Measuring Guitar Cables

There are plenty of exaggerated, and even outright fake claims about audio cables. I wanted to take a mixture of common factors that might impact a guitar cable, some based on sound engineering principles, and others that might be a bit more tenuous, to see which are measurable with commonly available lab equipment. The first stage was to list various factors that I would like to test, the second to devise tests for them.

The Factors

Conductor Materials

At normal temperatures, the International Annealed Copper Standard lists the Top 4 best metallic electrical conductors relative to Copper as:

1. Pure Silver – 105%
2. Copper – 100%
3. Pure Gold – 70%
4. Aluminum – 61%

Copper and Aluminum are the most commonly used in cables. Silver provides a very small improvement in conductivity at significant cost increase. Silver also reacts with oxygen and hydrogen sulfide in air creating a film over the surface that could potentially offset the small conductivity gains. Some cables using different mixes of all silver or silver and copper are available at the high price end of the market. Silver plating is also sometimes utilized in audio cables.

Gold is expensive, and rated at less conductivity than copper. The main benefit in cables is its high resistance to oxidization. Fine gold wire, sometimes in alloys or doped with other elements such as beryllium has long been used in semi-conductor manufacture. In these microscopic wires, the quantity of gold is so tiny, that the manufacturing benefits outweigh the cost of the material. Gold plating is sometimes used on audio connectors to take advantage of its resistance to chemical reaction with the air.

Aluminum’s claim to fame is weight, or rather lack of it. Although conductivity is rated at 61% of Copper, aluminum is often used in high current applications such as long distance power transmission where the lower cost of Installing and maintaining Aluminum cables offsets any loss in conductivity performance vs Copper.

Resistance, Inductance, Capacitance

When we connect a guitar pickup to an amp via an insulated 2 conductor cable, we are creating a circuit with resistance (R) inductance (L) and capacitance (C). This RLC circuit behaves as a low pass filter. As we change different values of the components by adding more, longer and different cables, the center frequency and slope of the filter change. This goes some way towards explaining what many people refer to as tone-suck. Some of the high frequencies can be attenuated as a result of this filter effect.

Oxygen Free Copper

Like fat free foods, or drug free zones, Oxygen free copper is not completely free of oxygen. For example ATSM C10200 Copper is called Oxygen Free at 0.001% Oxygen. While very small percentages of Oxygen in metals maybe important in certain manufacturing processes, or very precise measuring equipment, there is no consensus that it is of benefit in audio cables. However, OFC is not especially expensive, so cable manufacturers will often use it anyway, as there is not really much downside unless you are aiming for really low cost.

Microphonics

Where vibrations from mechanical actions such as moving, hitting or stepping on the cable are transferred into the electrical signal. It’s mainly the mechanical construction of the cable that influences this. The Triboelectric effect where static electricity builds up from the rubbing together of dissimilar materials within the cable can also cause issues. Manufacturers can usually minimize this through optimum choice of dialectric materials that exhibit this effect less, or even adding an additional conductive layer to dissipate the static charge.

Skin Effect

Skin effect is a phenomenon where alternating current density tends to get distributed towards the outside circumference of a wire. This is certainly proven to occur. Some applications even use hollow cables: Since the skin effect results in a large percentage of the current being carried around the outside of a wire, the center is just removed altogether saving weight and cost. However the effect is frequency dependent, with the skin depth becoming smaller with higher frequencies.

At 10GHz the skin depth can be around 1 micron, which makes a hollow tube with a thin silver plating a perfectly reasonable solution for microwave systems. Audio frequencies, are not microwaves. Wikipedia lists skin depth in copper at 10kHz as 652 Microns. 22AWG wire is 643microns, meaning the skin depth is actually greater than the diameter of a typical guitar cable.

