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How to install 70 volt speaker system...anyone ?
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lukejs
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23rd May 2010
Old 23rd May 2010
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How to install 70 volt speaker system...anyone ?

HI.. I'm helping a friend install sound in his restaurant, and it looks like he's gonna need about 10 speakers ! How do you install that many speakers with one amp ? I remember opening up a JBL speaker and finding a transformer...and wondered what the heck that was for..... Can I just go out and buy off the shelf speakers and install a transformer and some basic electronics, and get away with 10 speakers hooked up to one amp ? Thanks for any advice !! Luke
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23rd May 2010
Old 23rd May 2010
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70 volt systems are pretty easy - you normalize 0dB to 70v RMS and use transformers to tap off the power you need at each speaker.

So, you get, say 10 10w speakers with transformers. Make sure you have 100w or more available at your amp and you're golden.

http://www.jblpro.com/pub/manuals/pssdm_2.pdf


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23rd May 2010
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There is tons of documentation on the WWW from major manufacturers. These are also called "constant voltage" systems, and come in several nominal voltages (100v export, lower voltage in schools, etc).

The general concept is to step up the amplifier output voltage to reduce losses in the long runs of speaker wire, and hang all the speaker loads in parallel. These individual speakers have step down transformers built in that are tapped (based on turn ratio) to draw X watts from the nominal 70V line. This transformer determines relative loudness between speakers to account for how close or far from listeners they are located, or to make different loudness zones. To have real time control of loudness between zones requires separate channels of amplifiers for each zone.

So basically you figure out how many speakers you plan to use, and how much total watts they will need, and then hang them onto a CV power amp that makes more power than you need.

In general you use a slightly larger power amp, that total output power needed to account for transformer and wiring losses.

JR
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23rd May 2010
Old 23rd May 2010
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Hi
After a fashion, yes.
The idea behind 70 Volt and perhaps more common in the UK, 100 Volt line systems is that you have an amplifier that puts out 70 Volts at maximum output which for argument's sake is say 120 Watts RMS.
You can now attach speakers via individual transformers which will take some power from this line. Each speaker may be set to say 10 Watts so you simply add up all the speaker's power settings and as long as it is not more than 120 Watts you are all set.
Typical speakers (or the transformers designed for this work) are rated in Watts RMS and usually have many 'tappings' to allow you to have greater or lesser volume from any given unit. A typical 'naked' 70 Volt line transformer would have labelled secondaries for 4/8/16 Ohm speaker (allowing you choice of actual speaker) and you would select the primary tapping to suit how loud you want. This is all relatively easy so that at install time you simply add the individual speaker 'wattages' together and ensure your power amp will give at least this much. Basically the 'hard' maths is done for you.
You can get power amps with transformers already fitted internally (your safest bet) or you can add an external 'step up' transformer to take say a 120 Watt power amp rated for 8 Ohms speaker, up to 70 Volts. Note that not all power amps are really happy driving a transformer and you should enquire about it's suitability if you go this route.
Note also that some of the bigger power amps available can actually produce 70 Volts RMS when 'bridged' (mono) mode. This would normally be for amps capable of 620 watts or more into 8 Ohms. I presume you won't need this much power in a restaurant but as power amps are getting so cheap it is worth thinking about.
Incidentally, the reason for going 'high voltage' is to reduce power loss in thin cabling allowing skinny wires around your restaurant. (more reading on theory if you wanted to). It is the reason the power is distributed on pylons at 132,000 Volts but transformered down to 240 (110) Volts for your home.
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lukejs
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24th May 2010
Old 24th May 2010
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Thanks for the replies

OK.. thanks ! I'm starting to get it now. I read where they say that my amp shoud give 20 percent more watts than I need , in order to make up for losses from the cable runs and the transformers... I like the idea of transformers with mulitple taps, so I could install a switch on each speaker to select from, for example, 3 different volume settings .. anybody know where I should look for such a transformer ? one that I could get about 50 watts to the speaker at maximum volume ? thanks !
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24th May 2010
Old 24th May 2010
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Hi
Canford Audio in the UK is one, but you are nowhere near the UK.
Sowter make some expensive ones. Look at the specifications as ones for 'PA' use are often 'speech band' only as they typically feed 'horn' speakers with no response below 200Hz.
Do an internet search for audio transformers and look for companies within easy reach.
Transformers are readily available up to about 200 Watts but beyond that there are probably better technical solutions (amplifiers remote mounted and feed at line level).
Matt S
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25th May 2010
Old 25th May 2010
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ripple_fx1 is offline
QSC makes an autoformer you can tack onto the outputs of any amp, Atlas Soundolier probably has the other end. 50w is gonna rock the house - background music and paging duties won't require quite that much power.
There are other attenuator accessories which will allow you to make zones and control speaker volume without need for a switch. Typically, you'd optimize coverage with smart speaker placement, set the taps for even SPL throughout the facility, and adjust volume at the preamp. There are plenty of good drop-ceiling speakers available now - makes wire runs, coverage and install a snap. Speaking of wire - plenum-rated (CL-2) unshielded and twisted.
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25th May 2010
Old 25th May 2010
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Quote:
Originally Posted by lukejs View Post
OK.. thanks ! I'm starting to get it now. I read where they say that my amp shoud give 20 percent more watts than I need , in order to make up for losses from the cable runs and the transformers... I like the idea of transformers with mulitple taps, so I could install a switch on each speaker to select from, for example, 3 different volume settings .. anybody know where I should look for such a transformer ? one that I could get about 50 watts to the speaker at maximum volume ? thanks !
You probably don't need 20% extra power. I'm sure some installs have no extra capacity but 10% is an adequately conservative target. Of course too much power is better than not enough. When I was working with these 300W was about the highest popular power point for dedicated 70-100V amps, and they ranged all the way down to 5W mixer-amps still with 70V outputs.

There are many speakers sold for this instal market with the transformer already built in. If you use a conventional amp and step it up to 70V, some of the add on step up modules use an autoformer instead of a true isolated output winding transformer to reduce cost. This is not a huge deal but professional installers prefer fully floating 70v lines since they are more tolerant of inadvertent shorts to one side or the other.

There are many companies making these products so the models to choose from is probably larger than you think.

JR
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