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Author Topic: 70V / 100V lines questions  (Read 8725 times)

Eytan Gidron

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70V / 100V lines questions
« on: March 08, 2008, 12:08:11 pm »

I am spec'ing a background music system with Turbosound speakers and Crest amps. The are two zones:

1) A large outdoor open area which I want to cover with 4 x Turbo TCS-121C 12" speakers at four corners. The cable length from the amp room is about 100M (330 ft). These speakers can be ordered with a 70/100V transformer option (the maximum tap is 120W).

2) A smaller space for which I want to use 8 x Turbo Impact-65T speakers. This area is about 50M (165 ft) from the amps. These speakers also have a 70/100V transformer (the maximum tap is 60W).

I have no experience with 70/100V lines, but it seems to me that because of the cable length I should go this route. My questions are:

- The speakers can work with both 70V and 100V lines. Crest has amplifiers for both 70V and 100V. Which should I use? Are there any advantages for using one type of system over the other?

- Which amplifier power ratings should I use? I would like to have the speakers operate at full bandwidth (without high-passing).

- Which cable gauge should I use?

Any advice would be helpful.

Thanks,


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Ivan Beaver

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Re: 70V / 100V lines questions
« Reply #1 on: March 08, 2008, 12:44:44 pm »

As far as what type, 100V would give you less loss.  Think igh voltage tranmission lines.  But here is the US the most common is 70V, as that is the highest that can be used under a low voltage license.

Most 70/100V transformers are going to have some low freq loss due to the transformer.

The wattage is simple, just add up the loudspeaker wattage rating as used with whatever transformer tap you use and add about 10% for spare.

The wire size will depend on different factors.  If you home run each loudspeaker, you can use a smaller wire guage.  If you are going to "daisy chain" them-one to another- the guage is going to depend on the particular wattage load you choose and the particular length of each run.

Think of wattage in terms of impedance.  On a 70V system 1 watt is 4900 ohms

So 10 watts would be 490 ohms, 100 watts would be 49 ohms.

You can do the wire gauge calculations from there.
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Hal Bissinger/COMSYSTEC

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Re: 70V / 100V lines questions
« Reply #2 on: March 08, 2008, 01:38:15 pm »

First off, since you mention full bandwidth and highpassing, Crest would not be my first choice for this application since I believe they employ output transformers. Line transformers which are necessary for either a 70.7 or 100 volt system can limit the low frequency response. You can't eliminate them from the speakers but you can at the amplifier by using one of the direct drive models such as the Crown CTS series that will drive a 70.7 or 100 volt line directly.

As for whether to use 70.7 or 100 volt, the higher the voltage the less current which means you can use a smaller wire gauge to convey the same wattage. So by all means use 100 volts if you can. Some places put restrictions on how 100 volt wiring can be installed, there are no such restrictions on 70.7 or 25 volt wiring. That's the reason you are given that choice.

Only you can determine the power levels required from your speakers for the coverage required. This is an outdoor venue and we have no idea as to the layout. You are probably going to have to pick a wattage per speaker to start with then adjust your wattage taps to get it to do what you want it to do. Choose an amplifier or amplifiers accordingly, I would oversize by 50%.

-Hal

John Roberts {JR}

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Re: 70V / 100V lines questions
« Reply #3 on: March 08, 2008, 03:53:02 pm »

Eytan Gidron wrote on Sat, 08 March 2008 11:08

I am spec'ing a background music system with Turbosound speakers and Crest amps. The are two zones:

1) A large outdoor open area which I want to cover with 4 x Turbo TCS-121C 12" speakers at four corners. The cable length from the amp room is about 100M (330 ft). These speakers can be ordered with a 70/100V transformer option (the maximum tap is 120W).

2) A smaller space for which I want to use 8 x Turbo Impact-65T speakers. This area is about 50M (165 ft) from the amps. These speakers also have a 70/100V transformer (the maximum tap is 60W).

I have no experience with 70/100V lines, but it seems to me that because of the cable length I should go this route. My questions are:

- The speakers can work with both 70V and 100V lines. Crest has amplifiers for both 70V and 100V. Which should I use? Are there any advantages for using one type of system over the other?

- Which amplifier power ratings should I use? I would like to have the speakers operate at full bandwidth (without high-passing).

- Which cable gauge should I use?

Any advice would be helpful.

Thanks,





As others have offered constant voltage systems often have compromised LF response because of the step up/ step down transformers.

For perspective imagine the size and weight of your amplifier's power transformer. That is sized to pass power at 50 Hz, now to pass that same power at 25 Hz, it would have to be twice as big and heavy. If you could find one rated to go that low. Most constant voltage designs don't even try to hit 20 Hz on low end at full power.

As has already been mentioned, you can get around the limitation of the step up transformer end by using direct drive. A 100V line is the same voltage as a conventional amp that delivers 1250W into 8 ohms. Your only LF limitation in that case will be the stepdown transformers on the turbosound speakers which hopefully will be of similar performance to the speakers (but check the specifications).

If you are driving 8x60W taps and 4x120W taps from a single line that's almost a kW, so a 1250W@8 ohm amp is not overkill. However for background music you may not want to run at full volume, especially in the smaller space.

A stereo amp gives you two zones, so roughly 500W on each side of a 2x1250W@8 ohm amp.

JR


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Eytan Gidron

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Re: 70V / 100V lines questions
« Reply #4 on: March 09, 2008, 05:22:22 am »

Thanks all for the replies,

I now have a much better understanding of what is needed.

So, according to Robert I could use 1 Crest 9200 (1300W at 8 ohms)? I have that in stock and it would be great if I could use it.
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John Roberts {JR}

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Re: 70V / 100V lines questions
« Reply #5 on: March 09, 2008, 09:56:08 am »

Eytan Gidron wrote on Sun, 09 March 2008 04:22

Thanks all for the replies,

I now have a much better understanding of what is needed.

So, according to Robert I could use 1 Crest 9200 (1300W at 8 ohms)? I have that in stock and it would be great if I could use it.



1,300W @ 8 ohms is 102V so that's close enough to 100V for me.

I would still suggest checking the LF response of the step down transformers on the speaker end and see if HP filters are needed in send to power amp, The transformers response should be rated at tap power, so at reduced volume you could handle somewhat more extended LF bandwidth, just avoid pushing the bass at high volume.

JR (aka John )

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Ivan Beaver

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Re: 70V / 100V lines questions
« Reply #6 on: March 09, 2008, 10:22:41 am »

Agreed.  It would also be a good idea to do an impedance measurement of the particular cabinets as some transformers start having a very low impedance at lower freq.  This could put a strain on the amp if trying to "push" much bass.

That is one reason why the bass is generally rolled off in 70/100V systems.

He could also put a cap in series with the load to roll off some of the bass, but would have to figure out where he wants that to happen, based on particular loads and freq needed.

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Danley Sound Labs

Hal Bissinger/COMSYSTEC

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Re: 70V / 100V lines questions
« Reply #7 on: March 09, 2008, 04:48:51 pm »

some transformers start having a very low impedance at lower freq. This could put a strain on the amp if trying to "push" much bass.

All transformers do sooner or later. I have found that the transformers included with these speakers are usually pretty good. This is also why I always say that for applications where you supply your own transformers, always look at the frequency response and the weight. Bigger and heavier means more iron and the better it will be at lower frequencies. I also recommend overpowering the system to help with the low end. Those transformers are going to require more power than the listed tap wattage if you want to go below their low frequency spec.(within reason) and see it at the speaker. That 1300 watt amp sounds like it will do it for your background music.

-Hal
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