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Author Topic: 70-volt wiring  (Read 8340 times)

Robert Anderson

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70-volt wiring
« on: July 10, 2006, 03:44:44 pm »

Forgive my ignorance - I am new to 70-volt systems.  

Is it true that I can wire an entire 70-volt system in parallel as long as all the sum of the taps does not exceed the wattage of the amplifier?  Or do I still need to try and break it up into series and parallel branches to keep the amp from burning itself out?

Thanks
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Brad Weber

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Re: 70-volt wiring
« Reply #1 on: July 10, 2006, 04:08:10 pm »

In a 70V (or more accurately 70.7V) system the speakers, or actually the speaker transformers, are all wired in parallel.  Within reason you may parallel as many speakers as you desire.  This can be one single branch line daisy chained from one speaker to another or you can branch off at various points and have mutliple branch runs as best fits the actual installation.  Whether split at the amp, at a speaker or in a junction box, in a 70.7V system all wiring including multiple branches should be in parallel.

In general you can add speakers whose tap values add up to the output rating of the amplifier, but you do have to be a little careful.  The cabling does have line losses and in long lines this may be a factor.  You also have should verify whether the transformer tap information is indicating the power at the input to the transformer or what is delivered to the speaker, in the latter case there may also be transformer losses to consider that raise the actual power taken from the line.  And be sure to use the 70V rating for the amplifier.  Suffice it to say that you usually do not want to put 490W or 500W of speakers on a 500W 70V amp, but 400-420W of load might be just fine even with fairly long runs.  On the other hand, with a 70V system you can also have a single 4W speaker on a 100W amp without worrying about providing too much power to the speaker.

Also be aware of any volume controls in the runs as they must also be selected to handle the attached loads.
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Ivan Beaver

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Re: 70-volt wiring
« Reply #2 on: July 10, 2006, 09:05:37 pm »

The biggest thing to understand about 70V systems is that the transformers used are basically impedance transformers.  A 1W tap on a 70V  line is 4900 ohms-five or take.  10W is 490ohms and 49 ohms is 100watts.

It is a common misconception about using really small wire for 70V applications.  As you can see a 100Watt load has an impedance of 49 ohms.  Often times the loads are higher (lower impedance) and the runs are long.  Often times we use 12 and 14 ga for 70V lines.
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Rick Johnston

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Re: 70-volt wiring
« Reply #3 on: July 14, 2006, 01:49:20 am »

I can't remember the last time I load-tested a constant voltage system (70.7 or 25) and found the speaker load to be equal to the amp's power rating.

Design specs usually call for 20% headroom, and many times I see 50%. That means a 100 watt amp shouldn't have more than 50 speakers (each tapped at one watt) connected to it.

A 100 watt amp may have 100 speakers, each tapped at one watt, but they're only 3dB SPL down in each coverage area if you tap them at 1/2 watt.

The system now has headroom, the amp's running cooler, and you don't have to deal with line loss. You can also expand the system easily by just adding speakers. If you need that extra 3dB of loudness, move up to a 200 watt amp and tap each speaker at 1 watt.

Keep in mind, too, that although a system is called a "70.7-volt constant-voltage system" it will rarely, if ever, reach its rated voltage during normal use. There's some more built-in headroom. (But it needs to test at full load during the sign-off stage.)

Wire size? Typically 14-gauge trunk lines from amp(s) to the field, 16-gauge zone drops from the trunk to each section of the building, and 18-gauge speaker lines within the zones.

(Er ... Have you seen the price of copper lately?!?  The thinner the cheaper!)

For added protection against failure, high-pass the entire system at 100Hz and don't boost the bass on any mixer amp or EQ.

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John Roberts {JR}

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Re: 70-volt wiring
« Reply #4 on: July 14, 2006, 10:37:08 am »

Rick Johnston wrote on Fri, 14 July 2006 00:49

I can't remember the last time I load-tested a constant voltage system (70.7 or 25) and found the speaker load to be equal to the amp's power rating.

Design specs usually call for 20% headroom, and many times I see 50%. That means a 100 watt amp shouldn't have more than 50 speakers (each tapped at one watt) connected to it.

A 100 watt amp may have 100 speakers, each tapped at one watt, but they're only 3dB SPL down in each coverage area if you tap them at 1/2 watt.

The system now has headroom, the amp's running cooler, and you don't have to deal with line loss. You can also expand the system easily by just adding speakers. If you need that extra 3dB of loudness, move up to a 200 watt amp and tap each speaker at 1 watt.

Keep in mind, too, that although a system is called a "70.7-volt constant-voltage system" it will rarely, if ever, reach its rated voltage during normal use. There's some more built-in headroom. (But it needs to test at full load during the sign-off stage.)

Wire size? Typically 14-gauge trunk lines from amp(s) to the field, 16-gauge zone drops from the trunk to each section of the building, and 18-gauge speaker lines within the zones.

