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Clearing Up The Confusion: Attributes Of 70-Volt (And Other “Stepped Up”) Systems

Addressing common misconceptions about transformer-distributed loudspeaker systems.

Most people are surprised to find that “70-volt” systems share many common traits as “direct connected” counterparts.

The most important trait of 70-volt (more accurately 70.7-volt) systems is the use of transformers on the loudspeakers (usually internal) to “step down” the signal voltage before applying it to the loudspeaker.

This is necessary because the signal voltage is “stepped up” at the amplifier. In fact, at SynAudCon we prefer the term “transformer distribution system,” especially since they can be based on voltages other than 70.7 volts.

The ratings and specifications are based on sine waves, but any audio signal can be played over the system. Myths regarding these systems abound. Some are rooted in fact. Others are not. Here are some of the more common misconceptions about transformer-distributed loudspeaker systems.

Myth: 70.7 volts is present all the time, just like 120 volts on a household electrical circuit. It illustrates the difference between a “rated” voltage and an “actual” voltage. Audio signals are dynamic, meaning that they are only present when something is playing over the system.

Contrast this to utility power distribution systems, where a voltage is always present at the electrical outlet. The 70.7-volt system gets its name from the RMS (think average) voltage of a pure tone (a sine wave) whose peak voltage is 100 volts, which is the highest voltage that, by code, can be present on the line. So feed a sine wave into the system and turn it up until the “70 V” amplifier clips and you will read 70 Vrms on the line, which is 100 Vpeak.

A system gets its name from the RMS value of a sine wave used to rate the system. Popular rated voltages include 25, 70.7, 100, and 140 volts.

 

Substitute speech or music and turn it up until the amplifier clips, and you will read a far lower voltage (typically 1/3, or about 25 Vrms, depending on the program material). Play nothing into the system and there will be no voltage on the line, other than residual noise.

Myth: These systems have poor sound quality and are only suitable for paging and “elevator music.” This is an undeserved reputation. Using good quality loudspeakers and transformers, these systems can have excellent fidelity, indistinguishable from conventional systems that use 8-ohm loudspeakers.

It’s true that there are many poor quality 70-volt systems in use, but that is due to the factors that can make any sound system sound bad – poor quality products, poor design, improper calibration, and/or user error.

Myth: These systems are low-frequency deficient because “bass” can’t get through a transformer. The transformers used in these systems do indeed saturate more easily at low frequencies, resulting in potentially poor bass performance.

This can be remedied by over-sizing the transformers or by simply high passing the system (filtering out the low frequencies), and it’s reasonable to expect good fidelity to about 80 Hz without taking special measures to extend it to a lower frequency (larger transformers, direct-connected subwoofer, etc.)

Now I don’t know about you, but I really don’t miss very low frequencies when sitting in a meeting room, airport or restaurant. In fact, they can be annoying. They’re not needed at all for speech systems. Music systems for most applications can still sound quite good if high passed at 80 Hz.

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