel rating was 400 W RMS into 4 ohms). The bridge mode
operation is accomplished by placing the mode switch in the bridge position and connecting the "load"
between the red binding posts of each channel, and using channel "A" as the input channel (all functions
of channel "B" input are defeated). What actually happens from the technical standpoint is that "B"
channel is supplied an input signal which is equal in level, but is 180 degrees out-of-phase from that of
the channel "A" input signal. Thus, the load, which is connected between the channels, must be twice the
single channel rating, and it "sees" the sum of output voltages of both channels. This technique then
allows the CS-800 to produce an output voltage of 80 V RMS at full rated output. This bridged amp is now
used to drive 70 volt systems directly without a transformer, resulting in a very simple and cost effective
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First, most bridged amps have an 8 ohm minimum load rating, thus the 70 volt system impedance must
be limited to 8 ohms. It can be shown mathematically that a 600 watt 70 volt system load is about 8 ohms.
Therefore, to keep from overloading the amplifier, we must limit the size of this 70 volt system to 600
our 67% rules for system limits we better limit it to only 400 watts. Now the strange part...this 400 watt system will be driven to 533 W RTMS (all 70 volt parts will be "overpowered" to 133%) as you will see in
the chart below. Fortunately, all this works out well; however, the typical matching transformers have a
33% power loss. Notice all these "crazy" 70 volt limits occurred because a bridged CS-800 was an 80 volt
line and not a 70 volt line. Following is a chart listing some standard bridged power amp ratings and the
resulting performance:
• 600 WATTS/8 OHMS 70 V RMS LINE REFERENCE PWR @ 100 %... 0 dB SPL
• 1200 WATTS/8 OHMS...100 V RMS LINE... OVER-POWERED TO 200%...+3 dB SPL
Notice that only a 600 WATT/8 OHM power amp actually delivers 70 V RMS. In all cases. The 8 ohm
bridged power amps can drive up to a 400 W RMS size 70 volt system, but will "power" each part as
indicated above. Generally, the 600 and 800 W RMS/ 8 OHM amplifiers driving 70 volt systems will
perform well (Peavey CS-800s find extremely wide usages here). But a 400 W RMS/8 OHM amp can
result in a very under-powered system. In this case, I would suggest selecting the next highest power
level tap on the matching transformer (if available) and limit system size to 200 W RMS. Amplifiers that
produce more than 80 V RMS should never be connected to normally wired 70 volt components, since
they will be over-voltaged and could possibly be damaged and/or could cause severe distortion due to the
possibility of transformer "saturation" at the higher voltages. In this case, a viable alternative is to select
the next lowest power tap on the matching transformer. WARNING...THE TRANSFORMER MUST BE
RATED FOR THE DESIRED POWER (it must be a 10 W unit but use 5 W tap for 10 W RMS). This wiring
"trick" changes a 70 volt system to 100 volts, so 1200 WATT/8 OHM amps could "drive" up to a 1200
watt, 100 volt system (assuming an ideal one). However, for loading we should limit the system size to
800 watts.
100 volt distribution systems find wide usage in many countries outside the western hemisphere. They
are however, very rarely used in the U.S.A. and Canada, and most local audio distributors have never
heard of a 100 volt distribution transformer. Again, this 100 V system is treated just like the two systems
we have already covered, except 25 V or 70 V becomes 100 V. Peavey does sell many products in world
markets and we address the 100 volt applications with the 100 V RMS tap on the AUTOMATCH ( just in
case you were wondering why). Both the CS-1000 & CS-1200 bridged are good 100 volt distribution
amps. You may be asking, "Why 100 volt systems?" The answer lies in another real advantage of
distribution systems themselves. This advantage is that the typical voltage distribution system generally
has less losses in long cable runs than those encountered in a typical "direct coupled" system, and/or you
can generally use smaller wire sizes in a distributed system. Just ask your local power company about
their distribution system. Have you ever wondered why power companies use such high voltage levels on
their long "runs" from the generating plants to where the consumers are? The answer is simply avoiding
LOSSES. A utility/power company sells power, electrical power. Electrical power is a product of the
voltage level times the current delivered. For a given power, THE HIGHER THE VOLTAGE, THE LESS
CURRENT IS REQUIRED, & current is what causes losses. LOWER CURRENT MEANS LOWER
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The typical power company will use 200,000 V RMS or even higher on their distribution lines. This allows
them to have less losses in the cables and also use a relatively "small" wire. Both these reasons are
dollars. The power companies apply this logic through the whole system. On that power pole in front of
your home is 10,000 V RMS or more. That voltage is applied to a distribution transformer which steps it
down to the 120 V RMS, which then feeds that plug or light which we all take for granted.
