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Author Topic: 70-volt wiring  (Read 8346 times)

Hal Bissinger/COMSYSTEC

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Re: 70-volt wiring
« Reply #10 on: July 15, 2006, 02:01:51 pm »

I'm familiar with Rane DSP boxes. Haven't looked into it but I'm wondering whether you could do a bunch of presets that would adjust the volume and along with each step the EQ as well.

-Hal

Rick Johnston

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Re: 70-volt wiring
« Reply #11 on: July 17, 2006, 07:11:18 am »

Hal Bissinger/COMSYSTEC wrote


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.  



John Roberts  {JR} wrote


... 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.


The output transformer (not autoformer) is another line of defense against failure. Consider the inevitable short to building ground in the field wiring: Eventually someone will be in the ceiling pulling cable, or adding HVAC, or any of a thousand other jobs. Or the guy whose desk sits under a speaker decides to climb up there and cut the wire to the speaker, leaving the loose end to contact the ceiling grid, steel structure, or a chunk of conduit. A transformer-isolated amp will handle that short to ground better than a direct-coupled amp.

It's all in the application, though. A background music/voice paging system doesn't need excessive bass response, so the $10 eight-inch speakers are fine. System costs are very low compared to, say, a high-wattage foreground music EVID system with several full-range speakers each tapped at 16 watts and a couple of 70v subs. In the latter case I'd use an over-powered direct-coupled amp (actually two) as well.

-- RJ
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Iain_Macdonald

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Re: 70-volt wiring
« Reply #12 on: July 17, 2006, 08:36:14 am »

Quote:

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?


Hi Brad,

It's been and gone!

Here in the UK when Shuttlesound was a privately owned company. (1987) We manufactured and sold a product called the Inflexor. This did exactly what you wanted. It worked well in a number of applications. The listener still got a "full" sound at low levels. The product was designed by Ben Duncan, and used the Robinson Dadson curves.

Best wishes.

Iain.
London. UK.
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John Roberts {JR}

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Re: 70-volt wiring
« Reply #13 on: July 17, 2006, 10:39:36 am »

Rick Johnston wrote on Mon, 17 July 2006 06:11




The output transformer (not autoformer) is another line of defense against failure. Consider the inevitable short to building ground in the field wiring: Eventually someone will be in the ceiling pulling cable, or adding HVAC, or any of a thousand other jobs. Or the guy whose desk sits under a speaker decides to climb up there and cut the wire to the speaker, leaving the loose end to contact the ceiling grid, steel structure, or a chunk of conduit. A transformer-isolated amp will handle that short to ground better than a direct-coupled amp.

It's all in the application, though. A background music/voice paging system doesn't need excessive bass response, so the $10 eight-inch speakers are fine. System costs are very low compared to, say, a high-wattage foreground music EVID system with several full-range speakers each tapped at 16 watts and a couple of 70v subs. In the latter case I'd use an over-powered direct-coupled amp (actually two) as well.

-- RJ



I'm familiar with all the rationalizations and since the customer is always right (even when wrong) they got their full transformers.

Considering how many CV systems are in use around the world it seems like a significant expense to burden the ultimate purchaser (rarely the same person who insisted on the transformer). Transformers are larger, heavier, more expensive, less efficient,  worse audio performance (bandwidth and distortion), and so on.

I am well aware of the significant pain and expense of service calls, but it does seem overly generous to spec a more expensive approach to make the system more tolerant of "customer" caused problems. Perhaps the installer is also considering the extra tolerance of sloppy wiring when doing their initial install.

Whatever. It's impossible to know other peoples motives when we so rarely inspect and understand our own.

JR


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Don Boone

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Re: 70-volt wiring
« Reply #14 on: July 17, 2006, 04:27:29 pm »

70.7 volt systems were originally a way around the required limits for low voltage (meaning no conduit) system. In olden days anything below 36 volts was considered low voltage. 70.7 volt distribution met that requirement IF the output was balanced, 35.35 volts per side. Thus the requirement for a real transformer.

Don
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John Roberts {JR}

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Re: 70-volt wiring
« Reply #15 on: July 17, 2006, 06:54:17 pm »

Don Boone wrote on Mon, 17 July 2006 15:27

70.7 volt systems were originally a way around the required limits for low voltage (meaning no conduit) system. In olden days anything below 36 volts was considered low voltage. 70.7 volt distribution met that requirement IF the output was balanced, 35.35 volts per side. Thus the requirement for a real transformer.

Don


I never heard that one, but CV was already a mature technology by the time I started messing with it. FWIW I also don't recall ever seeing a center-tapped 70v (35+35) output winding on anything I was competing against. The output transformers did routinely have taps for lower voltages but nothing close to 35V. One could ground a 25v tap and reduce the peak potential wrt ground but they both wouldn't be below your 36v max. Of course you could probably just use the 25V tap, which is probably why it was there (I also recall it being a requirement for some school installs).

The folks busting my chops for a floating transformer output didn't ask for and were satisfied without a CT.

JR
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Rick Johnston

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Re: 70-volt wiring
« Reply #16 on: July 22, 2006, 09:50:04 am »

I've always understood that in the early days, 100 volts was considered the maximum that a wire could carry in a building without having to be in pipe. 100 volts peak-to-peak means 70.7 volts RMS.

