High-impedance vs. Low-impedance Systems
Mar 25, 2011 10:52 AM, By Jeffrey Miranda, president, NeoLogic Sound
While providing technical support for a well-known commercial audio manufacturer, I encountered many customers unsure of the differences between low- and high-impedance speaker systems. The differences, while only minor, are significant when it comes to the design and installation of a commercial sound system.
High-impedance 70V speaker systems offer a variety of advantages over their low-impedance counterparts. 70V systems offer a larger degree of flexibility when it comes to expandability. For many applications requiring multiple speakers, high-impedance (70V) systems are the absolute best way to go.
High-impedance systems are also known as constant-voltage systems, meaning at maximum output, the voltage stays the same. It allows for long-distance runs without signal loss. This voltage stays constant and is not affected based on the number of speakers. It’s imperative to fully understand these differences; I have seen too many blown amplifiers because of improper loading. This results in wasted time and money, and can often lead to a frustrated customer. Because these systems use a constant voltage and step-down transformers, they can be used for longer-distance runs. This would be particularly well-suited for paging and background music-type systems for hospitals, hotels, and large retail and restaurant spaces.
In a low-impedance system, most amplifiers are only rated to handle a load down to 4Ω. Some provide ratings to go as low as 2Ω; however, reliability in some lesser expensive amps can be extremely compromised at a 2Ω load rating. If you had an application that required eight speakers, and each speaker was rated at 8Ω, you would effectively require four stereo amplifiers—using two speakers on each amplifier channel, wired in parallel. This would therefore present each amplifier channel with a 4Ω load. It has always been my practice to design systems running at 4Ω. I have used 2Ω on occasion; however, this was when I specified large touring amplifiers that were proven to handle these loads reliably.
High-impedance 70V speakers feature a transformer and what are called “tap” settings. As an example, a speaker transformer might be rated up to 20W at 70.7V. It would have incremental tap settings: 0.5W, 2W, 5W, 10W, 15W, and 20W. You would then be able to set that tap setting based on the amount of power you needed at that particular speaker location. For most applications, you would leave all speakers at the same setting; however, you might find a situation where you would set it differently based on the speaker location.
Consider this example: You are designing a sound system for a hotel. The hotel has multiple speakers throughout its lobby and lounge areas. In addition to this, there are speakers located to pipe background music into the lobby’s two restrooms. Using the same tap setting as the other locations would produce far too much sound. So you might decide to put a lower tap setting on the restroom speakers, therefore allowing you to wire these locations directly into the other locations. In essence, you are feeding multiple areas with a single amplifier, while also setting individual attenuations for each speaker location. While the tap setting is not an attenuator per se, it works similarly. In calculating the load, you would add up all of the tap settings, based on the number of speakers. If you had 10 speakers total in the system, with a 20W tap, you would require an amplifier with a minimum load of 200W. As a rule of thumb, I always add at least 40 percent on top of that to allow for headroom and future expansion. Using my rule, you would need an amplifier rated for roughly 280W at 70.7V.
If you are looking to design a system, consider using high-impedance amplifiers and speakers, which will allow you to run to speakers at greater distances without suffering signal loss. Keep in mind that when designing a high-impedance system, you would need to use both amplifiers and speakers rated for this high impedance. Some also offer a 100V rating, but most commonly used speakers in the U.S. would be 70.7V. Places like hotel ballrooms, convention centers, retail spaces, and restaurants are well suited for high-impedance systems. If we can better understand the principles behind high- and low-impedance systems, and when a high-impedance system would be better used, we can offer our customers a more flexible and cost-effective system.