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.
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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.
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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.
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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.
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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