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Taming 25V Systems To Excel In The Audio Mix Key insights and actionable advice for the education campus. By Jim Schwenzer Ashly Audio


chools and education facilities of all types—elementar y schools, middle schools, high schools and college campuses—present unique challenges for audio professionals. With many different uses and users spread across a range of different audio types and platforms, audio systems within education environments can grow complicated quickly. Because of the mix of various audio genres involved with a typical campuswide audio installation, it would be best to enter such a project with experience that includes multi-zone paging, room combining, theatrical production and even large stadium sound reinforcement. That said, at the ver y core of nearly ever y education facility is the multi-zone 25V Constant Voltage Audio (CVA) paging system. Schools tend to be one of the last bastions for systems based upon 25V CVA. The reason for their prominence in schools is that 25V systems are safe and affordable, as lower-voltage cabling can be pulled through ceilings and walls safely without conduit. Yet, 25V is notably the least-elegant utilization of CVA that one could imagine. With 25V, systems lose all the benefits afforded to the high-voltage and high-impedance design of 70V and 100V CVAs. With 25V, the effects of wire resistance line loss and intensified frequency-related impedance issues become a real concern. Because of these challenges, there is a direct correlation between the attention to detail one gives the planning and implementation phases of a 25V CVA and the system’s 40 Sound & Communications May 2019

quality of performance and ultimate reliability. Adequate headroom, proper high-pass filtering and clip-limiting accommodations being accounted for in the early stages will pay off later on with a notable decrease in ser vice calls. What follows are a few best practices and important considerations for working with 25V CVA.

Headroom In the world of CVA, the term “headroom” is a bit misleading. What it really represents is the margin before reaching the point where load impedance drops below the amplifier’s safe operating range. It is a term that’s typically used to gauge the available region left to climb, but it’s now repurposed to indicate how far there is to plummet before arrival at the point of no return. Because we deal with theoretical impedance in the world of CVA, it is best practice to assume a certain margin of

error in the calculation. Electromagnetic inefficiencies and component tolerances always factor in, and, as such, prudent design would typically include a safety margin of roughly 20 percent. For example, if the amplifier is rated for 100W, you load it with a speaker load of 80W or fewer.

Frequency Limiting “Theoretical impedance” is frequency dependent. Therefore, it becomes crucial that the audio sent to the transducers is limited to the range of intended operation. The speakers involved with a 25V CVA distributed audio system are not generally intended for full-frequency/high-fidelity audio (20Hz to 20kHz); more likely, they’re intended to reproduce accurately between approximately 80Hz and 14kHz. Outside of that intended operating region, the impedance begins to drop. Any attempt to force the (continued on page 110)

Profile for Sound & Communications

Sound & Communications May 2019, Vol 65 No 5  

AV innovation and application has affected the education experience in more ways than in any other vertical market these days. Each May, So...

Sound & Communications May 2019, Vol 65 No 5  

AV innovation and application has affected the education experience in more ways than in any other vertical market these days. Each May, So...