by Ted Burmas
Ignoring wireless systems, the instrument cable is one of the few essential pieces of gear in most musician’s rigs. An instrument cable has one main function: to pass the instrument’s signal to the next piece of gear in the signal chain.
If this is all it needs to do, you may ask, “Why should I invest in higher quality instrument cables?” After all, even the lowest-cost cables can pass signals through them. This is where the details of the application, as well as construction, of the cable need to be considered. The goal of this article is to explain the characteristics of a quality cable to help you decide if it is worth your investment.
Characteristics of a Quality Instrument Cable
In my opinion, a quality instrument cable should have the following characteristics:
1) It should reject unwanted noise
2) It should be flexible
3) It should be durable and reliable
4) It should be sonically transparent
5) It should be easy to store
The Basics
Figure 1 shows the anatomy of a standard instrument cable. It shows the two main components: the bulk cable and the plug. The bulk cable has a conductor for ground and a conductor for the signal, separated by an insulator and covered with an insulating jacket. Each plug terminates its tip and sleeve to the two conductors of the bulk cable via solder. Additionally, there is a barrel and a strain relief to protect what is inside it.
The Bulk Cable’s Conductors
A conductor is a material that easily passes an electrical signal through it. It turns out that the metal copper is an excellent choice for this application for several reasons: 1) it is highly conductive, 2) it is easy to work with as it is relatively soft and flexible and, 3) it is relatively inexpensive. Many instrument cables tout the use of “oxygen free copper.” This basically means the copper was processed in a way to increase its purity and thereby improve its conductivity. I have my doubts if this affects sound quality in any way, but since it is readily available, I don’t see a reason not to use it. Other metals can and have been used in instrument cables, but since the majority use copper, we won’t discuss them.
Bulk Cable Construction
Most musicians prefer an instrument cable that is flexible to allow easy movement, as well as rugged, due to the abuse it will take on stage, in the studio, and at home. Both flexibility and durability of the bulk cable are usually improved by bundling many thin copper wires together into a spiral pattern, as opposed to a single thicker wire (which can be quite stiff). This can be done for both the signal and the ground.
In many environments where musicians play, electromagnetic energy can creep into the signal. It would be beneficial if the bulk cable was made in a way to help reject this interference. One effective way to reject unwanted electromagnetic energy is with a ground shield built into the cable itself. A cable with a shield designed in this way was shown in Figure 1 and is called a “coaxial cable.” The more complete the coverage of the shield, the more effective the rejection can be. Many techniques are used, from a foil, to a braid, to a spiral of thin copper wires, or a combination of these. No one approach is superior to another; they each have their pluses and minuses.
As many electric instruments, such as guitars or basses, have high-impedance passive pickups, the build-up of charge in the cable from movement can also be a source of noise. One way to combat this is with a conductive film contacting the insulator between the signal and the ground shield to dissipate this charge.
For Sonic Nuance Electronics instrument cables, I use Mogami w2524 bulk cable, which uses a spiral copper shield, combined with a conductive sub-shield for 100% coverage and has virtually no handling noise. Figure 2 shows the elements of this bulk cable. It is flexible, durable and rejects unwanted noise – several of the desirable characteristics listed earlier. In addition, after using thousands of feet of cable, I have found this bulk cable to be very consistent from roll-to-roll and is readily available, as shown in Figure 3.
Fig. 2
Fig. 3
Bulk Cable and Sonic Transparency: Passive Systems
This is where things get a bit weird. At best, most people are able to hear frequencies from 20Hz to 20kHz (or approximately from the notes Eb0 to C7). Let’s look at some numbers to investigate how a cable can affect the tone of an instrument in this frequency range.
