Re: Mike on Mics

[Mike Curtis <wd6ehr@xxxxxxxxxx>]

> Have you ever picked up radio stations on a cheap guitar cable?

Sure have.  Cheap cables lack a proper ground/shield and allow signal into
the center conductor.

There are numerous reasons why we sometimes pick up radio signals:

 1. at radio frequencies the shield is a significant portion of a wavelength
(or multiple) and is not at true ground potential throughout;

 2. a cheap cable is not well shielded and current is in fact induced in the
center conductor;

 3. the amplifier is not properly RF-bypassed, so RF gets into it and is
rectified by nonlinear components;

 4. unbalanced lines are susceptible to common mode signals.

> I stick by my point. If you don't believe me, try running signal to your 
> amp with just one wire and no ground wire. It won't work. 

If this were true, the telephone system repair business would be in big
trouble. In telephony, we use a device called an "inductive amplifier".  It
typically has a high impedance input amplifier, an internal speaker, a
switch, and a SINGLE probe tip.  There is no ground or any other physical
connection.  By touching ONLY ONE probe connection to a circuit, we can
monitor signals on the pair.  In fact, by placing the probe NEAR the wire,
we can hear signals on the pair.

This device most often looks like a straightened "banana" (in fact, that's
its nickname), and has no other electrical connection to anything else.

So a single connection DOES work :-)

However, in all honesty, I must admit that it does have a "return" - stray
capacitance ("inductive" is a misnomer from days past) This is not the same
as a ground, though, and picks up a lot of hum, etc., whereas a true ground
and electromagnetically and electrostatically shielded cable would not.

You mention ground LOOPS.  These are actually loops and not TRUE grounds. 
They don't FUNCTION as grounds, which is why they cause problems.

All these points about ground, 3rd prong, shock, hum, etc., eloquently make
the point that ground must be an electrically neutral point to work
properly, and that any deviation from the ideal causes problems of varying
magnitudes.

> Scientists disagree over whether the electrons move +to- or -to+.

There's no disagreement of any consequence, and hasn't been for a good many
years.  As far as I'm aware, all knowledgeable scientists are quite
convinced that electron flow is from negative to positive.  Engineers still
use "conventional current flow", but this is not due to a belief that
current _really_ flows "backward".  It's simply because the math requires
it.  Mathematically, we can't have a deficit represented by a positive
number and a surplus by a negative.  When electronics was in its infancy,
scientists erroneously believed that current flowed from positive to
negative.  The math was set up to work with these parameters - and they do
work.  Even though the actual current flow is wrong, the math involving
conventional current flow still works, and is used every day by electrical
engineers.



 -- mike