Ten "70 V System" Myths
“70 V Systems” Needn’t Be Confusing
Of all the topics covered in SynAudCon courses, few are as misunderstood as “70 V” systems. Most people are surprised to find that these systems share many common traits as “direct” connected systems. The most important trait of these 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 V.
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.
1. 70.7 volts is present all of the time, just like 120 V on a household electrical circuit.
This 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 V, 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. 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.
Figure 1 – The system gets its name from the RMS value of a sine wave used to rate the system. Popular rated voltages include 25V, 70V, 100V, and 140V.
2. These systems have poor sound quality, and are only suitable for paging and elevator music.
This is an undeserved reputation. If you use good quality loudspeakers and transformers, these systems can have excellent fidelity, indistinguishable from conventional systems that use 8 ohm loudspeakers. It is true that there are many poor quality 70 V 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, or user error.
3. These systems are low frequency deficient because you can’t get “bass” 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). It is 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 the very low frequencies when sitting in a meeting room, airport or restaurant. In fact, they can be annoying. They are not needed at all for speech systems. Music systems for most applications can still sound quite good if high passed at 80 Hz.
4. Due to the lethal voltages present on these systems, the wiring must be in conduit.
It’s not the voltage that is potentially lethal. It’s the current. The higher the voltage, the more current is “pumped through” something that finds its way into the circuit. It is best to treat these circuits with the same respect you would render an electrical outlet, even though in practice the voltage and available current are far lower. Many of today’s larger power amplifiers (1 kW and higher) can produce higher voltages than 70 V, along with many amperes of current. These are potentially much more dangerous than the typical 70 V system. None the less, keep your tongue off of the loudspeaker’s terminals and don’t use 70 V distribution for your hot tub loudspeakers without strict adherence to the electrical code.
5. You can’t install one of these systems unless you have an expensive impedance meter.
The impedance meter is handy, but not essential. In fact, the whole reason for the way that these systems are specified is to avoid impedance ratings and impedance measurements.
The objective is to avoid overloading the amplifier (too many loudspeakers, too large tap size, or both). You can stay within the amplifier’s safe load limits by adding up all of the power taps used on the loudspeakers, and making sure that the amplifier’s rating is larger. In scenarios where you are replacing an existing amplifier, an impedance measurement can assure that the replacement amplifier is large enough for the application, meaning that the minimum impedance of the line is greater than the minimum impedance that the amplifier is rated to drive.
6. The system won’t work right unless there is 70 volts on the line.
The voltage rating of the amplifier and the power taps on the loudspeakers assume the presence of a 70 Vrms sine wave. You can operate the line at any voltage lower than the rated voltage. It is common practice for small systems made up of identical loudspeakers to “max out” the power taps on the loudspeakers and then adjust the amplifier for the desired playback level, with no regard for the rated line voltage. Keep in mind that this may complicate future expansion, since the power tap on the loudspeaker assumes the presence of the full distribution voltage. In other words, your 10 W loudspeaker will not get 10 W unless the system is operated at 70 V.
7. It is incorrect to refer to a 70.7 V distribution system as simply a “70 V” system.
Technically true, but why make it complicated? There is no practical difference between the two voltages.
8. These systems are only for “ceiling speakers,” and one shouldn’t mix/match loudspeaker types.
The 70 V system is just a means of signal distribution. It has nothing to do with the type of loudspeaker used. While 70 V distribution works well for ceiling loudspeakers, the amplifier doesn’t know or care about “what” or “where” with regard to loudspeakers. Do you have separate circuits in your house for toasters, TVs, and lamps? Of course not. These can all share the same circuit, so long as they don’t draw more current than is available. In the same way, it is perfectly acceptable to mix and match loudspeaker types on a 70 V line. In fact, it is one of the greatest benefits of this means of signal distribution.
9. 70 volt systems have lots of phase interference and comb filtering.
High density ceiling loudspeakers do indeed exhibit significant phase interference and comb filtering. This is one of the reasons why they can produce even sound coverage. In general, the more loudspeakers, the more interference, the more even the coverage. A common mistake regarding ceiling loudspeaker systems is to install too few loudspeakers. In essence, if interference is inevitable, more is usually better than less.
This “carpet bomb” approach to sound coverage works well for some applications, such as meeting rooms, airport gates, and clothing stores. A more surgical approach to coverage that minimizes phase interference is preferred for theaters, auditoriums, and studios.
That said, the interference has nothing to do with whether or not the loudspeakers are on a 70 V line. There would be just as much interference if each loudspeaker were 8 ohms and had its own amplifier.
10. You need a “70 V amplifier” to drive a “70 V system.”
That’s the easiest way to do it, but what you actually need is an amplifier that can produce the distribution voltage (sine wave rating) and the needed current based on the impedance of load, which in turn is determined by how many loudspeakers (and their power tap) that you daisy chain onto the line. The “70 V system” (and other standard distribution voltage schemes, such as 25 V, 100 V, etc.) assure that you can meet these conditions by simply adding the power taps and buying an appropriate amplifier.
I can use a conventional (non-70 V) amplifier to drive the line if it allows the desired SPL to be reached. Since a conventional amplifier can be loaded to a fairly low impedance (8 ohms) it should be able to easily drive a zone of loudspeakers with transformers (usually a fairly high impedance). An additional factor is that transformer loads (70 V loudspeakers) are inherently more reactive than transformer-less loudspeakers, and some conventional amplifiers may not like that. For this reason, it is good practice to use an amplifier that is designed to drive a 70 V system.
Done properly, the transformer-distributed loudspeaker system is a nice arrow-in-the-quiver of the sound system designer. They have served our industry for nearly a century, and will likely do so for another. There is simply no other way to accomplish what these systems can do
- Drive many, many loudspeakers from a single amplifier.
- Use a relatively thin cable to do so.
- Set the volume of each loudspeaker independently
If your system needs these attributes, transformer distribution is the way to go. pb