*WHITE PAPER* The Sound of Music - How & Why the Speaker Cable Matters
G'DAY
I’ve spent a sizeable amount of the last year putting together this white paper: The Sound of Music and Error in Your Speaker Cables
Yes, I’ve done it for all the naysayers but mainly for all the cable advocates that know how you connect your separates determines the level of accuracy you can part from your system.
I’ve often theorized what is happening but now, here is some proof of what we are indeed hearing in speaker cables caused by the mismatch between the characteristic impedance of the speaker cable and the loudspeaker impedance.
I’ve included the circuit so you can build and test this out for yourselves.
Let the fun begin
Max Townshend
Townshend Audio
Rule of thumb says that cable becomes transmission line (reflections from impedance boundaries) when signal propagation one way is longer than 1/8 of the fastest transition time. When perfect square wave is applied all cables behave like transmission line, but it doesn't happen in real life. Rise time of the signal is roughly 0.35/BW, being 17.5us for 20kHz bandwidth. 17.5us/8=2.188us a propagation time of 437.5m (assuming speed of 5ns/m). Designers do this calculation in any digital design to determine if wires or traces need termination. I would worry about transmission line effects in audio when using speaker cables longer than 437.5 meters, unless I'm missing something? (Another commonly used test is to compare length of the cable to 1/10 of the signal wavelength. 20kHz audio signal has wavelength of 10km, assuming signal speed of 200,000km/s. 1/10 of it is 1000m) |
Sorry Mr. G'DAY, you are miles from the truth. I also spent much time into speaker cables research & analysis. 1. The speaker side is not a part of the analysis. It is only looking into the Amp's output through the cables. 2. The most important character of C (capacitance) should be zero, when the two cables (black and red) are separated and conducted by two separates. Problem solved. 3. The way to go (tested with multiple cases over here: https://forum.audiogon.com/discussions/no-one-actually-knows-how-to-lculate-what-speaker-cable-they-...about two years ago, and here: https://forum.audiogon.com/discussions/how-to-select-a-good-speaker-cable 4. What matters is R (resistance) as it's series to the Amps' R out or DF. If that is kept low the sound improves significantly. Try it. |
This white paper is embarrassing in the level of errors, and erroneous conclusions.
Seriously?. This is not at all what is happening. If that was the case, the graphs would go up and down with frequency to match the impedance curve of the load as it mismatches with the cable like the peaks in impedance at 1.5KHz, and 10KHz, but they don’t. Why is that? I know why. The author? Not so much. Now let’s have a good look at figure 12. The oscillations are 5 oscillations per usec. That’s 5MHz. No speaker puts that out. No human can hear it. Now how long is that cable? It’s 7 meters. * 2 = 14 meters. Waveform propagation speed is mainly related to dielectric, so let’s estimate speed at 0.6C = 180,000,000 meters per second. That’s about 13Mhz if due to transmission line reflections. Now what about the 18ohm impedance? Well it would also have transmission line reflections close to 13Mhz, but it appears to have oscillations close to 50 or 100MHz which if due to transmission line effects would require exceeding the speed of light. For cable 6, Fig 3, with Zo 476ohms, driving the dummy speaker load with a step input from a square wave (the simplest transient) gives rise to severe ringing that has many oscillations.This is due to the transient reaching the mis-matched speaker load where only a small fraction of the signal is absorbed by the load. The remainder of the signal is reflected back to the source (the amplifier) where it is reflected back to the load. Again, only a small fraction of the now-diminished signal is absorbed by the speaker, with the remainder reflecting back to the source and so on. Over time, all the reflections will eventually be absorbed in the load. Oh come on, really? That is not how characteristic impedance works at all. I sort of feel bad for the op putting this out. This is not going to go over well. Your simulation is highly flawed. So, lets go back to the conclusion: The results show that the principal factor determining the error of a cable is its geometry. Cables with very widely spaced conductors have the greatest error, closer-spaced conductor cables have less error, and very closely-spaced, flat conductor cables have the least, or near zero error. No, this is not the principal factor at all, nor is it what your results indicate. Geometry (spacing) does play a roll in what is a determining factor and what all your results show. Everything in your article points to 1 and only one 1 item. Inductance. Not characteristic impedance. Plain, simple inductance. Space conductors far apart, and the inductance is high. Space conductors close together and the inductance is low. Put two flat conductors really close and the inductance is very low (and the capacitance very high which can make some amplifiers unhappy). The graphs in figure 3 - all inductance. The oscillations in figure 12 - have nothing to do with transmission line effects, they are just a factor of the high or low source resistance in the simulation damping out the load oscillations slow or fast and impacting the frequency. Reflections causing roll-off? They are at 13Mhz approximately. In your simulation they settle out completely after 10 microseconds (>100KHz bandwidth). For low-level interconnect signal transmission, typical cables have an impedance of between 50 and 100ohmsanddrivea 10kilohm to 20 kilohm load. There are reflections from the load, but the source resistance is typically the same as the cable impedance, so the reflections will be absorbed in the source resistance and there will be no further reflections. This is known as “back matching”and usually occurs by default in audio and is de rigueur in video. Source impedances in audio equipment single ended are typically 600-2000 ohms, some higher. That is not anywhere near 100 ohms or 50 ohms.
Foamed polyethylene is better than basic PTFE which is why it is so common in high frequency cables. Reason PTFE is used in high frequency cables is dimensional stability. Polyester is not at all a good dielectric. It can be worse than PVC, or better, but never good. |
This will not work well for a cable and can have huge errors. An LCR meter cannot isolate the L and C when taking a measurement on a cable and hence the measurements for L and C end up wrong. With a good LCR meter (the DM4070 is not), you can adjust the measurement frequency and use the change over frequency to extract the accurate L and C values. Of course if you start with a good LCR meter, it will have an impedance measurement function, which will allow you to measure the impedance with the cable shorted and the cable open-circui which can then be used to accurately compute the characteristic impedance. APPENDIX B: Three Methods of Deriving Characteristic Impedance |
