[Boatanchors] 500 ohm vs. 600 0hm

[J Forster]

I read somewhere that the origin of 600 ohm circuits was in the telegraph days.
600 ohms was roughly the impedance of the open wire transmission line composed
of copper wires on glass insulators, air spaced, on telegraph poles.

When the telephone came along, the same transmission line design was used, and
the impedance of POTS circuits is still "48 Volts behind 600 ohms".

With the advent of underground and multipair cables, the same 600 ohm impedance
was preserved to reduce reflections.

With the advent of automatic exchanges and multiplex at the central office,
there was no longer a copper pair from subscriber to subscriber, so a relay coil
was used at the central office to terminate the subscriber loop and the same
relay indicated an "off hook" condition. The battery was at the exchange and
audio was picked off and multiplexed to the destination central office where the
process was mirrored.

The 500 to 600 ohm headsets likely came from commonality with early telephone
earpieces.

Early broadcast audio likely also drew on telephone audio technology. It is
usually balances with respect to ground for common mode rejection. POTS lines
are balanced for the same reason, but do not have an overall electrostatic
shield. Both audio and telephone lines are usually twisted to minimize the net
projected area and giving immunity from external AC fields.

If you want to characterize a balanced 600 ohm audio input, it is done as
follows:

Common Mode Impedance...  600 ohms (measured line-to-line)
Differential Mode Impedance...  high  (> 1 M) (measured either input to ground)
Common Mode Rejection Ratio...  60 dB minimum

Note that a good audio transformer, like a UTC A or Ouncer, will easily meet
these specs. It can also be done w/o a transformer using an instrumentation
amplifier.

Best,
-John

==================

.

Stanley Adams wrote:

> Is historically embedded in the history of the separation of radio
> broadcasting from those early pioneers in the telephone system, such as
> Western Electric and others.
> 500 ohms was the accepted standard in all wire plants, and it came to be
> that 600 ohms became important for the broadcasters.  In fact, the classic
> dBm, is centered in 600 ohm history.
>
> There are plenty of historical web sites that will speak to both impedances,
> and the difference between them is not worth or hoot nor holler.
>
> If a device is marked either, then you assume that you either need to add a
> transformer or one is built already into the equipment.  The circuit is
> balanced in reference to ground, with both sides equal impedance from the
> ground.   You can make a IC chip audio chip without using a transformer and
> trick it to think that it is 600 ohms, but the actual impedance to ground
> with any power out is closer to 20 ohms.
>
> Unbalanced circuits are of course 70 v usually, what we term as high
> impedance consumer equipment 2-10K input and or output.  You can run a 10k
> into a 600 ohm transformer but you really need to terminate the input with a
> 600 ohm resistor so that the high frequency content is not emphasized.
>
> Stan Adams
> Just my thoughts
>
> _______________________________________________