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Old 19th November 2002, 07:35 PM   #1
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Default 8 Ohms to 100V line system

Hi there

I need some info urgently on a particular PA system.

At the moment the amplifier has speakers connected to it via the 8ohm output.

System is now being changed to 100V distributed line system with transformer mounted speakers. In some countries 70V is used but 100V is also available which we are using.

As we already have a 100W amplifier with 8ohm output, is it possible to convert this 8ohm output to 100V output.
Using standard formulas we can work out the voltage at the 8ohm output as about 28.3V. So if we use a transformer and step up the voltage to 100V, will that work?

What are the pros/cons of doing this way.

Alternatively we could purchase another amplifier with 100V output but would appreciate any help if the first method could be made to work.

Info on 100V system:

http://www.atlas-soundolier.com/techLib/

http://www.atlas-soundolier.com/techLib/70voltRules.cfm

Thanks

gunmasterg9
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Old 19th November 2002, 09:03 PM   #2
lohk is offline lohk  Europe
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No.
You have to use a suitable transformer (the autoformers at your link) to convert a "normal" hifi or pro amp to 100V system. It seems better to use a special 100v system amp though.
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Old 19th November 2002, 09:26 PM   #3
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Any particular reason for saying no, lohk?

Actually I have been able to obtain a transformer which I have been told by the supplier that it will accept 8 ohm input and the output is 100V.
On the transformer the outputs are tapped 50, 70 and 100W.

I think the only difference between an autoformer and a transformer is that an autoformer allows stepup as well as stepdown configurations.

I may be wrong so would appreciate other comments.

Thanks

gunmasterg9
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Old 19th November 2002, 10:24 PM   #4
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You will need a transformer or autoforner in order to run the 100volt line. It isn't necessary to purchase a commmercial mixer amp or power amp to be able to do this. You must remember that the correct autoformer/transformer must be used. In your case you need one capable of working with a 100watt amp. The second thing to remember is that your load cannot exceed the rating of your amplifier. In other words your load must be less than 100watts, the prefered load would be in the neighborhood of 60-70 watts. The last thing to remember is that stepdown transformers must be used at the speakers. The only exception to this rule is if you have an amp large enough it can be run in bridge mono and drive the speaker load directly. An example of this would be a Altec 9440 running bridge mono(800watts) driving a 70 volt line direct. Don't try this because your amp is only 100watts and not capable of driving the load directly. I hope this helps!

Joe
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Old 19th November 2002, 10:35 PM   #5
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There are differences between autoformers and transformers. Autoformers only have 1 winding and work on a ratio. Transformers have a primary and a secondary winding. Autoformers will pass DC, transformers won't. The best choice for any application is a transformer as it will provide more protection and isolation.

Joe
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Old 19th November 2002, 10:42 PM   #6
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It sounds like you have an autoformer not a transformer. A transformer would have common,4ohm and 8ohm on the primary side and 0 or common, 70volt,or 100volt on the secondary side.
On a autoformer you have to match the output wattage or ratio as your taps clearly show. If you have a model number I can source the diagram and let you know for sure.
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Old 19th November 2002, 11:26 PM   #7
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gunmasterg9,
100V line systems transformers are specified so that the output of the transformer is 100V rms when the specified input is fed to the primary.
That means different turns ratio for different input power ratings.
IOW, when 50 W is supplied to the primary 50W winding, the output will measure 100V, and when using the 100W primary the output secondary will still be 100V, but double current is available at the secondary.
"Using standard formulas we can work out the voltage at the 8ohm output as about 28.3V. So if we use a transformer and step up the voltage to 100V, will that work?"
Yes, that is all a standard 100v line transformer does.
As a note, I have used and repaired PA amplifiers with torroidal 100V line output transformers, and these sounded rather better than conventional EI lamination output transformer PA amplifiers.
One thing to be wary of is that if your amplifier will be running continuously at maximum (100W) power, and you may need to provide fan forced cooling, as most domestic 100W amplifiers will not handle running at full power continuously.
Also you may surprised how loud 100W is spread over a large array of speakers (supermarket ceiling for example), so it is unlikely that you will in fact run at full power long term.

Eric.
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Old 20th November 2002, 12:59 AM   #8
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Thanks for the replies

Okay, max power required would be about 60W.

One other point I need clarified.
Inductive loads

When an autoformer is connected to the 8ohm output does the amplifier see a purely inductive load?

How does that differ when the 8ohm output is connected directly to 8ohm speakers. Are the speakers also purely inductive loads?

Do I need to use a capacitor when connecting the autoformer to the 8 ohm output?

gunmasterg9
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Old 20th November 2002, 07:32 AM   #9
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Best to use an isolating transformer as the coupling/stepup, and resistive speakers should reflect a resistive load back to the amp.
It may however be good to put an RC network across the amp output - 15 ohm or so, 5W in series with say 1 uF polypropylene.
Speakers are inductive with series resistance, and low value of shunt capacitance.

Eric.
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Old 20th November 2002, 08:14 AM   #10
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Don't forget that a 100 V system can be a difficult load. On some 100 volts amps I've seen had HF-protection (oscillation protection). I suspect that the amp must be designed to drive 100 V transformers.

I turned up the treble on one amp, ioioouuoio, self oscillation!
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