Passive System: The Model
The specifications for electromagnetic pickups are difficult to find and there is a huge variation in designs. On top of that, inductance and capacitance are rarely listed. While the numbers don’t tell the whole story, for the sake of our investigation, let’s assume a typical passive instrument with the volume and tone controls at 100% has the following numbers:
Pickup Specs:
Resistance: 10k ohms
Inductance: 2H
Capacitance: 100pF
Using specifications for 20ft of Mogami w2524 bulk cable and using a simplified model, we obtain the following numbers:
Bulk Cable Specs:
Resistance: 0.3 ohms
Inductance: 4 uH
Capacitance: 800pF
Note that the resistance and inductance are orders of magnitude smaller than that of the pickup, so they can be safely ignored for our investigation. Also realize that the numbers for resistance, inductance and capacitance for the bulk cable scale with length.
The load presented to this system is usually an amplifier, buffer or active DI with a high input impedance. Inductance and capacitance for the load are usually negligible and can safely be ignored.
Load Specs:
Resistance: 1M ohm
The combination of passive pickup, instrument cable, and load form a passive network. A simplified model of this passive network is shown in Figure 4. In a passive network, all elements interact with each other to some degree. Setting the volume and tone controls to max in our passive instrument helps to simplify the network since they essentially are removed. This is one reason some musicians prefer the tone of their instruments with the volume and/or tone controls not set to max.
Fig. 4
I’ll spare you the math and electronics theory, but the passive network in Figure 4 is essentially a low-pass filter with a resonant frequency.
Simulation Results
Simulation results from this model for various length of cable are shown in Figure 5. The results show that the cable definitely has an effect on the filter’s frequency response. Note that the resonant peak and the amount of low-pass filtering is affected by the length of cable, which is dominated by its capacitance. Cable capacitance in the model has no effect on lower frequencies.
Fig. 5
It is important to realize that this model says nothing about what frequencies the instrument actually presents to the pickup. Those frequencies are affected by the strings, the wood, the type of frets, type of magnets, if the musician has fried chicken grease on his fingers, and so on.
What Frequencies are Presented to a Pickup?
The fundamental frequencies from a bass with a low B to an electric guitar’s high E string, 12th fret, goes from about 30Hz to 660Hz, well below the 20kHz maximum accepted audio range humans can hear. This is also well below the resonant peak of our low-pass filter in Figure 5. However, there are many overtones that musical instruments create and there is no simple answer to “what frequencies are presented to the pickup?”
To get a rough idea of the frequencies that can be presented to an instrument cable from a set of pickups, a five-string bass with a low B and a six-string guitar were recorded and their frequency content measured through short cables. Both instruments had relatively new strings, the volume pots at 100%, and the tone controls off. Figure 6 shows the results from the bass and Figure 7 for the guitar. Note that there is virtually no energy in the upper frequencies where our simulation results showed the cable would have any effect.
Fig. 6
Fig. 7
Active to the Rescue
Now, let’s say our passive instrument is given an active preamp (aka a buffer) with an output resistance of 100 ohms with negligible capacitance and inductance (a reasonable assumption using op-amp based preamps). Our pickup-to-buffer-to-instrument cable combination will now have virtually no filtering or resonance effect in the audible range. This illustrates one of the key benefits of active instruments: cable capacitance has a negligible effect on tone. This provides a level of consistency for the musician in knowing what tones to expect, regardless of the instrument cable length. In fact, the musician can run very long cables before there is any effect on the frequency response in the audible range. It is important to note that some active preamps use a passive network for the volume and/or tone control, which increases the output impedance as these knobs are set to anything other than maximum. However, the majority of active instruments use active volume and tone controls, with a low output impedance, so this will not be a concern.
Note on Piezo Pickups
Piezoelectric pickups are very high impedance (in the mega ohms). Unless you want to filter out most of the high frequencies, it is usually best to have as short a cable as possible before going to a buffer or preamp. Piezo pickups are especially sensitive to cable capacitance. Since capacitance increases with cable length, keeping the cable short is very important. Many instruments with piezo pickups (acoustic guitars, mandolins, ukulele’s, etc.) have built in preamps, and thus the cable capacitance of the instrument cable has negligible effect on tone.
