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Old 1st April 2005, 08:02 PM   #1
edave is offline edave  United States
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Default Powering speakers over long distances?

Hello everyone-
I'm sorry if this is a common topic, I tried some basic searches through the forums and couldn't find anything right away, so I'm posting the question here-
Here's my dillema. The hall I live on has decided to upgrade our current audio system, which has speakers located in all 3 bathrooms (for shower music), and 2 lounges. Essentially we've got a two-channel system that has several different speakers playing off it. Currently everything runs off of one input, with a receiver-type amp powering one lounge, and a PA-style 70v amp powering the other 4 locations. What we want to do is switch over to a 7 channel system so we can send different music to different locations. While we've taken care of the problem of having different music across 7 channels, using a computer w/ a 7.1 channel card and some software hacking, I've gotten into a disagreement with a guy on our hall about how it will work to seperately amplify our different locations, that are a long ways from the source. Since we will need to amplify every location individually, we will need to get three additional amps (since we have five locations, and two amps already). The three additional amps need to provide power to speakers that are roughly 25', 50', and 100' away from where they are. He believes there is no way a normal amplifier could handle wires that long using 16-18 gauge wire. We need would need to provide around 15-20W rms at each location, figure we're using 4-8ohm speakers. My question is for the forum is if it is realistic that we could get some crappy 30-40W amps and get away with doing this? We really don't have the much money to spend, so a lot of the simple options like just getting a powerful 8-channel amp is out of the question. We're not looking for high-quality audio, so having some power/frequency loss is alright. If anyone could give me some rough guidelines on how long distance speaker wiring works, or any other instruction, that would be greatly appreciated!
Thanks so much,
Dave
PS- I have an EE background, so feel free to be technical, or even propose things like building our own amps, would be cool
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Old 1st April 2005, 08:47 PM   #2
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edave,

basicly there are 2 ways of doing this.

1. using speakers which are remote from the amp needs a 70 or 100V system. This is made of a transformer connected to your amp which secondary is 70 or 100V. At the speaker it is the other way round. So you need one transformer per speaker and one at the amplifier.
I have seen these transformers at Radio Shack.

2. using long signal cables from your computer to the remote amps. These cables can be common coax, like 75 or 50 ohm.
But to drive these low impedance long runs of cable you will need a suitable buffer. This can be made from video buffer/opamp ICs. If you terminate the end of the cable with a resistor equal to the cables impediance, you can use very very long runs without loosing signal.

hope this helps,
Dick.
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Old 1st April 2005, 11:12 PM   #3
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AFAIK, a 70V system requires relatively expensive tranformers at each speaker node so that is out for a "student" system.

Using the PC multi-channel output is a brilliant hack and can easily make use balanced signal runs (with some more cheap hacks)

PC channel line out
single-ended to balanced convertor A>1
cable
balanced buffer A>1
cable
amplifier
speaker

Since you are a school, you can make a bunch of balanced buffers using all the spare op-amps in the lab. The balanced signals will help bridge the distances without too much loss. Since you're not too worried about quality, don't bother with expensive XLR jack and hard wire the 3-conductor runs directly. Even the amplifiers can be hacked together using what ever is available, including old car amps, PC speakers, or any of the previously used equipment.

:)ensen.
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Old 1st April 2005, 11:35 PM   #4
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You're doing it the hard way. Just put a radio in each room. You can tune each to different stations. No speaker wires.

Now if they are FM Broadcast Band radios and you get seven low power liscense free FM transmitters you can control the content!

Forgive me if you need something far more complicated.
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Old 2nd April 2005, 05:39 AM   #5
Enzo is offline Enzo  United States
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I like the radio per room idea too.

For that matter, powered speakers at each location can be fed wirelessly, or you can run a balanced signal cable out to each. They even make remote speaker systems that transmit the signal through the AC power lines in the building. Plug the speaker into the wall and listen to it. You don't need low power licenses for this, commercial products are on the market already, look at Best Buy or the Shack and you will probly find them.

HAvving the amplification at each speaker allows control of volume adn whatever right there. WHo wants to walk down the hall from the shower to the control room just to turn it up or down?

RUn seven pair line to each room from central, then mount a selector switch in each room, so they can select which program they want from there. This again requires the speakers be powered or amplified somehow right in the destination room.

If you want a central location and speakers individually programmed from that location, it is not much diferent than having all the same, just need more amplifiers.

70v systems are great for serving an indeterminate number of speakers off one line without concern for impedance. Also, the wire does not have to be immense. But nothing says you cannot use 70v for just one speaker.

20 watts for a bathroom speaker would be deafening. 10 watts is more than enough, and coincidentally the common 70v transformer is usually good for 10 watts.

You can take the seven amp outputs, and run them up to 70v, and put 70v speakers at each location.

DO you really care that much about efficiency? And I assume this is not a critical listening environment - no one will be sitting on the john thinking about the soundstaging and separation - apparently it will be mono anyway. So with that in mind, you can run #16 wire 100 feet and ignore the cable losses. If it takes 20 wats at the head end to get 10 watts into the speaker, who cares? Just wire it up as if it were across the room.

70v transformers are not expensive, in fact they are extremely cheap.
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Old 2nd April 2005, 10:06 AM   #6
edave is offline edave  United States
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rcavictim - FM would be a great way of doing it. Unfortunately, I live in "sturdy" building built in the 1920s, I think more common phrase is "concrete bunker" nowadays.
Purple- I really like this idea. Hunting around, I found that TI offers two chips, ( the DRV134 and INA134) that do the single-ended to balanced conversion and back.... and luckily I can get free samples from them....
My one question concerns the balanced pair cabling- does it need to be a twisted-pair STP, or could I get away with having 3 twisted wires, or even just a twisted pair and one strand out of a cat5 cable? I don't think noise is going to be that much of a problem (and quite frankly, even if it was, I think our aux speakers are bad enough you'd never know ).

Thanks so much for everyone's help!
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