John Williamson wrote:
On 07/01/2018 23:22, wrote:
On Saturday, January 6, 2018 at 7:54:20 AM UTC-5, Scott Dorsey wrote:
The panel you have should have at least five pairs of screw terminals,
each of which would be connected to the output of an amplifier. There
should be a rotary switch to select which pair goes to the transformer,
a step down transformer (maybe a DC blocking cap if it's for a system
with DC intercom signalling), and a speaker and meter. Little else on
the panel, I believe.
https://www.dropbox.com/sh/t7xz8ftr2...71B8xO1ua?dl=0
Not that it matters to this application, but that sort of equipment in
the UK normally runs on a 100 Volt line, unless it's in a hazardous area
such as a coal mine, in which case it's 25 Volts to eliminate the
possibility of sparking.
In the US, the NEC says that wiring above 100V peak are considered class I
signalling circuits and have to be treated more or less like power wiring.
Which means in most locations it has to be in conduit. 100V or less and
it can be treated as Class II and the wiring can just be out free.
So... we have 70V RMS constant voltage audio systems whose peak current is
sort of expected to be 100V. Which is kind of unrealistic because it's
making assumptions about the waveform, but that's how it's usually interpreted.
So those systems can be in class II wiring.
In the EU the rules are slightly different so they tend to run systems of
100V RMS because that's where their legal line is drawn.
Now... there are SOME localities in the US where the law is more strict
than the NEC. Hampton, Virginia schools for instance have 25V constant
voltage systems for intercoms because anything higher than 25V RMS there
needs to be in conduit.
It's all more legal silliness than actual technical constraints.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."