To this day, some local codes don't require pipe while others do.

John Roberts  {JR} wrote on Mon, 17 July 2006 10:39

I'm familiar with all the rationalizations and since the customer is always right (even when wrong) they got their full transformers.

Let's not forget the original benefit of transformers: They step up the voltage to run long distances on smaller wires, then step it back down to run the speakers. The resultant higher impedance also means that more speakers can be added to a distributed CV system versus a low-impedance direct-coupled system.

Regards,
Rick Johnston

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John Roberts {JR}

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Re: 70-volt wiring
« Reply #17 on: July 22, 2006, 10:33:54 am »

Rick Johnston wrote on Sat, 22 July 2006 08:50

=John Roberts  {JR} wrote on Mon, 17 July 2006 10:39]I'm familiar with all the rationalizations and since the customer is always right (even when wrong) they got their full transformers./quote

Let's not forget the original benefit of transformers: They step up the voltage to run long distances on smaller wires, then step it back down to run the speakers. The resultant higher impedance also means that more speakers can be added to a distributed CV system versus a low-impedance direct-coupled system.

Regards,
Rick Johnston




And let's not forget I'm talking about refusal to use 70V auto-formers instead of 70V transformers. The primary difference being isolation from ground or amplifier common. Isolation does provide the benefit of tolerating inadvertent grounding of one of either + or - speaker line. Whether this benefits the customer or installer is a judgement call. Since the decision was strongly influenced by  installers and consultants, transformers it was.

All customers pay more up front to mitigate against shorts occurring during installation or later. FWIW transformer common was always terminated right next to ground on the output barrier strip so it could easily be shunted together.

JR
 
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Hal Bissinger/COMSYSTEC

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Re: 70-volt wiring
« Reply #18 on: July 22, 2006, 01:42:27 pm »

Well, in vacuum tube days the output transformer was there by necessity and the secondary common was usually grounded for safety. A primary to secondary short could put anywhere from 400 to 700 volts DC on the speaker wiring otherwise.

Today the isolation properties of an output transformer are a requirement of some electrical codes that date back to vacuum tube days and may be the basis for them being desired by some otherwise.

The transformerless output amps I am familiar with (Crown CT series) will go into protection if one side of the output gets shorted to ground. The same problem exists with a transformer that has its common grounded and the other side becomes grounded.

My concern with a floating output is the possibility that the wiring in a building could be crossed with line voltage, effectively putting that voltage between the speaker wiring and ground. It would go undetected and pose a real safety issue for someone who comes along to service a speaker, the amp or wiring.

Then again I believe there are some codes that take issue with the fact that the amp itself might not be properly grounded in which case a speaker wiring fault could put line voltage on the amp case and controls if one side of the output were grounded to the chassis. So in this case a floating output is reqired even though a hazardous voltage on the output and wiring could exist.

I don't think there is any real "best way" here and I do think, since in may instances an output transformer is an option, performance and economics are the current design factors.

-Hal  

Rick Johnston

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Re: 70-volt wiring
« Reply #19 on: July 23, 2006, 10:02:46 am »

John Roberts  {JR} wrote

And let's not forget I'm talking about refusal to use 70V auto-formers instead of 70V transformers. The primary difference being isolation from ground or amplifier common. Isolation does provide the benefit of tolerating inadvertent grounding of one of either + or - speaker line. Whether this benefits the customer or installer is a judgement call.

Is there an electrical advantage to using an autoformer instead of a transformer? Both cost about the same AFAIK.

Hal Bissinger/COMSYSTEC wrote

The transformerless output amps I am familiar with (Crown CT series) will go into protection if one side of the output gets shorted to ground.


One of our installations has eleven CTs4200's and six CTs8200's in 15 head ends (one in each building in the complex). No single amp channel has more than 80 watts total load. In three years since the install, only three individual amp cards have failed. Two in the same amp at the same time were due to heat buildup inside the rack. (The air conditioning in that head end's room failed.) The third failure was caused by an errant forklift in a shipping area smashing the PC board in an attenuator -- causing both sides of the amp line to short to each other and to the +24vdc side of the music/page relay line.

I'd say that's a pretty good track record for that line of amps.
Quote:


My concern with a floating output is the possibility that the wiring in a building could be crossed with line voltage, effectively putting that voltage between the speaker wiring and ground. It would go undetected and pose a real safety issue for someone who comes along to service a speaker, the amp or wiring.


A war story: Service call at a manufacturing plant. A very simple 70v system. Biamp CMA-350 with ten 15-watt horns in one building. One 12-gauge PVC jacketed line from that amp ran through underground conduit to another building with six more 15-watt horns. The amp had failed. I put my Simpson meter on the line to check for shorts to building ground and instantly blew the meter's fuse. Replaced it and it instantly blew again. Checked for AC voltage: 118 volts. Come to find out some maintenance guy had opened up one of our junction boxes in the ceiling to connect AC power and a switch leg for a new light -- to our line.

Fortunately, I learned a long time ago to touch only one wire at a time -- even if it's "only" a line-level signal cable.

Regards,
Rick Johnston
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