There are circuit designs that are bandwidth limited ( on purpose ) so as to minimize the potential for high frequency oscillation. While doing this WILL reduce in-band performance to varying degrees, the manufacturer is more concerned about reliability and longevity than they are about sonics and being "technically correct". This is a perfect example of what we would call "real world trade-offs". Having said that, some designs are REALLY bandwidth limited and i'm not sure if this was done on purpose ( for added stability ) or if it is just a by-product of poor design.
As far as oscillation taking place at 200 KHz, i think that the Goertz Impedance Compensation ( Zobel ) Networks are tuned to come into play at 150 - 160 KHz ( give or take ). I can't recall the values of the parts used, but i know that they are different than what Nelson Pass has recommended in the past, what Jon Risch currently recommends and what Bob Carver feels is most appropriate. Obviously, what these highly educated EE's have to say should tell you something about how they feel about circuit design i.e. they all differ. I know that Carver's Sunfire amplifiers have impedance compensation built into them that comes into play at appr 80 KHz. Given the fact that the early Phase Linear amps used to blow up / sound horrible when connected to highly capacitive cables, i'm guessing that he's selected that frequency based on his past experience with trying to solve such problems.
As far as zip cord having a higher inductance / lower capacitance level, that is the very reason that the nominal impedance of these cables is so high at audio frequencies. As inductance is lowered and capacitance is raised, the nominal impedance is brought down closer to what the amplifier wants to see as a load. This has to do with what we call "transmission line theory" and equates to improved power transfer, more linear transient response and a wider bandwidth.
Most zip cord based designs will end up at somewhere between 80 - 120 ohms. This will vary with the gauge of the conductors and the amount of spacing between the positive and negative. This is very similar to most of the Nordost designs. Kinda funny how cables of similar impedances can give you such different sonics i.e. Nordost is lean and light in the bass / warmth region and "zip cord" is fuller sounding with rolled off highs.
When moving up to "fancy geometries", even Kimber, which is a relatively low inductance / higher capacitance cable design, clocks in at appr 20 ohms. This is for their heavier gauge cables with the Z ( impedance ) going up as gauge is reduced. The 4 TC/VS/PR series is closer to 40 ohms, give or take.
The only "audiophile approved" cables that i know of that are relatively current in production with a low nominal impedance would be Goertz and Dunlavy. Obviously, Dunlavy is no longer marketed under that name. For the record, Dunlavy started off as an RF engineer and then switched over to audio later in life, so he was well aware of transmission line theory and optmizing power transfer & bandwidth. While he thought that the differences in cables could not be heard during DBX conditons, he wanted to make a cable that he thought was "technically correct" for the specific job being done.
From a technical standpoint, both the Goertz and Dunlavy cables have impedances below 10 ohms, which is where you'll find the nominal impedance of most speakers at. The Goertz varies from 2 ohms to 10 ohms depending on the model ( heavier gauges are lower impedance ) and the Dunlavy's come in at about 6 ohms.
There have been other designs in the past, such as the infamous "amp blowing" Polk's, etc... that have also been quite low in impedance / high in capacitance. I'm sure that there are others currently being offered, i'm just not aware of them. I remember reading something about Coincident Technology and their cables being of a low nominal impedance / high capacitance design, but can't recall any specifics.
Obviously, some amp designs will oscillate at a lower frequency and have a lower threshold in terms of what it takes to excite them into oscillation than others. If one is in doubt, it is best to use a Zobel and not have to worry about it. Since some amplifiers already have Zobel's built into them from the factory, you might want to contact the manufacturer and see what they say. If a manufacturer is adamant against using such speaker cable designs, this tells me that they do not know enough about circuit design to help stabilize the circuit. It also tells me that the rest of the circuit is probably not very good either based on that lack of knowledge.
My experience is that using a low impedance speaker cable offers the best performance. Like anything else in the real world though, one may have to take precautionary measures to avoid the side-effects of such idealized designs when combined with wide bandwidth amplifiers of varying levels of stability. As i previously mentioned, some amplifier designs / systems may not benefit as much as others, so it would be foolish to to use a product that cost WAY more money yet offered no real audible or measurable results.
With that in mind, i would suggest that one should try some different cables out with various geometries / electrical characteristics and see what works best for them in their system. If you can't tell a difference, use the cheapest stuff that will work now and remain trouble-free in the future. Sean
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