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[Christian Tepfer]

In order to achieve good sound (as good as it gets) use high quality transformers and cable.

[Eric Madson]

This can be achieved with a consumer piece, but as everybody else indicated, you should look at purchasing a pro 70V amp.

determine the rated output of your amp and find a relevant transformer at the following link.
http://www.eidusa.com/Electronics_Trans_Audio_117_119.htm

ex: 60watts/8 ohms= EID-119Y60

wire the amps output to the txfrms 8ohm-common taps. Wire the speaker output side to the 70v-common side. This will give you the correct voltage ratio for a 70v tap at each speaker. Each speaker requires a 70v/watt tap and the total wattage of all the speakers tapped cannot exceed 60 watts. The txfrmr is rated for 50-10k but I would try to High pass the audio so it sees nothing below 80hz. Transformers do not like low frequencies because they saturate and look like a short to the amp. Jensen also use to manufacture some good audio transformers but I cannot find any on the web.

[Hal Bissinger/COMSYSTEC]

Looks like we are going around in circles. This was posted earlier by Don Boone:

Some consumer receivers can't tolerate a 70 volt transformer on it's outputs. Decades ago I remember a DIY install that used a JVC receiver from Circuit City driving a 70 line. The receivers would last about a month. After the third replacment Circuit City cried foul and the customer had to buy a proper TOA 70 volt amp.

-Hal

[Eric Madson]

I believe Don is referring to "Directly" driving a 70V line. If you step the amplifiers rail voltage up to 70V you have a known voltage output making the tap settings relative.
If you have a "consumer" or "pro" piece that is only rated at 100w@8ohms then the max rail voltage is only about 28v. If you try to tap off multiple 70v speakers for a total of 100w you will load the amp down past its rating.
If you step the output voltage up to give you 70v at rated out you will have no issues.
I have done this countless times with no problems. The only issues I have had is cheaper transformers that don't like lower frequencies. If you high pass you should not saturate the transformer at higher power levels. Make sure you purchase a good quality Audio transformer.

Maybe Don can clarify

[Don Boone]

I was referring to connecting 2 good quality transformers on the 8 ohm speaker outputs of a JVC consumer receiver and connecting 12 70 volt ceiling speakers. The JVC would die within a month.

BTW 100 watts of 70 volt speakers would be a 50 ohm load. The JVC could have handled that load with no problem as long as the bass was turned down. The level would be pretty low though.

Don

[Eric Madson]

BTW 100 watts of 70 volt speakers would be a 50 ohm load.


Yes, assuming the amplifier is capable of reaching 70v.

[Hal Bissinger/COMSYSTEC]

Yes, assuming the amplifier is capable of reaching 70v.

Well yes. A 100w amplifier with a 70 volt output will deliver 70 volts across a 50 ohm load or more. A 100w amplifier with an 8 ohm output connected to the proper transformer (8:50) should deliver the same 70 volts at 100 watts, again across a 50 ohm load or more. So the amplifier would not need to deliver 70 volts, but as you say only about 28 volts at 100 watts. This is actually the way most constant voltage amplifiers operate. They use a transformer to produce the 70.7 or 100 volt output but they are also designed for it.

If you have a "consumer" or "pro" piece that is only rated at 100w@8ohms then the max rail voltage is only about 28v. If you try to tap off multiple 70v speakers for a total of 100w you will load the amp down past its rating.

No, not true. You will be putting a 50 ohm load on that 8 ohm output. Of course you will only be supplying about 16 watts to all of your speakers. The amplifier and speakers will operate quite happily though, assuming that's what you want.

Keep in mind also that consumer gear is not held to the same testing standards as commercial gear. The output power of consumer amps is usually hype. So that instruction book wattage you think you are getting could be much less. If you hang a transformer off the output and use it to feed a constant voltage system based on that assumed power output you may very well wind up overloading it.

-Hal