The Plugs
I prefer plugs that use metal for both the barrels and the strain relief clamps, for durability. I also prefer nickel-plated plugs over gold-plated, as the gold is usually very thin and wears off eventually to the metal below (which is often nickel, anyway). The plugs I use for Sonic Nuance Electronics instrument cables are nickel plugs from G&H. They have copper cores for added durability (copper is ductile and will bend, rather than snap, like harder metals when bent). The effect of capacitance and resistance of the plug on sound quality is negligible, in my experience. The consistency of the plug’s dimensions can effect “contact crackle” noise though (the sound of an intermittent contact).
Every instrument cable I have ever seen that has had issues has done so at the junction of the plug and cable. Therefore, the durability of the instrument cable is intimately related to the integrity of this junction. Besides the attributes I have mentioned about my favorite plugs, I use Techflex polymer sleeving (held on by the plug’s metal clamps), adhesive lined heat shrink, and hot glue, as shown in Figure 8. The polymer sleeving adds another level of durability in terms of impact and strain relief to the instrument cable (as the polymer sleeving web is pulled lengthwise, it compressed the cable inside of it, transferring much of the tension strain into compression over a large surface area of the cable.)
Fig. 8
Soldering
Solder provides a gas-tight bond between the plug and bulk cable’s conductors that shouldn’t shake loose in normal conditions. Some people feel the particular composition of metals that make up solder can affect tone. I haven’t experienced this myself, but I wouldn’t immediately dismiss the idea. Let me explain why with a quick story.
Years ago, a fellow engineering friend and I once did a listening test for a friend who owned an audiophile shop. We were asked to listen to the difference between a $1,000 IEC power cable to a $1 one on the same hi-fi audio system. The owner was trying to decide if he should carry the expensive product and wanted our opinions. We were not paid for this, so we had no financial incentive either way. I was extremely skeptical going into the test. Defying all logic and physics, all three of us did hear a definite improvement in the sound quality with the (ridiculously) expensive power cable. To this day I cannot explain why this was or how it was even possible from a technical standpoint. I went into the listening test a skeptic and left with a powerful lesson. While specifications and engineering knowledge are a good guide, you ultimately have to trust your ears when it comes to audio.
Back to soldering. While I have my doubts that the type of solder affects tone, I am sure that a bad solder joint can worsen both tone and durability. A quality instrument cable should have the solder joints done right. Most instrument cable manufacturers use specialized tools to make and test the solder joints.
You may be wondering why I have not addressed so-called “solder-less” plugs. Let me put it this way, if I offered solder-less plugs, my instrument cables wouldn’t have lifetime warranties…
Closing Thoughts
There are many excellent instrument cables on the market today. A quality instrument cable may be good insurance in terms of durability and reliability for years to come. The sonic transparency due to electromagnetic and handling noise immunity, as well as low capacitance (less so with an active instrument), may be important to you, as well.
Someone once said that buying quality only hurts once, and I have to agree. As cost doesn’t always equal quality, I hope this article will help you to identify a quality instrument cable when you are making a decision for your next purchase.
More Closing Thoughts
I went through an extensive period of torture testing when coming up with my instrument cable recipe as shown in Figure 9. Figure 10 shows the final result. I also use a Riptie velcro cable wrap to make the instrument cable easy to store and in transit. Although I have some custom tools made for the process, I show the steps I go through to make each cable at https://www.sonicnuance.com/cable-manufacturing-steps/. If you are a DIY person who likes to learn new skills, I encourage you to try to make your own instrument cable by using these steps as a guide. However, I wouldn’t recommend it if your goal is to save money, as you may be in for an unpleasant surprise when you add the cost of components and your time.
Ted Burmas is the owner and designer at Sonic Nuance Electronics which designs and manufactures transparent and rugged live performance tools.
Fig. 9
Fig. 10
