[Boatanchors] 600 ohm lines / 70.7 volt distribution, etc.

[Dave Maples]

All: Just to fill in some gaps here, 600-ohm terminating impedance
originated in earlier-generation analog telco switchgear.  Those connections
were 2-wire bidirectional connections, of course, so both inputs and outputs
had to be designed to terminate that 600-ohm line correctly.  At the
distances involved in telephone transmission, incorrect terminating
impedances can cause frequency-response changes, etc.

Interestingly enough, standard terminating impedance for residential phone
lines is now 900 ohms, not 600.  That change was made many years ago, I
suspect with the introduction of the 500-series (rotary) phone sets.  Anyone
have some more history here?

Maximum voice-band power level to put on the standard phone line was -4 dBm
(-4 dB below 1 milliwatt across the terminating impedance of the line).
Other maximum levels apply to out-of-band energy, energy around 2600 Hz (the
old disconnect tone), etc.

As was stated earlier, the commercial audio industry developed the
high-voltage transmission system in order to simplify distributed speaker
installations.  Standard line voltages are 70.7 VAC at rated power, and 25.2
VAC at rated power.

The 70.7 V number was chosen because when calculating power and resistance,
P = (E*E) / R as was already noted.  E*E in this case ~= 5000, which is easy
to remember.  Don't ask me why the lower voltage was chosen to be 25.2 VAC;
E*E in that case = 635, which certainly isn't easy for me to remember :-).

Both 70.7 V and 25.2 V wiring is governed by Article 725 of the National
Electrical Code.  When you look at a commercial amplifier and see a "CLASS 2
WIRING PERMITTED" note on it, that refers to Class 2 signalling wiring as
defined in Article 725.  The lower voltage may have been created
specifically to avoid having to run wiring in conduit; I'm not sure about
that.  Again, more history here?

Maybe this will help some.

Thanks,
Dave WB4FUR