Speaker Cable (Part 1)

If you are going to buy a pair of speaker cables, what would be your concern? Thin cable sound smoother than thick cable? Silver has better performance than copper?  High tech cable with proprietary structure give you more fidelity? Yeah, so many questions. But, before make a decision, please consider what means a good cable.

Cable changes everything

Just imagine, you got a high fidelity audio amplifier and a pair of monitor loudspeakers. They were connected by speaker cables in this way.

Figure 1. Audio amplifier and loudspeakers were connected by speaker cables

You want to play the music exactly the same as the original source. Now, the only thing could alter the sound will be the speaker cables. If the cables have more lows, it will allow you to savor a richer bass, it will also be a dull sound. If the cables have more highs, the sound will be sharper and clean, but you sacrifice low end. Therefore, the cable quality is quite essential to the sound quality.

Figure 2. How speaker cables change the frequency response

Resistance problem

Before talking about how the resistance of cable can affect the sound quality, let us take a look the impedance curve of a speakers. Below is an impedance curve of B&W Nautilus 805, measured by Stereophile. As you can see, the impedance is fluctuated along with the frequency, from 10Hz to 50kHz. I will use this model for the later analysis.

Figure 3. B&W Nautilus 805, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Now we are going to connect a pair of thin and long cable to the 805. Below is the environment:
  • Cable core material = copper
  • Cable length = 12 m
  • Diameter of the cable = 24 AWG (a thin core similar to a cat5e cable)
  • Termination resistance = 0.1 ohms
  • Ambient temperature of  = 20°C
The cable resistance plus the termination resistance is around 1.1 ohms. It seems that the little resistance is not a big deal, but it would greatly alter the frequency response. If we connect an ideal cable (0 ohms) to the 805, we will get the black dotted line (ie. No deviation from the 805’s frequency response) as shown below. 

Figure 4. Equivalent impedance (ohms) and Deviation from ideal response (dB) vs frequency (Hz)

Our goal is to keep the deviation as small as possible (close to the black dotted line) in order to maintain the highest fidelity. In real life, we couldn't find any cable with 0 ohms. Let’s look at the orange dotted line in Figure 4. As the impedance of 805 is fluctuated along with frequency, the deviation is also fluctuated too. 3 “saddles” were found at 40Hz, 200Hz and 20kHz. That indicates the thin cable will turn the volume down by 1dB to 1.5dB at these frequencies. For 805, you may found the low end is missed, voice is brighter and treble is rolled off. Normally, the speaker cables should be thicker. For a 3 meter 16AWG cable, the result would be better (blue dotted line). But still, 3 “saddles” were found at 40Hz, 200Hz and 20kHz.

How to prevent?
  1. Choose thicker wires
  2. Keep the cable as short as possible
  3. Solder the cable to the connector directly (ie. Banana plug) to prevent the connection have gone bad over time

No comments:

Post a Comment