Studios, live sound venues and homes all have magnetic fields that can cause interference. Magnetic fields are produced whenever current flows through a wire. AC power cables, transformers, power supplies, computers, portable chargers, motors and light dimmers are among the devices that can emit strong magnetic fields. A microphone cable can pick up magnetic interference if it happens to pass near one of these devices. Cables can also pick up magnetic fields when they run adjacent to AC power. Top-quality star-quad cables can reduce this magnetic interference by at least 20 to 30 dB. This is almost always enough to reduce the interference to inaudible levels.
Balanced microphone cables are commonly used to carry low-level microphone signals. Magnetic interference is often a problem with microphone feeds because of the very low signal levels produced by microphones. Microphone outputs must be amplified by 20 to 60 dB to reach a line level. When a microphone feed is amplified, any magnetically-induced interference will also be amplified. In our opinion, star-quad cables should always be used for microphone feeds.
Balanced line-level interconnect use microphone cables to carry signals that are typically 20 to 60 dB higher than microphone feeds. Magnetic interference is usually not noticeable on a line-level interconnect unless the interference is severe. But, severe levels of magnetic interference can occur when line-level cables pass through crowded equipment racks, long cable trays, or in close proximity to lighting systems. We highly recommend using star-quad cables for balanced line-level interconnects. In our opinion, star-quad cables provide low-cost insurance against magnetic interference problems.
It is also unwise to equip a facility with a mix of star quad and standard microphone cables. In most cases the standard cables will work for the line-level interconnects, but these cables will inevitably wind up connected to a microphone. If all of the cables are star quad, they will be interchangeable between microphone and line-level applications.
There are a variety of different balanced microphone cables on the market. All microphone cables are designed to provide shielding against radio-frequency (RF) interference. The conductors are twisted and then wrapped with foil, a spiral of copper wires, or with a full copper braid. Of these, the braided shield is the most effective against electrostatic RF fields. However, none of these electrostatic shields are effective against magnetic interference. They are not magnetic shields!
Foil or copper shields cannot block the magnetic interference produced by devices that are relatively close to the cable. Magnetic fields easily flow through foil and copper shields causing interference.
Star-quad microphone cables are specially designed to provide immunity to magnetic fields. These microphone cables have 4 conductors arranged in a precise geometry that provides immunity to the magnetic fields which easily pass through the outer RF shield. Four conductors are arranged in a four pointed star configuration and the wires at opposite points of the star are connected together at each end of the cable. When the cables are wired in this manner, the + and - legs of the balanced connection each receive equal induced voltages from any magnetic field. This configuration balances the interference to the + and - legs of the balanced connection. The key to the success of star-quad cable is the fact that the magnetically-induced interference is exactly the same on the + and - legs of the balanced connection. The star-quad geometry of the cable keeps the interference signal identical on both legs no matter what direction the magnetic interference is coming from.
An equal signal on both legs of a balanced circuit is known as a common-mode signal. Balanced inputs are designed to reject common-mode signals, and this means that common-mode interference will be rejected. Balanced inputs use a transformer or a differential amplifier to reject common-mode signals while passing the normal-mode signals that carry the audio. Star-quad cables produce a nearly-perfect common-mode interference signal when exposed to a magnetic field. This common-mode interference is almost entirely rejected by the balanced input.
Normal (non star quad) microphone cables have two conductors surrounded by an electrostatic shield (just like the shield on a star-quad cable). Remember that the shield has no function from a magnetic standpoint, so we need to look at this cable as if it were just two closely-spaced wires. One wire carries the + leg and one wire carries the - leg.
If the source of the magnetic interference is closer to one wire than the other, the closer wire will receive a larger induced voltage. If the induced voltage does not match between the + and - legs, it will not be fully rejected at the balanced input. The induced voltage in the two legs will only match when the two wires are both an equal distance from the source of the magnetic interference.
In theory, it is possible to rotate a 2-wire cable to minimize the interference, but this is not a practical way of solving magnetic interference problems. Minimum and maximum sensitivity are only separated by a 90 degree rotation of the cable. We can't run around the stage or the studio moving and rotating cables every time we encounter magnetic interference. We need cables that provide immunity to magnetic fields coming from all directions.
We can change the spacing between the two wires in a traditional microphone cable and this will change the magnetic susceptibility. If we put more space between the wires, we make the problem worse. With a wide spacing, one wire will see a much larger induced voltage than the other. If the induced voltages don't match, they will not be rejected by the balanced input.
If we move the two wires in our microphone wire closer together, the difference in the distance to the source of magnetic interference is reduced and the magnetic susceptibility is improved. If we could put both wires in exactly the same location, both wires would see exactly the same induced voltage, and the interference would be completely rejected by our balanced circuit.
