Dual Differential / Balanced?

Thanks kirkus for your comments.

Bill Whitlock's paper is excellent. I like the Rane page on balanced operation (http://www.rane.com/note110.html) as a succinct description of the balanced line standard, AES file 48.

Speaking of Bill Whitlock, since the interface-related noise that is being discussed may in many cases be caused or contributed to by ground loop effects, you'll probably find pages 31 through 35 of this paper to be of interest. To whet your interest, its introduction states that "this finally explains what drives 99% of all ground loops!"

Excellent link, Al . . . a great comprehensive document that covers many aspects of Whitlock's papers over the years.  The dedication to the late Neil Muncy at the beginning is nice, too . . . his AES paper from the mid-1990s is the definitive work on one of the most prevalent equipment design issues in the audio industry.

I also read Whitlock's AES Paper that's the source for the content of pages 31 through 35 of the presentation paper . . . and this is some brilliant and thorough work as well.  I've long been suspicious of the randomly-arranged conglomerations of THHN that populate the conduit on commercial jobs, even though it's not too clear what's the best route to pursue during construction to avoid it . . . asking union electricians to twist THHN into pairs before each pull seems like a great way to get kicked off the jobsite.  The ideal solution would be a pre-twisted cable with fillers and an overall jacket -- something that could be easily specified on the prints, relatively easy to pull through the conduit, and with established fill tables to make sure the conduit sizing and labor costs are predictable.  In absence of this . . . using multiple smaller conduits or runs of MC where necessary may be the best route, or at least over-size the ground wire(s) for some brute-force reduction may be all that can be achieved in the real world.

I do think that we're lucky in that the "conduit transformer" issue is far lower on the list of worries for a residential single-room audio system than it is for i.e. a large commercial building with high-power active line arrays or digitally-steered columns placed hundreds of feet from their source electronics.  The main susceptibility for high-end audio systems would be where multiple dedicated branch circuits are employed . . . and this can be eliminated as an issue by using Romex, running all circuits to a single multi-gang non-metallic wall outlet box, and connecting all the grounds together at this wall box in addition to the panel.  This eliminates the chance of any voltage differential between the third-prong AC grounds in the system, yet still provides the benefit of fully separating the current flow between the circuits.  Also, I've seen many breaker panels where the connection bar(s) are shared between ground and neutral connections for the various circuits, and this can needlessly impart random voltage differences between the grounds of different outlets, and additional ground-to-neutral noise.

My only slight disagreement with Bill Whitlock would probably be that I don't share the same level of distain for unbalanced interconnection, in the context of the systems that we discuss on Audiogon.  Here, we're usually talking about audio systems that don't share grounds with equipment in other rooms, and can easily be plugged into AC outlets that are all on a single power strip, or ganged together in the wall . . . and the audiophile's idea of a "long" interconnect is . . . maybe 15 feet?  I've just seen so many ill-conceived applications of the 3-pin XLR connector on high-end audio gear that I frequently feel I need to look at a schematic before deciding if it's even usable.   Much of the time I think equipment with poorly-designed "balanced" inputs would work so much better with simpler circuitry, and a plain 'ol RCA jack for the input.

 . . . amplifiers are quite good and accept a balanced input correctly (many high end audio amps do not, likely because the designers don't know that there is a standard for balanced line operation, defined by AES file 48).

Ralph, I'm assuming that by "a standard for balanced line operation" you're talking about the 600-ohm termination resistor . . . and I'm sorry to pull a Kryten here, but AES48 says nothing about signal impedances; rather, it's a culmination of grounding practices derived mainly from the work of Neil Muncy, whom I referenced above.  None of the AES Standards documents mention balanced signal impedances . . . but the Bill Whitlock presentation for which Al provided the link references IEC standard 61938, according to which "all professional and broadcast line amplifier inputs" should be >= 10K ohms.  He also goes on to address the origins of 600 ohms as a specification, and a few reasons why he feels it's inapplicable to modern audio systems . . . and I can't find any reason to disagree with him on this point.

I know you've maintained that the addition of a termination resistor makes cable characteristics non-critical . . . but I'm still unclear as to what electrical mechanism you feel is responsible for this.  In my own experience, while some of my system iterations over the years have seemed more sensitive to cables than others, I haven't noticed a correlation between this and low termination impedances.  I also have 100K, 600 ohm, and 150 ohm termination impedances all a mouse-click away from each other on the Audio Precision system . . . and I've had my share of times where some measureable artifacts that end up being related to cables and interconnection remain completely unfazed by the setting.  So if you have any details on this that I haven't thought of, I'd be very interested to know.

^^ The 600 ohm resistor was used in the old days as a termination device that ran double duty. Part of it was to keep the impedance low such that cable artifacts were swamped by the low impedance. The other part was that the output transformers used to drive 600 ohms need to be loaded at 600 ohms to get flat frequency response (many transformers will express the inter-winding capacitance rather than their turns ratio if not loaded sufficiently).

So that 600 ohm standard was around for a while but was not included in AES file 48. For this reason, if one follows only file 48 one might still run into minor differences in the sound of cables, but if the equipment in use has lower output impedances much of that will be swamped. IOW I am saying that AES file 48 does not cover the entire topic (but its got the lion's share of it for sure). Since there is still a lot of equipment in use that supports the older 600 ohm standard, we made sure our gear did too so it would be compatible.

Now I've not looked at the newer Audio Precision stuff, but what they were making in the early 1990s did not conform to AES file 48. I'd be interested to see if the later stuff does. How it did not conform back then had to do with the fact that it referenced its inputs on its balanced connection with respect to ground (pin 1) instead of each other (pins 2 and 3).  I have to say this really surprised me because then as now the Audio Precision equipment represents a high standard of quality in audio test equipment!

i don't think that is a critical issue. Other factors are equally important. I happen to have fully balanced, dual mono amps, but other stuff sounds great, too..