Can non-70v amplifiers be used with 70v ceiling speakers?

Jan 14, 2011
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This may belong in the install forum, but in the near future I may be asked to make an old conference boardroom ceiling audio system work again. A competitor has proposed to completely re-build the ceiling system, but I want to use the existing components.

Other than the ceiling units themselves, the only remaining component is what looks like a consumer-level mixer/amplifier from the early 80s, which I'm guessing was used to power the speakers. Assuming the ceiling speakers comprise a 70v system, can I use a regular pro-audio amp to power them?
 
Re: Can non-70v amplifiers be used with 70v ceiling speakers?

I think a 650-700W at 8-ohm amp will approximately equal 70.7V and the taps on the speakers will be accurate.

If doing that then you can load the line up to considerably more "watts" than 700 as most amps at 8-ohms don't have a problem maintaining the same voltage at lower impedances.

With the costs of cheap power nowadays it's not a bad way to go -removes the need for an output transformer on the amp design making it lighter and smaller. You still have to be careful of LF though as saturated line speaker transformers and attenuators start to resemble a short-circuit.

Using a smaller amp may work but the overall output might not get high enough. If the speakers are tapped at low wattages then moving it to the higher tap might give you enough.
 
Re: Can non-70v amplifiers be used with 70v ceiling speakers?

Simple answer, Ohm's law says that you can direct drive an 8 Ohm load at 70.7V based on P=V**2/R, which is (70.7)**2/8 or 625W, so an amp rated around 625W into 8 Ohms can direct drive a typical 70.7V system. As already noted, especially with lower cost speaker transformers it may be desired to apply a high pass filter to avoid saturating the transformers at low frequencies. Lower output amps can also often be used to drive 70V systems by adding external output transformers.

In regards to reusing the existing speakers and wiring, I understand wanting to save the client money and also wanting to get the work but will you potentially be assuming responsibility for the apparently approximately 30 year old existing work and equipment? Have you verified that everything is working properly? Is the existing work properly documented or will you be doing that?
 
Re: Can non-70v amplifiers be used with 70v ceiling speakers?

This may belong in the install forum, but in the near future I may be asked to make an old conference boardroom ceiling audio system work again. A competitor has proposed to completely re-build the ceiling system, but I want to use the existing components.

Other than the ceiling units themselves, the only remaining component is what looks like a consumer-level mixer/amplifier from the early 80s, which I'm guessing was used to power the speakers. Assuming the ceiling speakers comprise a 70v system, can I use a regular pro-audio amp to power them?

I am not familiar with any 70V "consumer" level amps. Most are designed for the dedicated duty, while most install applications are very cost sensitive. Yes a conventional amp could be substituted but to get the voltage swing you want, you will probably be leaving money on the table with an oversized amp for the power needed. As mentioned elsewhere in this thread you need to be careful about too much LF content that can saturate magnetics in the step down transformers on each speaker.

Fixed sound installers also like the transformer isolation of dedicated instal amp outputs because they are more tolerant of inadvertent shorts to ground on either speaker line, as long as only one leg gets grounded at a time.

Before you jump to replace with a non-instal amp, you may want to price what using the correct amp would cost. They should not be very expensive.

Note: you will not need very much power for a modest boardroom system.

JR

PS: Does the old install amp work?
 
Re: Can non-70v amplifiers be used with 70v ceiling speakers?

In regards to reusing the existing speakers and wiring, I understand wanting to save the client money and also wanting to get the work but will you potentially be assuming responsibility for the apparently approximately 30 year old existing work and equipment? Have you verified that everything is working properly? Is the existing work properly documented or will you be doing that?

This would be my concern too. Do you even know if the speakers work? What was good 30 years ago likely doesn't compare to many affordable, modern, options. Ceiling speakers and wiring really aren't very expensive and look at it this way: you may save the client money up front but what happens if the speakers sound like arse or are broken? Then they have to pay the money that they would have up front anyway to have you come back out and replace those, instead now they are paying more in labor as it's always cheaper to do things at once.

Being cheap may get you some bids up front, but in the long run you'll only be hurting your reputation and doing your clients a diservice. If they aren't happy with your work do you really think that they'll call you next time or recomend you to others?
 
Re: Can non-70v amplifiers be used with 70v ceiling speakers?

Use an autoformer such as a QSC AUDIO OT-600
600W 70-volt autoformer to match most power amps to a 70 volts system.