70-volt distributed loudspeaker systems are ubiquitous in almost all types of buildings and industrial facilities. However, they are also surrounded by numerous myths, approximations, and simplifications. It is essential to set the technical record straight in order to ensure reliable and compliant designs.
1. 70 volts or 70.7 volts?
The correct value is 70.7 volts RMS. This figure is not arbitrary: it corresponds to a peak voltage of 100 volts PEAK, or 70.7 V RMS = 100 V / √2 (not to be confused with 100V RMS distributed systems!). However, in practice, this voltage is rarely reached. It represents the maximum amplifier output (at full power), not a constant voltage on the audio line.
2. Why do we talk about “constant tension”?
The term »Constant Voltage» is misleading, as the voltage varies according to the audio signal. It is not a fixed 70V line, but an audio alternating current (AC) line, whose amplitude varies constantly. The real advantage of these systems is the consistency of the impedance seen by the amplifier, as all speakers use transformers. This simplifies the load on the amplifier and facilitates parallel wiring of multiple speakers.
3. There is no such thing as a 70V speaker... really.
What is known as a «70V speaker» is actually a traditional speaker (often 8 ohms) with a built-in transformer. This transformer allows a fraction of the power to be drawn via a “tap” (e.g., 1 W, 5 W, 15 W), while maintaining an impedance suitable for the overall system. It is therefore the transformers that form the heart of the distributed system, not the speakers themselves.
4. Alternating current and Class 2: where is the limit?
The voltage of 70.7 V RMS is alternating current (AC), and it is still considered Class 2 because the total power is limited. This reduces safety requirements as long as the installation complies with these limits. However, several modern systems exceed Class 2 power ratings, which requires Class 1 compliance (cables in conduits, circuit protection, etc.).
5. No, you cannot run 500 meters with 22 AWG...
A persistent myth claims that 70V systems allow for unlimited distances with very small gauge cable. This is false! As with any audio system, voltage (and therefore signal) loss increases with cable length and conductor resistance.
But more importantly: Losses are expressed in decibels (dB). It is the loss in sound power level that counts, not just the voltage. A cable that is too long or too small can cause audible loss, even if the system is functioning “technically.”.
💡 For the curious: The loss in decibels (dB) is calculated by taking ten times the decimal logarithm of [1 + (2 × round-trip cable resistance ÷ system impedance)].
But don't worry... we'll take care of the calculations for you 😉
Conclusion: rigor and planning above all else
70V systems are powerful, efficient, and suitable for large buildings, provided you understand how they work. It's time to move beyond the shortcuts and beliefs inherited from a time when installations were often based on undocumented empirical practices!