The analogy between this system and constant voltage audio distribution systems should be apparent
and should answer the question "why" 100 volts, and "why" the typical audio distribution system has less
losses in long cable runs than those encountered in a typical "direct coupled" system. It should be
obvious that delivering 70 V RMS over a long cable run to a line matching transformer which presents a bad of 500 ohms (10 W), can be accomplished using a relatively small wire as compared to delivering 9 V
RMS the same distance to an 8 ohm speaker "direct coupled" (10 W). The significant difference between
the two systems is simply "loading." For example, assuming a 100 ft. run, the 70 V RMS cable can be a
#27 AWG pair with losses of 0.21 W. The 9 V RMS cable should be a #10 AWG pair, but 100 ft. of #10
AWG is too expensive, so we use #16 AWG and base 1 W. You may be wondering how I came up with
the wire sizes and the losses. There are many good publications that cover this topic in great detail. Most
deal with the wire size required for certain length runs which must carry so much current. This information
is important for any system, including 120 VAC electrical wiring & regular power amp speaker loading.
Choosing the right size wire for the job is also very important in any distribution system.
It is not the intention of this paper to cover cable sizing. We will leave such for another time and another
paper. I will make just one point about cable "sizing." Most folks just think that 70 volt systems
automatically can use very small wire sizes. Somehow converting that 40 volt amp (CS400) up to 70 volts
"magically" cuts way down on those cable losses. Always remember that wire size is directly related to
the level of current it has to carry. In the above example we raised the voltage by we
lowered the current by four sevenths. That means we can use four sevenths of the wire size originally
required without the conversion to 70 volts. That's about two (or three) wire sizes smaller...that's all.
Example: We are driving an 8 ohm loudspeaker array with that CS-800 and about 100 ft. of cable. It is
determined that such a run should use #10 AWG cable. Now, using the 70 volt conversion transformers
(Peavey Automatches) at both ends (one at the amp to convert up and one at the speaker to convert
down) will allow us to use #12 AWG cable...big deal. You will find that the cost of the two Automatch
transformers and the #12 wire is more expensive than using the #10 cable in the first place. And, yes, you
must use #12 with that 70 volt system, not that #18 that you hoped to use. Right here in Lauderdale
County we have a certain football stadium that uses a 130 watt Packaged P.A. with our Automatch
transformer to drive a 70 volt loudspeaker array about 1000 ft away in the scoreboard. They use a #27
telephone wire...that's about 60 watts down the drain.
Most background music systems require a convenient means for adjusting the volume level at each
speaker or at small groups of speakers. This feature is usually provided through the use of a "local" line
control. Selecting the correct line control can be as important as the choice of the line matching
transformer in the typical distribution system. The term "attenuator" includes potentiometers (called
"pots"), pads, and auto-transformers. All three find usage as level setting devices in typical sound
systems, but each performs a slightly different function.
