I am doing research on installing a small constant-voltage speaker system using a ten year old TOA BG-115 Integrated Amplifier, and I am trying to determine what the reason is for the existence of 25 volt wiring for distributed audio background music / announcement building speakers.

TOA Integrated Amplifier BG-115 TOA BG-115 audio controls TOA BG-115 Audio output transformer, with taps for 8 ohm, 25v, 70v

Speakers, with integrated multitap transformer and 25/70v voltage selection: http://www.specotech.com/index.php/products/audio/speakers/surface-mount/item/572-wb86t

And what this is for anyone who is not familiar: https://en.wikipedia.org/wiki/Constant-voltage_speaker_system

The more common distributed audio design is to use 70 volts. The only specific answer I have been able to find so far is that "some building safety codes require the use of 25 volts". Yes, but why, exactly?

As far as I understand the main benefit of 70 volt is operating at a higher voltage allows audio signals to travel much further through the distributed wiring network to the speakers.

25 volt will apparently have a correspondingly shorter operating distance for the same amount of line losses, and will lose more signal to resistance. How much this matters is unknown.

Apparently 70 volt can give you quite a shock even compared to 120 volt AC due to the high frequency components of audio signals. That is why a plastic cover is over the audio transformer output terminals of this amplifier, to reduce the risk of shock.

Is this part of the reason for the use of 25 volt systems, less or no shock hazard?

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    \$\begingroup\$ I'm voting to close this question as off-topic because this isn't related to sound design. This question should be on the electrical engineering SE. \$\endgroup\$ – Mark Dec 18 '16 at 11:58
  • \$\begingroup\$ This question involves audio systems which I see as a frequently asked question. If you have a problem with it, I recommend you go through this site and vote to close all other topics that involve wiring or electronics of any kind, regardless of whether it is related to audio systems. \$\endgroup\$ – Dale Mahalko Dec 18 '16 at 12:04
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    \$\begingroup\$ nothing personal, but it's off topic. \$\endgroup\$ – Mark Dec 18 '16 at 12:11
  • \$\begingroup\$ Whoops it is 70v, not 75v. This an audio distribution system and is commonly used in large retail stores, hospitals, and schools for background music and message announcements. This uses a higher voltage to distant speakers because the resistance losses are greater for regular low-voltage 4/8 ohm over 100s of meters of wire. The speakers all use step-down transformers to connect to the distribution wiring. \$\endgroup\$ – Dale Mahalko Dec 18 '16 at 19:22

In a nutshell - cost. 25V distributed speaker systems most often use the existing 16 Ohm transformer winding found in older PA amplifiers.

In addition, building the matching transformer that mounts on the rear of every speaker in the system costs less if it has to handle only a 25V input rather than beings required to handle the 70V or 100V systems most often used today. Less wire / turns of wire equals lower cost.

One final thought - a 25V distributed speaker system usually requires only Class 2 wiring.

Note that many 70V systems can also use Class 2 wiring, so long as the total power capability of the amplifier is 100 Watts or less.


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