Constant-Voltage-4
Voltage Variations — Make Up Your Mind
The particular number of
70.7
volts originally came
about from the second way that constant-voltage
distribution reduced costs: Back in the late ’40s, UL
safety code specified that all voltages above 100 volts
peak (“max open-circuit value”) created a “shock
hazard,” and subsequently
must be placed in conduit
– expensive – bad. Therefore working backward from a
maximum of 100 volts peak (conduit not required), you
get a maximum rms value of 70.7 volts (Vrms = 0.707
Vpeak). [It is common to see/hear/read “70.7 volts”
shortened to just “70 volts” – it’s sloppy; it’s wrong; but
it’s common – accept it.] In Europe, and now in the
U.S., 100 volts rms is popular. This allows use of even
smaller wire. Some large U.S. installations have used as
high as 210 volts rms, with wire runs of over one mile!
Remember: the higher the voltage, the lower the cur-
rent, the smaller the cable, the longer the line. [For the
very astute reader: The wire-gauge benefits of a reduc-
tion in current exceeds the power loss increases due
to the higher impedance caused by the smaller wire,
due to the current-squared nature of power.] In some
parts of the U.S., safety regulations regarding conduit
use became stricter, forcing distributed systems to
adopt a 25 volt rms standard. This saves conduit, but
adds considerable copper cost (lower voltage = higher
current = bigger wire), so its use is restricted to small
installations.
Figure 4. Transformer & Line Insertion Losses
Calculating Losses — Chasing Your Tail
As previously stated, modern constant-voltage am-
plifiers either integrate the step-up transformer into
the same chassis, or employ a high voltage design to
direct-drive the line. Similarly, constant-voltage loud-
speakers have the step-down transformers built-in as
diagrammed in Figures 2 and 3. The constant-voltage
concept specifies that amplifiers and loudspeakers need
only be rated in watts. For example, an amplifier is
rated for so many watts output at 70.7 volts, and a loud-
speaker is rated for so many watts input (producing a
certain SPL). Designing a system becomes a relatively
simple matter of selecting speakers that will achieve
the target SPL (quieter zones use lower wattage speak-
ers, or ones with taps, etc.), and then adding up the
total to obtain the required amplifier power.
For example, say you need (10) 25 watt, (5) 50 watt
and (15) 10 watt loudspeakers to create the coverage
and loudness required. Adding this up says you need
650 watts of amplifier power – simple enough – but
alas, life in audioland is never easy. Because of real-
world losses, you will need about 1000 watts!
Figure 4 shows the losses associated with each trans-
former in the system (another vote for direct-drive),
plus the very real problem of line-losses.
Insertion loss
is the term used to describe the power dissipated or
lost due to heat and voltage-drops across the internal
transformer wiring. This lost power often is referred to
as
I
2
R
losses, since power (in watts) is current-squared
(abbreviated
I
2
) times the wire resistance,
R
. This same
mechanism describes line-losses, since long lines add
substantial total resistance and can be a significant
source of power loss due to I
2
R effects. These losses oc-
cur physically as heat along the length of the wire.
You can go to a lot of trouble to calculate and/or
measure each of these losses to determine exactly how
much power is required
3
, however there is a Catch-22
involved: Direct calculation turns out to be extremely
difficult and unreliable due to the lack of published in-
sertion loss information, thus measurement is the only
truly reliable source of data. The Catch-22 is that in
order to measure it, you must wait until you have built
it, but in order to build it, you must have your amplifi-
ers, which you cannot order until you measure it, after
you have built it!
The alternative is to apply a very seasoned rule of
thumb:
Use 1.5 times the value found by summing all
of the loudspeaker powers.
Thus for our example, 1.5
times 650 watts tells us we need 975 watts.
IN
AMP
TRANSFORMER
INSERTION LOSS
(TIL)
I
2
R LINE LOSS
(TIL)