Power pots (or Rheostats) have often been used to control the volume level of a single speaker. A typical
pot used with an 8 ohm speaker is 50 ohms/5 watts. As a general rule, pot values should be at least 5
times (but no more than 10 times) that of the speaker impedance (or line impedance) value, and have a
power rating of at least 20% of the expected speaker power level. If the resistance value is too high, the
result will be loss of "range," where no sound will be heard until the pot is turned almost completely up, at
which time full volume will be produced. If the value is too low, excessive power will be drawn from the
power amplifier and "dissipated" in the pot itself, and it could be destroyed by "heating." Figure 4 shows
the wiring arrangement for the typical pot application. Notice that when the pot is full CW, the speaker is
connected directly to the source and the pot itself is additional "loading" on the source - the total load
being 8 ohms in parallel with 50 ohms (approx. 6.9 ohms). When the pot is full CCW, the speaker is
removed from the source, but the pot itself is still providing loading on the source (pot value 50 ohms). At
the mid-range setting the pot powers the signal level and provides a loading of about 31 ohms (25 + 25
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"in parallel with" 8,25 + 6=31).
The additional loading must always be considered when using this approach, and the bad variation can
cause problems in applications where the source must have a constant load value, such as between the
8 ohm secondary taps of a line matching transformer, and an 8 ohm speaker. In this case the line
matching transformer characteristics change significantly when the 8 ohm taps are terminated with 50
ohms, and thus A POT SHOULD NOT BE USED HERE. However, a pot can be successfully used in the
primary (70 volt) side. A 5000 ohm, 2 watt pot finds wide range usage in single transformer applications,
and a 2000 ohm, 3 watt pot can be used for controlling up to 5 units. Generally, pots should be avoided
when controlling more than 5 transformers, since the value required will cause excessive pot dissipation
(heating). In this case, the 5000 ohm pot dissipates about 1 W RMS of power.
All such loading must be added to the total power calculation to determine the power amp requirement,
but unlike the matching transformer case, the pot loading values do not have to be "padded" upward due
to bad changes at low frequencies.
L-pads avoid many of the problems associated with pots as these controls contain dual offsetting
resistance elements and are designed to provide a constant impedance to the source. Figure 5 is the
wiring arrangement for an 8 ohm, 10 watt L-pad. Note that this unit uses an 8 ohm, 10 watt resistance
element connected between the source and the speaker, and a separate 40 ohm, 5 watt resistance
element connected across the speaker. At the full CW setting the 8 ohm element becomes a low
resistance value and the 40 ohm element is actually "open," the wiper not making contact. At this seeing,
then, the only load on the source is the speaker itself, the L-pad basically being "out of the circuit." This is
the first real advantage of L-pads...THEY DO NOT PROVIDE ANY ADDITIONAL SYSTEM LOADING.
Hence, when used they don't increase the power amplifier requirements.
At the full CCW setting, the 8 ohm element becomes an 8 ohm load to the source and the 40 ohm
element becomes a low resistance value effectively shorting the speaker. At the mid-range setting, the 8
ohm element is 4 ohms (in series with the speaker), and the 40 ohm element is 20 ohms (in parallel with
the speaker). 20 ohms across an 8 ohm speaker yields approximately 5 ohms. Hence, the source loading
is about 9 ohms (5 + 4). This, then, is the second advantage of L-pads...THEY PROVIDE A RELATIVELY
CONSTANT IMPEDANCE TO THE SOURCE, and hence the device of choice between the 8 ohm
secondary taps of the line transformer and the 8 ohm speaker.
As expected, L-pads also have some major disadvantages. First, they are much more expensive than
single element pots, although current prices are becoming more competitive. Second, you must match
the impedance of the pads to the loudspeaker impedance, and they also have a MAXIMUM POWER
RATING THAT CAN NEVER BE EXCEEDED. This makes using them for multiple speakers almost
impossible. Third, the L-pad has a very poor maximum attenuation value (generally less than 30 dB), and
as such doesn't turn the speaker "off" completely, the resulting "leakage" being annoying. A pad called
the T-pad addresses this L-pad disadvantage by using three offsetting resistance elements, the 3rd
element being in series with the speakers. This type pad has a greater maximum attenuation value than
the L-pad, but T-pads are extremely expensive and are not readily available. The pot approach generally
can attenuate signal levels more than 50 dB.
