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100 volt line conversion.

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100 volt line conversion.

Post by Rebel Rafter » Fri Jun 03, 2016 7:47 pm

Hi all, RR here. Does anyone here know anything about 100 volt line audio systems? I know what they're used for, for feeding loads of speakers around a building without putting speakers directly in parallel and placing too low an impedance on the amplifier and burning it out. But what I'd like to know is how do I convert say a 50 or 60 watt RMS power amp from low impedance speaker o/p only to 100 volt line? Can I use a line matching transformer "in reverse" at the o/p or do I need a special transformer made for the job? Obviously the said transformer would have to be rated at a bit more than the amp o/p and all the step down matching transformers for each speaker would have to be set at wattages that added up to no more than the amp o/p power. And would an ordinary amplifier do such as a typical all DC coupled transistor job typically found in older music centres and hi-fi or does the amp need to be special too? I know some specialised amplifiers have been produced that can drive 70 or 100 volt lines directly but aren't they the exception rather than the norm? All the 70 and 100 volt line driving amplifiers I've seen have step up transformer(s) fitted, some of them multi tapped like the ones in my friend's old American jukebox. RR.

 
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Re: 100 volt line conversion.

Post by Refugee » Fri Jun 03, 2016 8:08 pm

They are just an ordinary transformer.
One with 2X 110V on the primary and the same voltage at the mains transformer in the amp on the secondary should do.

 
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Re: 100 volt line conversion.

Post by Ed Dinning » Fri Jun 03, 2016 8:53 pm

Hi RR, using a 100v line speaker transformer "backwards" will work; but these are only normally rated at about 10w. They are normally wound for a variety of speaker impedances and power out to the satellite speaker, allowing the "local" volume to be set. They are quite a common component and I have a few of them here.
I recently supplied one to a member of another forum who wanted to try his 100v line amp with "normal" speakers. He was pleasantly surprised at how well it worked.
Using a mains transformer will work, but will give much reduced fidelity.

Ed

 
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Re: 100 volt line conversion.

Post by Terrykc » Fri Jun 03, 2016 10:44 pm

Refugee wrote:They are just an ordinary transformer.

No! There is a difference between 'ordinary' - I assume you mean mains - transformers and audio transformers.

Refugee wrote:One with 2X 110V on the primary and the same voltage at the mains transformer in the amp on the secondary should do.

I don't see the relevance of the mains transformer secondary here.

It could be 350-0-350V for a valve amplifier or, say, 40-0-40V for a solid state amp with similar power ratings!

The transformer at the amplifier end should produce 100V RMS at maximum output - whatever that is. Ideally, the total power consumed downstream should equal the maximum rated output of the amplifier.

The usual formulae for power and resistance will give the RMS figure for the output of the amplifier and, therefore, the transformer input.

From this, the transformer ratio to generate the 100V line should be obvious - all that is now needed is to source an appropriate Audio transformer for the purpose. As this is unlikely to be available 'off the shelf', might I point your in the direction of Ed Dinning, our acknowledged expert in this field?

 
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Re: 100 volt line conversion.

Post by crackle » Sat Jun 04, 2016 3:48 pm

You can often find 100v line OP transistor amps of 100w output for a few quid at bootsales.
They dont often go for much money as they are not looked on as HiFi. and are usually mono anyway.

Mike

 
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Re: 100 volt line conversion.

Post by Ed Dinning » Sat Jun 04, 2016 5:11 pm

Hi RR, note that a 100v line op trans will be a very different beast if it is for a valve or a transistor amp.


Ed

 
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Re: 100 volt line conversion.

Post by Pamphonica » Sat Jun 04, 2016 9:30 pm

Watch out for the loadings on your amp. The matching techniques are quite different.

A 100V line system chucks out 100V at max output, irrespective of load up to the permitted max wattage of speakers connected. The PA speakers have tapped transformers inside to present whatever load is needed to get the required watts out of the speaker, based on a standard 8 ohm speaker inside the housing.

The audio amp always sees about 8 ohms irrespective of power rating of speaker corrected and has to chuck out a suitable voltage to deliver the required power.

Example: A 100W PA speaker (100V line type) should present 100 ohms load to the PA amp. R= V^2/W.
To use this speaker with a standard 8 ohm output audio amp, the matching transformer will have to match 8 ohms at the amp to 100 ohms for the speaker and be rated at a lot more than 100W. Impedance ratio about 12:1

If you use a 10W PA speaker it presents a 1000 ohm impedance to a 100V-line PA amp. R=V^2/W again.
So if you tried the 100W matching transformer on a 10W speaker it would present an 80 ohm load to the amp, which it might not like, ESPECIALLY if it was a valve amp!

So you need to match the PA speaker correctly to the amp, based on the speaker power rating.

I don't know if this helps, but it might be worth trying out the maths first before using a matching transformer.

Put simply, if you have a 10W PA speaker make sure you use the 10W tap on the matching transformer, to match correctly to 8 ohms.

Jeremy

 
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Re: 100 volt line conversion.

