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. |
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 |
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... |
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. |
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? |
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 |
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 |
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. |
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 |
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. |
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. |
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. |
Rick Johnston wrote on Sun, 23 July 2006 09:02 |
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. /quote Is there an electrical advantage to using an autoformer instead of a transformer? Both cost about the same AFAIK. Regards, Rick Johnston |
Don Boone wrote |
Rick you might be right about the 100v thing unless telephones were exempt. Talk battery is 48 volts and ring voltage is 90 volts. But I heard the 36 volt thing sometime during my education at the Burn & Learn School of Electronics. |