Research from University of St. Andrews in the UK shows measured power loss due to skin effect with internal impedance taken into account in a 4mm copper wire to be 0.02dB at 25KHz. That’s a power loss too small to recognize in a practical audio system, at a frequency too high for humans to hear. Once you get down to frequencies within the range of human hearing, any power loss due to skin effect was recorded as virtually zero in common household copper wire.

Directionality

Some guitar cables maybe considered directional because they have two internal conductors for signal and a separate screen. The screen is disconnected at one end. This can help in some circumstances reducing noise from ground loops. These cables will normally be marked which end has the shield disconnected. This can only work with cables specifically designed for this with a separate shield. Most shielded guitar cables use the shield as the return and so cannot be disconnected without breaking the circuit.

A second type of directionality suggests that the audio performance is better with some cables oriented one way or the other. One day if I can ever afford any of the cables that claim this, I’ll test them.

The Tests

In the test, I have seven different cables varying in price from less than $6 to over $250. There are a variety of different core materials and jack platings, as well as the same cable with different lengths, so we can compare the impact just of the length of the cable.

Contenders

Neewer – This was the lowest cost 10’ guitar cable I could find on Amazon Prime at the time of purchase. I paid $6.95. The cable has a braided Tweed style cloth jacket and large jack plugs. The tip of the jack plug is gold colored, although I could not tell if it is actually Gold plated. The sleeve of the jack appears to Nickel.

Rapco Horizon – The Horizon Standard is available at Guitar center for $7.50. I took this one from our collection in the lab. This cable has 116 reviews on Musicians Friend, so I imagine it is a very popular low cost, no frills instrument cable.

Quantum Audio Designs – The first of the Oxygen Free Copper cables. This one was purchased from our local music store for about $15. It has heat-shrink over the jack and first few inches of the cable.

Best-tronics Pro Audio – I tested a 10ft TRS cable. TRS cables are sometimes used with active pickups, dual magnetic/Piezo, and other types of guitar that require the additional conductor. The cable utilizes OFC conductors, and Switchcraft ¼ Nickel jacks. Best-tronics will custom make cables to order to any length. Price is around $24 for TRS 10’.

Kirlin Stage – The Kirlin Stage is described as ‘exclusively designed for live performance’ The 18 AWG OFC conductor is thicker than the others on the list. The large metal jack plug has gold plated contacts, and a black painted metal body. The Literature lists a couple of China Patent numbers: ZL 201230626606.8, and ZL201320086667.9, if you want to try looking them up. Price is $25 at Musicians Friend.

Zaolla Silverline – The Silverlines are the only Silver cables that I tested here. They are described by Zaolla as using a mixture of Silver and Copper conductors: ‘all Zaolla Silverline instrument cables feature a solid Silver center conductor and an enamel coated, stranded copper ancillary conductor in a unique hybrid configuration’. The Jack plugs are listed as having both Rhodium and Silver plating layers. Similar to Best-tronics, Zaolla can build cables to custom lengths on request. I tested a 15’ cable and a 2’ cable from the same line. A 15ft cable is priced on zaolla.com at $254.95, and a 10’ at $199.95.

The Tests

A cable connected to a magnetic guitar pickup creates an RLC 2nd order low pass filter. This creates a small boost to the signal at the resonant frequency, and then gradually attenuates the signal at higher frequencies past that. By adjusting the amount of capacitance and resistance in the circuit, you can control the center frequency and slope of the filter. This is how a passive speaker crossover works, for example.

Since the resistance and capacitance of your guitar pickups, tone and volume controls, as well as the number and length of cables all work together to impact this, every rig is going to be different. This is potentially even true in the same guitar. If it is a multi-pickup guitar where the pickups have different inductance and resistance, just switching between pickups will change the characteristics of the filter.

With this in mind, I measured as best as I was able, just the resistance and capacitance of the cables and jack plugs themselves. If you characterize the rest of your circuit by looking up the numbers or measuring your guitar pickups, you can then plug these numbers into an online RLC filter calculator (or do the math yourself if you prefer) and predict the effects.

