Re: [LAU] Behringer and Linux

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. 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