Re: Mike on Mics

[Mike Curtis <wd6ehr@xxxxxxxxxx>]

> 	Try disconnecting the ground from your phone and see if the 
> inductive amp still picks up anything. 

This has nothing to do with harmonica, which is why I took it off line.

It works just fine ungrounded :-)  I use it to troubleshoot totally
ungrounded and electrically isolated systems daily.  My DBA is "The
Telephone Man".  I troubleshoot and repair telephone systems for other
companies, when they give up on them.  But ask any telephone man to
demonstrate his toner and inductive amplifier.  In fact, you can even use a
"butt set" (the telephone on his tool belt) in monitor position (high-Z) to
monitor toner signal with just a single wire.

Raw telephone cable is not grounded, yet I can connect a toner (battery
operated oscillator) to one end of the cable and hear tone on the other end. 
In fact, I can hear the toners signal by simply putting the banana NEAR one
of the toners leads.

Telephone LINES are not grounded (they're BALANCED line.) If you doubt this,
check the green and red (tip and ring) leads on your telephone modular
connector (assuming an RJ-11, or RJ-14 and line one) with a megohmmeter.  A
megohmmeter is Telco spec for this test, because there should be _N_O_
ground on a telephone line.  Telephone EQUIPMENT is grounded, mainly for
safety purposes.  The protector (the box outside your house where the line
is tied down) is grounded, but this is for electrostatic discharge and
accidental high voltage protection.  Protectors used to be carbon inserts
with very narrow air gaps to ground.  Today, more sophisticated components
are used, but the purpose is the same.

But this has little (electrically) to do with the lines themselves.

The telephone company has had a lot of trouble with traditionally schooled
electronics techs.  They often have difficulty comprehending balanced lines,
as their world is "unbalanced" (no pun intended.) In a balanced line, each
wire carries a mirror image current (or voltage if you are measuring that)
of the other.

> I notice you mentioned a "pair" of 
> phone wires. Why a pair if you only need one wire? I would think the 
> phone co would use just one if it would work.

Because inductive amplifiers are troubleshooting devices designed for on
premise diagnosis.  They're effective only over short distances.  But that
wasn't the issue.  You claimed that a single wire connection would not pass
signal.  No distance was specified.

Actually, you can do this test yourself.  Plug in your amplifier, and a
guitar cord.  Without touching anything else, touch your finger (just one
:-) to the plug tip (just a single wire.)

hmmmmmmmm  ;-)

Telephone circuits are 600 ohm nominal "balanced" line, and do not use
ground.  (This is the same as used on amplifiers running line level balanced
lines and line level signals.) The 2 wires in a telephone circuit are known
as "tip" and "ring", so named after their positions on the old branch
exchange plugs.  Long ago, a ground was used for ringing voltage return,
between ring and ground, but this rather archaic practice has been long
disbanded.

Grounding does not directly involve telco lines.  Protectors and other
equipment are grounded, but this is for safety (and sometimes minimization
of common mode), and electrically has nothing to do with the actual
telephone circuit.  In fact, a ground present on the telephone circuit will
be registered as a line fault by the Telco line tester, which is run on all
lines every night.

Anyway, the original issue was ground current.  If you break the circuit,
there will be voltage differential.  But the shield is considered to be at
ground potential.  If we measure voltage RELATIVE TO GROUND, then ground
will of course be zero.

We might compare the principle to a man going 55 MPH in his car.  that's
relative to the ground, too (literally.) but if we're in a radar-equipped
police car going 55 mph from the other direction, he's going 110 mph
relative to us, and that's what the gun will show.

If we instead factor in the Earth speed around the sun, and the Sun in the
Milky Way, and the galaxy in the cluster, etc., you can easily see that our
55 mph really isn't.

But if you're standing on the ground, it most certainly is 55 mph RELATIVE
TO US.


 -- mike