And last, the constant impedance to the source characteristic is often not a desirable feature in
applications that really don't require this. The constant impedance "thing" means a constant system
loading. There are many versions of L-pads available for usage at 70 volt line levels. In the old days of
tube type power amplifiers, you learned to NEVER MISMATCH A TUBE POWER AMPLIFIER. That large
"output transformer" was not very forgiving of higher than normal impedance loads, and that "open circuit"
was a disaster. Such conditions usually destroyed the output transformer itself due to the extreme high
levels of 'inductive energy' that had no place to go at full power (clipping) operating levels, and would
simply "arc" over internally causing the primary winding to short. Hence, born out of necessity, the L-pad
came about to maintain a CONSTANT 70 VOLT SYSTEM IMPEDANCE which was required for good
amp reliability. Unfortunately, many folk still think that "YOU GOT TO GO WITH THE PAD."
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Contemporary power amplifiers no longer suffer from such problems, and are very "happy" to drive higher
than normal loads or even open circuits, and under these conditions run far cooler than when fully loaded.
Thus, the 5000 ohm pot is the economical contemporary choke. Although it has the disadvantage of
adding system loading, it is usually not the case, since generally not all the pots are at the maximum
setting. Hence, system loading is actually less, and the amplifier likes it. Most experienced installers
simply ignore the pot loading on a system. NOTICE YOU CAN GET IN TROUBLE IF ALL POTS ARE
TURNED UP TO THE MAXIMUM! A very popular item used in many installations is a speaker that has
the line matching transformer already mounted to the unit itself and has the 8 ohm output taps already
connected to the speaker. Here, the installer simply connects the desired power taps. In this case, a 5000
ohm pot wired into the primary circuit works out real well. Using an 8 ohm L-pad in the secondary circuit
here would be difficult.
The third type of attenuator is the auto-transformer. This type operates in a manner similar to an ordinary
line matching transformer, with each step or change in volume being accomplished by switching to a
lower or higher ratio primary tap with a rotary switch. The type is generally very expensive and is not used
very often in contemporary applications.
As you can see, constant voltage distribution systems are not as complicated as many would make them
out to be. With a few simple rules, and just plain common sense, you can avoid the pitfalls that are lurking
out there. I would suggest, however, before you set yourself up as that expert installer and tackle that
really big distributed system, that you try a small one like our little office scenario. Like any other
electronic endeavor, adequate test equipment is a necessity. There are several really great portable
impedance measurement devices available from your local electronic distributor that can check the actual
impedance of a distribution line and/or the distribution transformer, and will tell you what the actual
loading is at any point in the system. Such measurements cannot be accomplished with a simple
I have found the following publications to be very helpful on this subject:
AUDIO CYCLOPEDIA by Committee / Howard Sams publisher
SOUND SYSTEMS ENGINEERING by Don Davis / Howard Sams publisher
Some summary type info (from another source).
The sum of the total transformer wattage taps used for all speakers in a distribution line must never
exceed the amplifier's rates output.
It is preferred to load a constant voltage amplifier to 80 % of it's rated output. This gives the system a
20 % headroom which increases system performance. Over loading an amplifier will cause the power
or output fuse to blow. In some cases overloading can blow the output stage or even the output
transformer of the amplifier. In any case over loading will reduce the life expectancy of the amplifier.
In a constant voltage distribution system it is important to observe speaker phasing. Especially when
more then one speaker are installed in the same area.
If a line matching transformer offers both 25v and 70 volt taps, make sure the proper line voltage is
used. Wrong voltage may result in over-loading an amplifier or it will give you reduced or increased
sound level.
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