About Us

In article writes:

Get a length of insulated wire, connect the ends to pins 2 and 3 of an
XLR, plug it into a mic pre and try the effect of having an open loop,
closed loop and then twisted tightly all along its length when held close
proximity to an AC power transformer.

Once again, the Pretentious Pundit of Pro Audio changes the subject
when he's caught in a misconception. This is an excellent
demonstration of the principle of loop area (again, I recommend Steve
Lampen's book) but it doesn't tell you anything about the performance
of a differential amplifier.

Using twisted wire is one technique of reducing the common mode
voltage applied to the inputs of a differential amplifier. However it
does nothing to reduce the common mode voltage that is a function of
the source.

Connecting a loop of wire between the two terminals of a differential
input and inducing current in the wire is the perfect test case, and,
in fact, is a good test of the common mode rejection of the input.
Assuming that the wire is uniform, the voltage drop at each input will
be determined by ONLY the impedance of each input relative to the
amplifier's (internal) reference point. If they're identical, they
will sum to zero and you have complete common mode rejection. If the
impedances of the two inputs are unequal, the voltage drops will be
unequal and you will have some output from the amplifier - less than
complete common mode rejection.

I suggest that YOU try the experiment. But don't make any assumptions
about things being perfect, because they won't be. You have a tendency
to ignore things that get in the way of proof, declaring them
"insignificant." That may be the way it works in product development,
but it's not the way it works in science. It's important to make the
distinction, and it's particularly important when trying to explain
theory to a novice.

Bob Olhsson used to use a tag line to his messages which read
somethign like "In theory, theory and practice are the same. In
practice, they are different." Keep that in mind. You'll be a better
engineer.

I would just ignore the snide remarks and leave his discussion except
that I hate to see someone with the right information be put down with
irrelevant and inaccurate responses, and more important, that the person
asking a legitimate question gets misled if he got disgusted with the in-fighting
and stops reading the thread.

--
I'm really Mike Rivers )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me he double-m-eleven-double-zero at yahoo

Thread Tools
Show Printable Version
Display Modes
Switch to Linear Mode Switch to Hybrid Mode Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
What are they Teaching	Michael McKelvy	Audio Opinions	199	October 15th 04 07:56 PM
What is "Counter mode" + "0" on Sony DAT?	Corinna Vinschen	General	4	June 12th 04 08:58 AM
Stereo crosstalk at high frequency on my mixer	Dreamist	Pro Audio	3	March 29th 04 12:53 AM
AC Power Conditioner (Cont.)	Martin Glasband	High End Audio	0	December 24th 03 08:11 PM

Menu

About Us