Speaker systems: When is 25 volt used vs 70 volt?

Question

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.

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?

Answer 1

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.

Answer 2

There are several reasons for 25 volt, as stated above, also, there is a small niche market for in POOL speaker systems, where code forbids anything over 30 volts. In-pool speaker manufacturers usually spec the commercially available 25 volt amps like yours.

Answer 3

25 volt audio is primarily used for School Paging/ Intercom systems. This provides the correct speaker impedance for intercom audio at lower wattage levels. When there is a 2 way intercom audio connection to the classroom speaker, the speaker acts as a microphone during listen function of the intercom audio connection (half duplex).

70 volt audio is used primarily in one way paging audio systems.

Answer 4

25 volt systems were used a lot in vintage juke boxes and older PA systems. It is useful when matching an existing system. You will see 25 and 70 volt on PA amplifiers and juke boxes from the 1960's and 70's. 25 volt has slowly disapeared, and most amplifiers just have 70 volt line now. 100 volt in europe.