(Er ... Have you seen the price of copper lately?!?  The thinner the cheaper!)

For added protection against failure, high-pass the entire system at 100Hz and don't boost the bass on any mixer amp or EQ.





Some of the de-rating is to make up for losses in the wiring/transformers.

The advice about HPF and avoiding bass boost is right on because all those transformers hanging on the 70V line can saturate if hit with too much low frequency content.

JR

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Hal Bissinger/COMSYSTEC

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Re: 70-volt wiring
« Reply #5 on: July 14, 2006, 04:12:46 pm »

The advice about HPF and avoiding bass boost is right on because all those transformers hanging on the 70V line can saturate if hit with too much low frequency content.

I have had considerable success without HPF and with bass boost by using much larger than necessary direct coupled amps (to eliminate that transformer) and high quality transformers on the speakers. The speaker transformers are critical for decent LF response.

Most speakers like the JBL Control series have large transformers built-in that will give acceptable LF performance but don't expect the $10 8" speaker/transformer package to be good for anything but voice.

If you have to use that type of speaker for music choose one with a good LF response to begin with then use a transfomer with as much iron as you can find. These will normally give the lowest LF response possible.

-Hal

John Roberts {JR}

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Re: 70-volt wiring
« Reply #6 on: July 14, 2006, 06:14:34 pm »

Hal Bissinger/COMSYSTEC wrote on Fri, 14 July 2006 15:12

The advice about HPF and avoiding bass boost is right on because all those transformers hanging on the 70V line can saturate if hit with too much low frequency content.

I have had considerable success without HPF and with bass boost by using much larger than necessary direct coupled amps (to eliminate that transformer) and high quality transformers on the speakers. The speaker transformers are critical for decent LF response.

Most speakers like the JBL Control series have large transformers built-in that will give acceptable LF performance but don't expect the $10 8" speaker/transformer package to be good for anything but voice.

If you have to use that type of speaker for music choose one with a good LF response to begin with then use a transfomer with as much iron as you can find. These will normally give the lowest LF response possible.

-Hal


I think most mfrs would love to accommodate you as eliminating the output iron eliminates a significant expense too, but when I was dealing with that market I had to please old line installers who wouldn't even accept auto-formers in place of the fully floating output transformers that are common in 70v amps.

Magnetic saturation is a product of frequency and amplitude so you can get away with some bass boost as long as signal levels remain modest. One of my inventions dealt with a modified tone control circuit for this market that would allow full bass boost as long as signal levels were low but then clamped (just the bass) when it got loud. It was only a simple clamp but since it was operating on just the low bass bandpass it was masked by the full range audio when working.

You can also add a capacitor in one leg of the classic Baxandall tone control to roll off the bass boost adaptively such that the LF cut still looks like a true shelf while LF boost has a bandpass.  

JR
 
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Brad Weber

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Re: 70-volt wiring
« Reply #7 on: July 14, 2006, 06:42:15 pm »

JR,

This brings up a question I've had for quite some time.  Why does it seem that nobody has developed an automatic loudness compensation device or DSP algorithm?  I would think this could be implemented in DSP, perhaps even using a room mic like ambient noise analysis.  If we can have the system adjust the level based on the ambient noise, why not also adjust the EQ based on the system loudness or an averaged input level to the device?

Just curious...
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Brad Weber
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John Roberts {JR}

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Re: 70-volt wiring
« Reply #8 on: July 14, 2006, 07:07:03 pm »

Brad Weber wrote on Fri, 14 July 2006 17:42

JR,

This brings up a question I've had for quite some time.  Why does it seem that nobody has developed an automatic loudness compensation device or DSP algorithm?  I would think this could be implemented in DSP, perhaps even using a room mic like ambient noise analysis.  If we can have the system adjust the level based on the ambient noise, why not also adjust the EQ based on the system loudness or an averaged input level to the device?

Just curious...


A proper loudness contour (ala Fletcher-Munson) could be very nicely done in DSP. I suspect there may be some issues with throwing significant HF and LF boost on playback systems that are not exactly models of high fidelity but in a properly integrated solution the LF saturation characteristic of the system magnetics could be accommodated.

If this was done well, you wouldn't know it was there. I suspect some background music providers might already EQ for anticipated modest playback levels.

If it can't be done already, it's just a matter of time. Larger installs are already resembling a computer system waiting for your plug-ins. It will filter down as DSP becomes cheaper and more pervasive.

JR
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Dave Barker

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Re: 70-volt wiring
« Reply #9 on: July 14, 2006, 08:57:56 pm »

DBX has the loudness control feature in the Zone Pro series of DSP boxes for commercial applications.  Works fairly well also.  I believe there are a couple of others that is just the first one that comes to mind and the one we use most.  I believe DBX calls it "Auto Warmth".
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