Many people have heard of 100v speaker systems, sometimes referred to as “constant voltage systems” but do not always have a full understanding of how they work, when to specify a 100v system, their advantages and also their disadvantages. Well buckle up, we’re about to dive in and demystify the mystery that is 100v!
In our other resource article on Speakers, Amps, Impedance and Power we discussed that all low impedance amplifiers (what we might call “normal” or “standard amplifiers”) have a minimum impedance load that they will safely run at. We also discussed that most of the time speakers will be wired in parallel with each other, and that the more speakers you wire in parallel on a single amp channel the lower the total impedance load on the amp.
So how do you achieve adequate sound coverage in a large public space?
Imagine a scenario where you have to design a speaker system to be installed in a public space. The system will only be used for announcements and low-level background music. As such the volume doesn’t want to be loud so would require 20+ speakers around the space to give adequate coverage. The amplifiers for this system will also be installed in a separate room which is quite far away from the space the speakers will be installed. This fairly common scenario would cause two separate problems for a standard low impedance system.
The first issue would be the power loss due to all the very long speaker cable runs that would be required on such an install, which would most likely be hundreds of metres.
Second, the 20+ speakers could not be wired to a single low impedance amplifier as the total load on the amp would drop well below the minimum impedance it will safely work at.
There is a way around this by using a combination of series and parallel wiring but the cabling would be a nightmare and probably end up looking like a plate of spaghetti, so not exactly practical!
The other way around this would be to use a very large number of amplifiers with only a few speakers on each amp. Neither of these solutions though will get around the first issue of the really long cable runs, plus multiple amps would also cost a fortune, again not very practical!
Enter 100v systems - higher voltage, lower current
Contrary to low impedance systems which run with a relatively high current and a low voltage going through your speaker cable, 100v systems run at a higher voltage and lower current. How does this help? Well I won’t rattle on here about Ohm’s Law and the discovery the German physicist Georg Ohm made in the 1800’s, but what he discovered was the important relationship between current, voltage, and resistance which is all described in Ohms Law.
In layman's terms you get far less power loss over long cable runs when using high voltage, low current.
As we have mentioned this unfortunately is the opposite to how standard low impedance amps work.
In a 100v or constant voltage system both the amp and each individual speaker are fitted with a transformer that has a high impedance primary coil and a low impedance secondary coil. The primary high impedance coil is the 100v line side and the secondary coil is connected to the speaker. This high impedance 100v system passes less current down the cable than a low impedance system would so there is very little loss regardless of long cable runs.
Are there viable direct drive high impedance alternatives to the 100v system?
It’s worth noting that there are a few modern amplifier designs like those from Crown’s DCi series that are capable of directly driving high impedance 100v systems without the need of a step-up transformer. Even with this type of amp though a special 100v speaker that includes a transformer is still required for a 100v system.
This method of stepping up and down the voltage and current via transformers is exactly the same way we receive power to our homes. From the power station electricity is run through miles worth of cables underground and over pylons at a very high voltage and a comparatively low current, so there is very little loss over those huge distances.
This super high voltage supply eventually reaches the small sub-stations near our homes and there it is stepped down by huge transformers to the higher current, lower voltage supply that we are all familiar with.
No impedance load worries with 100v speaker systems
The other great advantage of 100v systems is that we don’t have to worry about the impedance load on the amp the way we would with standard low impedance systems. As long as all the speakers are wired in parallel you can daisy chain as many speakers together as you wish.
The only rule you need to consider is that the total wattage of all the speakers combined does not exceed the total output power of the amp. Combining speakers with different power ratings is not a problem, again as long as the total power of all the speakers is not greater than the output power of the amp.
You will often find that many 100v speakers will have wattage taps on the transformer. A 30w speaker for example might also have taps at 15w, 10w and 5w. If you select the 15w tap you can then run twice as many speakers from the same amp as you would on the full 30w. This is also helpful if you need a particular speaker to be lower in volume than the rest, just set it on a lower wattage tap.
100v line systems - so why don’t we use them all the time?
This all sounds amazing, so why don’t we just use 100v line systems all the time? Well there are disadvantages. For a start off most 100v speakers tend to be only low power. The higher the power handling of the speaker the bigger the line transformer needs to be and it quickly reaches a point where the transformer size would not be practical.
The other issue is that the frequency response of 100v system is generally limited too. The transformers used are not efficient over wide frequency spectrum. If the system needs to produce deep clear bass and super crisp transparent highs then constant voltage is the wrong choice. Their application is limited to just speech and low-level background music which is fine for numerous ceiling speakers in an office and reception space or retail outlet.
Conclusion - when a 100v line system is the right choice
So in short 100v line systems are a good choice when a large amount of speakers are required and the system will only be used for speech and background music at low volume.
100v systems are also a good choice when very long speaker cable runs are necessary as the power loss in the long cables will be minimal.
100v line systems are not suitable when high fidelity audio is required or high sound pressure levels.
I hope you have found this resource article helpful. As always if you need help specifying any audio system please do not hesitate to contact our audio team. We are happy to help via telephone, email and can also make site visits where applicable.
