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Discussion Starter · #1 ·
Hi I have 5 speakers running on a 70 volt speaker system. Interfacing this is a 70 volt power transformer rated at 100 watts. Each speaker is set for 5 watts. I have blown two home recievers, first one went the first time I hooked it up, the second one took about a month to finally go. Any recommendations on what type of amplifier I can buy that will be able to drive this system. I only use the reciever for playing AM/FM music through the system. I would like to just be able to use a regular reciever if possible, but now I am wondering if I also need an external amp?


Thanks in advance for your suggestions!
 

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Quote:
Originally Posted by gtv /forum/post/0


Hi I have 5 speakers running on a 70 volt speaker system. Interfacing this is a 70 volt power transformer rated at 100 watts. Each speaker is set for 5 watts. I have blown two home recievers, first one went the first time I hooked it up, the second one took about a month to finally go. Any recommendations on what type of amplifier I can buy that will be able to drive this system. I only use the reciever for playing AM/FM music through the system. I would like to just be able to use a regular reciever if possible, but now I am wondering if I also need an external amp?


Thanks in advance for your suggestions!

There are may amps that are inexpensive and will work for you. You can hook your receiver preamp out to the 70v amp and use it that way.
 

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Discussion Starter · #3 ·
I currently am hooking up the reciever directly to the power transformer. The transformer is used to impedance match the speakers. It is rated for 100 watts and the impedance is supposed to be 8 ohms. But my 100 watt per channel amplifiers struggle to work and "cut out" at about half way. I have had two of them fail. Any suggestions?
 

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In big systems, the usual problem is that someone adds a normal 8 ohm speaker without the speaker transformer.
 

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Hello, we are replacing an old 70v amp in our Lodge with a new Crown 160ma amp. The old amp plugs say 70 & 70 v. there was no listing for com. How do I determine which is ground in this system? does it matter? the new amp states one of the 2 wires should be com. The amp is about 30 ft from the first speaker.


Thanks Thor
 

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Normally neither speaker wire is grounded. But a multi-branch system (2 or more amp. channels) may have a common. Also each speaker transformer will have a terminal marked common.


Rane paper on 70V systems:


Constant-Voltage Audio Distribution Systems

* 25, 70.7 & 100 Volts

* U.S. Standards

* Just What is "Constant" Anyway?

* Voltage Variations -- Make Up Your Mind

* Calculating Losses -- Chasing Your Tail

http://www.rane.com/note136.html
 

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On a 70v system the output transformer of the amp usually does NOT connect chassis ground to the common terminal.


On some amps there is a removable jumper to tie the chassis ground and the output common terminal together but they are rarely used.


Question: when you connect to the 100 watt transformer, are you connecting you receiver output to the 70 volt primary side or the 8 ohm secondary side?


This will work BUT you need to connect the receiver's speaker output to the 8ohm side of the transformer and use the highest wattage tap on the transformer. You may need a higher wattage transformer such as a 150watt with a 75w tap on the 8ohm side. The transformer -equipped speakers are then connected to the the 70volt line you have created and tied to the 5 or 7.5 watt taps.


This is an old trick that has worked successfully thousands of times IF done correctly.


As for a receiver with a built in 70 volt output, I do not recall seeing a one piece unit like that.


TOA makes a couple very nice outboard AM/FM tuners and they make one of the best commercial PA amps for permanent installs in the business.
 
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