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Speaker Cable Faceoff 2 - Zobel Networks

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clip_image014.jpgThis RC network in more commonly referred to as a Zobel network. Usually this compensation is applied directly at the source (output of the amplifier). However, in this case, Goertz has implemented this network at the speaker side of their high capacitance low inductance speaker cables. Though the Zobel network is a good idea to minimize amplifier stability issues for high capacitance cables, Goertz chose non ideal values to resolve this potentially devastating issue.

Excerpt from Article on Loudspeaker Cable Characteristic Impedance
I selected this for the first simulation, as it is the most likely to cause amplifier instability with no termination zobel. Although the radical changes in phase are quite visible, it is a little difficult to see the effect on the amplifier. It transpires that this cable produces an output phase on the amplifier of that is well outside its phase margin, at around 192°.

 

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Figure 5 - Cable 3, No Far End Termination

As you can see, the cable has a large peak in the response at a little under 10MHz, and the phase response is savage. Each kink or discontinuity in the plot indicates a reflection, and note the phase angle - it shows 700° of phase shift at 100MHz!

 

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Figure 6 - Cable 3, Far End Terminated (4.7 Ohms)

Adding the far end zobel network, as you can see here the cable's response decay is perfectly smooth with an 8 ohm resistor. This is the optimum match, and is the value that should be used - not 10 ohms as supplied by Goertz (see below).

 

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Figure 7 - Cable 3, Far End Terminated (10 Ohms)

Even 100nF in series with 10 ohms restores the amplifier phase margin to normal (150°). As seen above, 8 ohms is preferable, but the phase margin is barely affected.

The speaker end response has a small "lump" with 10 ohms, and phase goes "wobbly" at above 20MHz. This is probably not a concern, and you will almost certainly get away with it. It is very evident that this particular cable should never be used without a zobel at the speaker end, and it is equally obvious that the vendor does not really understand transmission line theory, since the zobels supplied with the cable (and you have to ask for them!) are incorrect. This is not difficult to get right, and if they are off base with something a simple as a resistor value, I would be disinclined to believe their other material.

While the results aren't actually appalling, it is quite obvious that the performance is not as good as with the correct termination resistance. At this level (and since the impedance of the cable is quoted on the web site), I find it difficult to understand how they could have made such an error. Not that this manufacturer is alone by any means - "experts" will emerge from the woodwork, suggesting that 10 ohm, 100nF zobel is the panacea. It is - but only for 10 ohm cables!

To view Rod's article in its entirety, visit http://sound.westhost.com//cable-z.htm
(Note the characteristic impedance of the cable quoted on Goertz site does not match what it should be based on our measurements. Using classic SQRT(L/C) (which isn't exactly correct, but this topic goes beyond the scope of this article) we calculate about 8 ohms while Goertz calculated about 2.51 ohms.

Goertz claim to fame in their low inductance cable is an associated low characteristic impedance to better match the application of amplifier to loudspeaker connection. While characteristic impedance matching is essential for transmission lines (usually at RF frequencies), it is a lofty goal for this application given the frequencies in question and the fact that we are interfacing a low impedance source with a complex load impedance. Goertz also mentions that their cable is better at dealing with Skin Effect. In this case they actually have a point as illustrated in the graph below - but again, at frequencies far exceeding the audio bandwidth in question.

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While the 10AWG cable does have lower DC resistance and AC resistance up until about 20kHz, the Goertz AC resistance is ruler flat up until about 100kHz whereas the AV Cable AC resistance is almost 2x that of the Goertz. Again this is academic considering the intended use of the cable and bandwidth in question.

 

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