Many small venues such as bars, restaurants, schools, and
churches use a commercial speaker wiring format known as 70V. This
wiring format has a higher signal voltage that allows for many
advantages. Let's take a look at the differences, how it works, and
how to decide which format is best for your application.
A conventional speaker system has one or more speakers that
typically have an impedance of 8 Ohms, but 4, and 16 Ohm speakers
are common as well. The impedance is similar to resistance, and can
be used to determine how much load a speaker will put on an
amplifier. A quality amplifier will list what the allowed range of
load impedance is and what the maximum output will be at various
loads. For example, an amplifier may list that it can handle
speaker configurations between 4 and 16 Ohms. Hooking up any
speaker configuration that places a load that is outside this range
can cause damage to the amplifier.
The amplifier in our example states that its maximum output will
be 200 Watts at an 8-Ohm load and 600 Watts at a 4-Ohm load. As you
can see, by increasing the load from 8 Ohms to 4 Ohms (a smaller
Ohms value equates to a higher load, as it indicates less
resistance) the amplifier max output went from 200W to 600W. This
illustrates how a load even just slightly higher than the specified
range can work the amplifier much harder then intended.
Speakers can be wired in series, parallel, or mixtures of both
to allow for many different combinations that will result in the
desired load impedance. The more speakers that you want to
incorporate into your system, the more complex you wiring
configuration may become. Also, if a speaker were to become
disconnected or shorted accidently, the amplifier may become
overloaded and damaged.
A 70V speaker configuration simplifies this problem
significantly. The 70V system works by using a transformer after
the amplifier to step up the signal voltage. Another transformer is
used at each speaker to step the voltage back down to conventional
speaker levels. The higher voltage system is much less sensitive to
small changes in impedance. The transformers at each speaker
typically have multiple lugs that are labeled at the wattage they
will draw from the amplifier. Any number and combination of
speakers can be added to the circuit, so long as the total wattage
draw from the amplifier does not exceed the stated max 70V wattage
load. If a speaker is connected or disconnected, it will not throw
of the impedance balance of the reset of the network, like can
happen with conventional speaker voltages.
Another great advantage to the 70V speaker system is that it
allows for longer wiring runs and the use of smaller gauge speaker
wire. This is because the higher voltage from the step-up
transformer also translates to less current and as a result less
power loss over longer lengths of wire. This is similar to why the
electric power utilities use step-up transformers and run the power
long distances over high tension power lines and then uses
step-down transformers at the power poles right before they enter a
consumer's home.
One more advantage to a 70V system is in cases where you want to
have separate volume controls at each speaker, in a multi-room
setup for example. You can purchase 70V volume controls that are
placed directly before a speaker's step-down transformer. These
volumes controls are much more reliable and safer for your
amplifier then their conventional speaker level counterparts.
So how do you decide which setup is best for you? The more
speakers you want to use on a single amplifier channel, the
stronger the argument for a 70V system. Other factors, such as very
long runs of speaker wire also will support the case for a 70V
system. Generally any single channel that needs 5 or more speakers,
or has speaker runs over 75 feet may be better served with a 70V
system. System's with only one or two speakers per channel, such as
in a typical home theater setup, a 70V system offers few advantages
and would add cost and complexity.