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Registered Member #71
Joined: Thu Feb 09 2006, 08:23AM
Location:
Posts: 63
I've always heard about 70/100V speakers, and transformers designed to tap off these 70/100V lines..
Excuse my ignorance, but I dont understand. Why not just specify impedence, what is the voltage spec for? I have a 25W horn speaker that has '100V' labelled on it, but no indication of how many ohms it is, how do I wire this up to an amp?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
If you really want to know the impedance of a 100v line speaker: Ohms=(100v squared)/(power rating in watts). So your speaker is 400 ohms.
The voltage spec is because that's the most convenient way to specify a system that could have dozens of speakers, and more speakers getting added and removed all the time over its life. If you specified by impedance, you'd have to work it out all over again every time you added a speaker, to make sure the amp wasn't getting overloaded. And those (1/Rt)=(1/R1)+(1/R2) etc would get tedious pretty fast with a term for each speaker.
Specifying the line voltage means that you can just keep hooking more speakers in parallel, and as long as the sum of their rated powers doesn't exceed the rated power of the amp driving the whole mess, the amp shouldn't get overloaded.
Your horn speaker is probably 8 ohms inside with a built-in transformer to match it to the line, so I would open it, bypass the transformer and hook it straight to an ordinary stereo amp.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
70/100 systems are typically constant voltage distributed systems.
these are typically connected to an amplifier with a transformer. the primary side of the transformer is high impedance, and the secondary side (which connects to the speakers) is the matching impedance (i.e. 4, 8, 16 ohm) to the speaker.
You would connect the entire system (multiple speakers, impedances, etc...) in a way such as not to exceed the minimum impedance of the amplifier.
for a few speakers, you probably don't need any transformers. The transformers are just needed for large distribution speaker networks where you want to run the primary side at high voltage (relatively) and low current.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
My understanding is that, like mains power distribution, the power losses are smaller if you use a higher voltage. Even an ohm or 2 of cable resistance is a significant loss if you are running an 8 Ohm speaker. The problem get's worse if you are trying to feed a set of speakers in parallel. Building's "tannoy" systems often use this sort of thing. Imagine trying to feed a hundred speakers in parallel, the cable losses would be huge. If you connect the output of the amplifier to a transformer and step it up from a few volts to 100 then you can distribute this quite easilly. Each speaker is then connected to the line through a step down transformer. Also, by using different tappings on the transformers each speaker can have the volume set differently.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
My old school had a system like this. There was a speaker in each classroom and an amplifier and mic in the head master's office. One day I was at a flea market with my dad and we found an old power amp with a 100V line output. So on the last day before the summer holidays, me and some friends unplugged the original amp, hooked our own amp up in an empty classroom, connected a mic and some turntables to it, and played requests. We started taking donations for charity, but in the end we made most money through teachers bribing us to shut up.
I think if you tried anything like that nowadays, you'd be zip-tied to your desk and have your Ritalin dosage doubled.
Anyway, why is this relevant? I noticed that the 100V system let the wires be much thinner than regular speaker cable, and you could tap into the system at any point, the speakers all being in parallel. My power amp was just a regular solid-state amp with a step-up transformer built into it.
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100V speakers?
