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Engineer's Corner

ENGINEER'S CORNER #1

What Happened When the Engineer
Scratched His Head?

Flexygy 6

There's a lot of buzz out there about speaker cables. There's a bewildering variety of speaker cable on the market: twisted, straight, heavy gauge twin lead, woven etc, and some built like ordinary lamp cord. There are lots of technical and performance promises. After a bit of technical investigation, I learned the sad truth of the situation: simply put, too good to be true. It was when I came across a cable that professed to be a "true audio speaker transmission line," however, that I went ballistic, and uttered a common explicative.

Bull-doggedly, I went to work, determined to get at the truth.

I measured the characteristic impedance of every speaker cable I could get my hands on with a Time Domain Reflectometer (TDR) and a pair of tracking carbon potentiometers at both the source and the free end of each cable. If you are an engineer, you know this is the time-honored way to test broadband impedance and impedance uniformity. With one TDR test after another it became clearer and clearer that none of these "new" cable designs had produced nominal line impedance much lower than about 80 ohms. It was then I scratched my head again and the proverbial light went on.

The TDR tests reminded me of what was actually important in a loudspeaker cable. Here goes...

From my background as a hands-on, broadcast engineering executive and RF circuit designer, I knew that it was not physically possible to build a true transmission line at speaker nominal impedance. Even, if a true transmission line could be built, it would not be effective for high-current, power coupling, nor would it even be desirable given the nature of the typical loudspeaker load and the source impedance of modern amplifiers.

You ask why? IMPEDANCE. Measured in ohms, good old impedance indicates the amount of resistance to the flow of an electric current in a wire, source or load to an electric signal. It includes capacitance and any stray inductance, the so-called "imaginary" component in impedance, although for high performance cable, only the pure resistance should dominate.

First and foremost, to qualify as a true transmission line, a cable must couple the source impedance to an identical load impedance. This is the key requirement for radio frequencies, for balanced audio line circuits, and for many other radio frequency applications. Equally important, the impedance of the "true transmission line" itself must match the impedance of the source and the load. Good old geometry rather than magical construction material governs structural impedance. Quite simply the size, the shape and the topology of the wire used to fabricate speaker cable determine the impedance. The conductor size and the conductor spacing are the controlling factors that characterize the impedance of cables composed of two or more wires. Bizarre insulation, rare metals and sales promises have no influence at all!

Construction geometry SOLELY determines what is, for example, a 75-ohm coax, or a 50-ohm coax, or for that matter any "type" of cable. Cut open a cross section of 50ohm or 75ohm cable, and you'll see that the ratio of the inner conductor and outer shield is different. The closer the inner conductor is to the outer shield, the lower the impedance. Since it is not possible to have an inner shield and outer shield the same size, there is no coaxial line commercially manufactured with a rated impedance of less than 25 ohms capable of at least a 100 volt insulating "stand-off". This holds true for any conductor topology: side-by-side, twisted, braided, bent, folded, etc. Truly low impedance is even more difficult to obtain in many instances of exotic cable construction.

So what DOES make a great speaker cable?

It's all about minimizing two parameters at once, in pursuit of the ultimate goal for all metallic conductor cables:
  (1) ZERO resistance
  (2) Above all NEAR ZERO inter-conductor capacitance

While some high-end cables on the market do a fine job of reducing resistance, and some of the woven and braided designs look cool, many of these cables actually have grossly high capacitance per foot. The resulting MEASURED performance is clearly inferior, as is the resulting listening performance - except perhaps in the unique instance of connecting to a zero feedback vacuum tube amplifier.

There is a valid case for two types of speaker cable: a low capacitance, high resistance cable for coupling to vacuum tube amplifiers and a low capacitance, low resistance cable for coupling to solid-state amplifiers. Keep this in mind when cable / amplifier shopping.

If you're connecting to an unstable retro tube amplifier, cables providing high capacitance per foot are an ideal match for higher source impedance and stability. With vacuum tube designs, adding some resistance (up to 10% of the rated loudspeaker impedance) between the amplifier and the speaker actually makes the speaker sound better. Basically the destabilizing reactance of the speaker is damped by high-loss/lossy speaker cable.

For most of you with stable, state-of-the art, solid-state amplifiers, these woven, braided cable beauties spell disaster or at best are a questionable investment. Your MOSFET or Super Beta style amp, for example, is already designed to dampen the reactive instability of the speaker. More capacitance on the load line is no benefit.

If you're in the state of the art, solid-state amp camp, only one cable topology can successfully offer the lowest capacitance -- FLAT CABLE. Within a flat cable the proximity of the individual conductors can be minimized, as typically, only two of the multiple conductors are proximate to one another - and construction spacing can optimize even that.

Hence the birth of FLEXYGY 6 conductor speaker cable. The secret of this FLAT and FLEXIBLE cable lies in exactly what we've been talking about: cable topology and impedance.

FLEXYGY excels in its FLAT geometry and copper lay out. I decided to use copper, LOTS of ultra-fine drawn, high purity copper, where other cables use filler and plastic. I also selected more finely stranded copper than the competition. Many of the cables I tested use only a few limited copper strands. Examine a cross section of FLEXYGY and you'll see hundreds of fine copper wires arranged in conductor groups. The amount of copper in FLEXYGY's six conductors far exceeds most other high-performance speaker cables. The flat topology, the layout and the quantity of copper offer two unique and key performance benefits to the solid-state amp owner:
  (1) FLEXYGY's superior high frequency response
  (2) FLEXYGY extreme suppleness and flexibility

You'll really appreciate that added flexibility when you're in the throws of installation nightmares: under the Oriental rug, along the baseboards, behind your wall of sound unit, etc.

At the end of the day I'm feeling pretty good. FLEXYGY delivers world-class capability without breaking the bank account. Desirable cable characteristics can be had. The limitations that materials impose on impedance can actually be used constructively to create a cable product with the highest performance at an affordable price. Maybe I'm just a Boy Scout at heart but I always get my synergistic kicks out of "making it happen," "doing the right thing" - combining great innovation and performance with a realistic price does it for me. I'm feeling pretty great about FLEXYGY 6 conductor speaker cable, and I think you will too. There's no risk to trying, so why not give it a listen.

Tell 'em Donald sent you.

I am still scratching my head about a few other issues so stay tuned.

Donald - The Exponential Engineer

 

Read Engineer's Corner #2