What is a 70-Volt Speaker System?
70-Volt speaker systems (referred to as Constant Voltage Speaker Systems) were basically invented to fill the need for speaker distribution in large facilities. For Example: Let’s say that you need speaker coverage in a very large area or in multiple rooms that require 30-40 speakers – you would be able to power all 30-40 speakers with only one 70-Volt amplifier. It would be quite expensive to install (30-40) 8-ohm speakers for this type of application. 70-Volt speaker systems use matching transformers at each speaker that will have different wattage taps (see 70-Volt transformer below). The American standard is actually 70.7 volts, but is commonly referred to as just 70V. 70V speaker systems are the preferred standard for the majority of commercial sound system installations (restaurants, airports, malls, theme parks, etc.). Every audio technician will eventually be working with 70V systems so it would be useful to understand some of the basics. Constant Voltage = Equal energy at each speaker. Voltage and Resistance is higher, and the Current is lower.
Typical 8-Ohm vs 70-Volt System
There are certain needs for the different types of audio systems. If you are setting up for a live band performance – then it’s a no-brainer to use a standard speaker system (4-ohm, 8ohm). If you need to cover a whole lot of area such as a large Supermarket or large Outdoor area – the only way to go is to utilize a 70-volt system. There are many differences such as frequency response, fidelity, cable length, impedance matching, etc. We will now review the basics of both types of speaker systems below.
In this typical 8-ohm speaker system example (shown above) the Amplifier has two outputs (rated at 8-ohms each). The amplifier is feeding two 8-ohm speakers so we can say that this example system is “matched.” If for example you want to add two more 8-ohm speakers (one to each of the existing speakers) this would bring the “impedance” that the amplifier will “see” to around 4-ohms per channel. This 8-ohm amplifier will now be “seeing” 4-ohms at its outputs and will be working harder. If you wanted to add a 3rd speaker to each side it would bring the impedance down even lower. Although this sometimes works temporarily, there is a good chance that it won’t work for very long (and likely damage the amplifier as well).
Looking at the 70V system example below you will see that transformers are being used for distribution to the speakers. These transformers match the required impedances needed to make this system function properly. As you can see the output of the amplifier is 8-ohm’s and is feeding the 8-ohm side of the main 70V transformer. The main 70V transformer’s 70V side is feeding the speakers 70V transformer side. And lastly, the speaker transformers 8-ohm side is feeding the speakers 8-ohm input side. All Matched (8-ohm to 8-ohm, 70V to 70V, 8-ohm to 8-ohm)!
Although the example (shown left) has only two speakers on the diagram – there is the capability of adding many more 70V speakers.
For Example:
If the Main Transformer was rated at 100 Watts – you could have 8 – 10Watt speakers, or 16 – 5Watt speakers or even up to 80 – 1Watt speakers hooked to the system without any issues.
Below is an example of a larger 70-Volt system that was installed in a Super Market.
70-Volt System Technical Notes:
- To clarify what “constant voltage” actually means – The speaker lines do not always have a voltage that is 70-Volts. When there are quiet passages in the signal (or no signal at all) going to the 70V amplifier, then there is “less” or “no” voltage present on the line.
- With constant voltage systems, amplifiers are seen as “power / voltage generators” and speaker lines are seen as “power lines.” Think of this in terms of voltages and wattages, rather than impedances.
- 70-Volt transformers can affect low-end frequencies and can lead to poor bass fidelity, especially below 80Hz. With this being said – most 70-Volt system applications are not expecting extreme low-end frequencies (especially in supermarkets, malls, or office buildings). Even with these low-end frequency losses, the audio will still sound really good to the majority of listeners.
- The American standard is 70.7-Volts (referred to as 70-Volts), and the European standard is 100-Volts.
