[LAU] Behringer and Linux

[Chris Caudle]

> "ground loop" is almost certainly the problem.

A problem with "ground loop" is symptomatic of a fundamental
misunderstanding of Ohm's law and is a problem of poor layout in the
mixer.

Fons has explained reasonably well for a short email where the previous
email goes wrong, but there really isn't time to go into full detail of
why the problem should not exist at all.

The shortest version I can come up with is that any two pieces of
equipment will have some voltage difference between their cases. 
Connecting the equipment together allows current to flow between the
equipment along the shields of connecting cables, and across whatever
conductors form a path for that current to get back to the source (form a
complete circuit).  That path could include chassis, cables, printed
circuit board traces, anything conductive.  Any current flowing in a
conductor will have an associated voltage drop described by Ohm's law
(voltage is the product of current and impedance), and the job of the
design engineer is to understand and control where that current will be
flowing, and make sure that no audio circuitry uses one of those
conductive paths as a reference conductor for the audio.

> Both of these, are not really Behringer problems.

They most certainly are.

The design engineers at Behringer failed that layout task for at least the
headphone amplifier in the mixer Fons described.
It is two decades too late to make excuses for incompetence in dealing
with that particular problem.

This is what most consider the turning point in highlighting the problem,
a full issue in 1995 (twenty years ago this June!) of the Journal of the
Audio Engineer Society dealing specifically with grounding and shielding
concerns, still available as a special publication from the AES today:
http://www.aes.org/publications/specialpubs/journal_issues.cfm

SHIELDS AND GROUNDS: SAFETY, POWER MAINS, STUDIO, CABLE AND EQUIPMENT,
(special excerpt) The June 1995 issue of the Journal was a definitive and
comprehensive collection of information on this important topic. The seven
papers by Neil Muncy and other experts in the field have been reprinted
into a convenient guide for designers and practitioners.

And of course following the recommendations in AES standard 48 go a long
way toward alleviating the problem.  Only available for one decade, so
perhaps we could excuse the designers for not being familiar with it yet:
http://www.aes.org/publications/standards/search.cfm?docID=44

AES48-2005 (r2010): AES standard on interconnections - Grounding and EMC
practices - Shields of connectors in audio equipment containing active
circuitry
Printing Date: 2010-07-08
Publication History: Pub. 2005; Reaffirmed 2010
Abstract: This standard specifies requirements for the termination, within
audio equipment, of the shields of cables supporting interconnections with
other equipment, taking into account measures commonly necessary for the
preservation of EMC (electromagnetic compatibility) at both audio and
radio frequencies. The shielding (or screening) of audio equipment,
cables, and microphones can be critical for EMC. The improper connection
of these shields can cause common-impedance coupling in equipment. From XL
connector usage, where Pin 1 is standardised as the designated shield
contact, this has
been identified as the Pin 1 problem.

Bill Whitlock of Jensen Transformers has written a lot of good material (a
couple of the articles in the June 1995 JAES), but a recent web page
redesign at Jensen seems to make his online material difficult to find by
requiring a login to download the app notes.
The app notes at Rane are nearly as good, and are a good place to start
while waiting for your copy of the June '95 JAES to arrive in the post:

http://www.rane.com/note151.html
http://www.rane.com/note110.html
http://www.rane.com/pdf/whitlock.pdf

This stuff isn't brain surgery, and it's well past time to stop making
excuses for manufacturers who won't get it right.

-- 
Chris Caudle