Post by Rebel Rafter » Sat Jun 04, 2016 10:41 pm

Hi, from RR. Thanks for the advice. I've not seen ^ used in a formula before, what is that? is it V squared or is it a square root? I know some audio power calculations use a square root but I can never remember the formula, I did have it all written down somewhere but I've no idea where now. I'll have to do a "back to basics" exercise at some point. Or is it the V squared over R formula I seem to remember from college way back. ISTR there is three formulae for power calculations, V x I, I squared R, and V squared over R. Or have I got it wrong again... That wouldn't surprise me. I might have a book stashed away somewhere with it all in, but if I have I can't remember where, I've that many boxes full of stuff lying about. I suppose it's a bit difficult displaying a formula with a keyboard as there isn't the right symbols, at least not the ones I know. RR.

 
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Re: 100 volt line conversion.

Post by Pamphonica » Sat Jun 04, 2016 11:04 pm

Sorry. V^2 is shortcut for V-squared as I cannot put a 2 as a superscript. It came from good old Basic programming.
So V^2/W is the same as v-squared divided by watts.
Your formulae are perfect - power in watts = v-squared divided by resistance
(derived from power = v times i and i=v divided by r from good old ohms law)
Sorry, I seem to have gone all algebraic. Must be time for bed!
Jeremy

 
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Re: 100 volt line conversion.

Post by Terrykc » Sun Jun 05, 2016 12:45 am

Crackle wrote:You can often find 100v line OP transistor amps of 100w output for a few quid at bootsales.
They dont often go for much money as they ... are usually mono anyway.

Errr ...

So are amplifiers and loudspeakers, Mike, unless you have two of each ...

It is important to consider the application here. If it is a fixed installation, in a hall or theatre say, a decent stereo installation might require nothing more than one or two pairs of good line source speakers, which might well be fed at low impedance.

The 100V line system is primarily used for PA work and excels when the coverage area is large and cable losses are important. Stereo is unlikely to be a requirement in such systems whereas full coverage is.

Thus relatively thin and easily run flexible cables can be used - important, in particular, for temporary events - and the power tapping on each speaker allows it to deliver the amount of power appropriate to its location ...

Does this make sense to you?

 
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Re: 100 volt line conversion.

Post by Herald1360 » Sun Jun 05, 2016 11:28 am

:aac
Pamphonica wrote:
Example: A 100W PA speaker (100V line type) should present 100 ohms load to the PA amp. R= V^2/W.
To use this speaker with a standard 8 ohm output audio amp, the matching transformer will have to match 8 ohms at the amp to 100 ohms for the speaker and be rated at a lot more than 100W. Impedance ratio about 12:1


Jeremy



Exactly. So the transformer would need a winding ratio of 1:sqrt12 or about 1:3.5

A mains tx with 110v primary and a 30v secondary could be used backwards for low Fi results- probably OK for PA speech announcements.

 
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Re: 100 volt line conversion.

Post by crackle » Sun Jun 05, 2016 1:15 pm

My point was that buying an old amp with 100v line OP is much cheaper than buying an OP transformer or even a mains transformer. Being mono they are not popular with hifi and the 100v. Most common PA amps now days are low impedance OP and 2 channel, but often able to be switched to "bridged" mono use.
I have bought a few of these over the last few years, I gave an Adastra amp to Jeffrey just recently for the Radio Museum.
Mike

 
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Re: 100 volt line conversion.

Post by Terrykc » Sun Jun 05, 2016 3:45 pm

Herald1360 wrote:A mains tx with 110v primary and a 30v secondary could be used backwards for low Fi results- probably OK for PA speech announcements.

Don't forget that, as Jeremy has used a 100W speaker in his example, and such systems would normally be expected to use several such speakers, we are talking serious power here - perhaps 500W - 1kW or even more - which, in turn, means serious money and would rarely be used just for PA speech announcements, so why skimp on what will probably be the cheapest component in the sound system anyway?

Many 100V line systems will be used mainly for low level sound reinforcement: lots of low power speakers arranged for even distribution of sound over a wide area rather than the old lots of power - and volume - at one end philosophy. Power ratings might not exceed 10W and could be as low as 1W for individual units.

 
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Re: 100 volt line conversion.

Post by Pamphonica » Sun Jun 05, 2016 4:57 pm

Terry's point is a good one. 100V line is definitely a distributed power approach. For our village PA, I (currently) use 8 horns round the field and arena all set at between 10 and 30W. Driven by a 200W PA amp that works out very nicely for background music and safety announcements.

Most outdoor PA systems deliberately have a very limited bass response. The output transformers are designed for a low-end cutoff of probably no better than 100Hz. If you use these amps and speakers for music it's pretty tinny! Horns are of course even worse at fidelity but you do get good noise levels for speech! Big wide-range PA cabinet setups used for rock concerts are not usually 100V line. They are 4 or 8 ohm, often individually amped.

On the other side, the HiFi amp will have a much better lower-end frequency response, but PA speakers will not do it justice. I am sure someone like Ed Dinning or Mike Barker can advise on how you design transformers for better LF response.

Jeremy


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