Measurements

All measurements were taken at 68 degrees F. The cables were all kept in the same room as the measuring equipment for at least five hours, and the measuring equipment was left on for at least one hour before for temperatures to stabilize. I measured the resistance of each cable from tip to tip, and sleeve to sleeve. I measured the capacitance of each cable from tip to sleeve at each end. The results are shown in the table.

Results

The biggest single factor impacting change was the length of the cable. Total resistance of the cable assembly (including jacks) is reduced by more than half on the 2ft Silverline compared to the 15ft. Capacitance was reduced almost six times, which is directly proportional to the change in length within the margin of error.

The 15 ft Silverline had the lowest resistance on the signal conductor at 0.013 Ohms/ft. The 18 AWG Kirlin was close behind at 0.016 Ohms/ft. The 2ft Silverline measured a higher resistance per foot, but this was most likely due to the contact resistance at the measuring point on the jack making up a greater proportion of the number on the short cable. As cables get longer, the resistance of the cable becomes much more significant than the resistance of the jack.

The Horizon had the greatest resistance on the tip. At 24AWG compared to 22AWG for the Best-tronics and 18AWG for the Kirlin, this is not unexpected. The thinner wire should have a greater resistance and our measurements bear this out. The Horizon also had an order of magnitude higher resistance on the sleeve. The cable was not new, and was taken from our demo room, it possibly had some damage, which just goes to show that it is worth testing your cables on a regular basis, even if they appear to be working.

The resistance of the cable however, generally makes up only a small percentage of the overall resistance of the circuit. With the pickups, volume, and tone controls usually being in the hundreds of Kilo Ohms range, the capacitance of the cable usually has a more dramatic impact on the audible tone. Here again, the best way to reduce the capacitance of the cable is to shorten it. The short 2ft Silverline measured just 84pF, compared to 969pF for the Neewer which was the worst performing in our capacitance test. Calculating a theoretical pickup configuration, that moves the cutoff frequency of the filter from 12KHz to 3.6KHz, which would be a noticeable difference.

The lowest measured capacitance per foot of all out cables was the Quantum Audio Designs at 25pF/ft, followed by the Silverline 15 at 33pF/ft. and the Silverline 2 and Best-Tronics both at 42pF/ft.

Conclusions

Shorter is better. Although it’s obviously not practical to use a 2ft guitar cable, minimizing the number of cables and keeping down the length helps. Use as short a cable as possible from your guitar, and use a buffer first in line if at all possible. If your environment requires a really long guitar cable, consider using active pickups, or a wireless system instead.

The very low cost cables in the test did not perform very well. The Neewer had a much higher capacitance than the others which would be audible in most guitar setups. The Horizon had a higher than normal resistance reading on the sleeve (which may have been due to some damage).

The Zaolla Silverline was most expensive cable in our test, and had the lowest tip resistance and the second lowest capacitance so there’s no doubt it scored well. The Silverline was also noticeably lighter than the other cables. The 18AWG core in the Kirlin helped it turn in a low resistance measurement very close to the Silverline, although capacitance of the Kirlin was not great.

From $20 up, the differences between the expensive and mid-priced cables were small, especially in capacitance where it most matters. At around $20 ea, the mid-priced Quantum and Best-Tronics cables were close to or better than the highest price cable. I use mostly Best-tronics cables myself around here. Taking into account other factors such as mechanical reliability, etc as well as well as just the electrical characteristics, plus the fact they will custom make them to order, they work out great for me.

I wasn’t able to come up with a reliable, repeatable and unbiased test for microphonics that doesn’t require expensive mechanical lab machines that I don’t have easy access to. I thought about a small rubber hammer like the type used for testing microphonics in amplifier tubes, and fitting it to a micro-controlled stepper motor to repeatedly tap a cable with the same force. I’d be interested to hear any suggestions on how to test this.