The solution to magnetic interference is to build a cable so that both legs of the balanced circuit have the same geometric center. If both wires could occupy the same physical space, they would experience identical induced voltages forming a common-mode signal that would be completely rejected by our balanced input. Since we can't put two wires in the same place, we have to get creative.
The solution is to replace each wire in the microphone cable with a pair of wires. This means that our microphone cable will need 4 wires (two for each leg). The geometric center of a pair of wires lies in the empty space between the two wires:
If we arrange our two pairs of wire in a star quad configuration, the geometric centers of both pairs will be in exactly the same location (the center of the cable):
We have now effectively placed two wires in one location. Both pairs of wires will now respond identically to a magnetic field from any direction. The magnetic interference will induce a common-mode voltage, but will not induce a normal-mode voltage. The common-mode interference signal will be rejected by the balanced input, and our system will be immune to magnetic interference. It is a clever trick, and it really works well!
The opposite points of the star must be shorted together at each end of the cable. Each shorted pair forms one leg of the balanced circuit. The cable Benchmark uses has a pair of white wires and a pair of blue wires. If you look carefully at the cross section of the cable you will see that the white wires are on opposite points of the star. Likewise, the blue wires are opposite each other. The white wires should be connected to the + leg (pin 2 of the XLR connectors), and the blue wires should be connected to the - leg (pin 3 of the XLR connectors). If any wire is incorrectly connected at either end, the cable will be very susceptible to magnetic interference. In most cases, an improperly wired star-quad cable will be worse than a standard two-wire cable.
Some star-quad cables have a different color code, which can easily lead to wiring errors. These cables use a different color for each of the four wires, and this makes it harder to determine which wires should be connected together. Some cables use the colors red, blue, white and green. The red and blue wires are on opposite points of the star and must be shorted together at each end of the cable. Likewise, the white and green wires are on opposite points of the star and must be shorted together at each end of the cable. These four-color cables can lead to wiring errors, so follow the cable manufacturer's wiring instructions carefully. Remember, a miswired star-quad cable will have more magnetic susceptibility than most 2-wire cables.
The star-quad geometry must be accurately maintained over the entire length of the cable. In the Canare L-4E6S cable, 5 filler strands keep the geometry well controlled. These fillers also provide a very significant improvement in the mechanical strength:
The accuracy of the star-quad geometry is a major factor determining the magnetic immunity of the balanced cable system. Some star-quad cables omit these important filler strands that maintain the geometry. For this reason, some star-quad cables are much better than others. Tests have shown that the magnetic immunity provided by Canare L-4E6S is typically 10 dB better than that provided by Mogami Neglex 2534 star quad cable.
Benchmark has selected Canare L-4E6S cable because of its strength, durability, flexibility, and superior immunity to magnetic interference.
Benchmark has recorded a lab demonstration that shows what happens when a standard two-wire cable is exposed to common sources of magnetic interference.
You will be able to hear the interference, see it on an oscilloscope, and view its spectrum on an FFT. A star-quad cable and a standard cable are exposed to the same sources of magnetic interference and the results are compared. This demonstration shows the dramatic difference between the two cables. The star-quad cable provided a 20 to 50 dB reduction in magnetic interference, keeping the interference below audible levels.
Note: The interference will play through the left channel while John narrates on the right channel.
This is really the wrong question to be asking. In most cases, star quad cables will not change the sound, they will only reduce interference. If no interference is present, any differences will only be a function of cable capacitance and the source impedance. Very long cables can introduce enough capacitance to attenuate high audio frequencies. In many cases, star-quad cables have some additional capacitance per foot. But, a loss in frequency response over a long cable run can be corrected with EQ. In contrast, interference cannot be removed.
Wikipedia: "Star Quad Cable"
How fast things can change!
It is March 23, 2020 and we are currently battling the worldwide COVID-19 pandemic.
This application note will be a departure from normal. I will make a few observations about the current situation and then look at the nuts and bolts of how we reconstructed our operations in less than 48 hours. Benchmark is 100% operational, but nothing looks the same as it did last week.
- John Siau
As an engineer I like to use "rules of thumb" to make quick estimates that help to explain the physical world around me.
These rules of thumb are easy-to-remember approximations that eliminate the need for complicated and needlessly precise calculations.
If you feel discombobulated by the complexities of high school physics, there is hope! I encourage you to step back and take a fresh approach.
If you learn a few simple rules of thumb, you can unravel mysteries of the physical world, amaze your friends, and yourself.
In this paper I will present 15 simple rules that I find useful when working with music and audio.
- John Siau
The Benchmark AHB2 power amplifier and HPA4 headphone amplifier both feature feed-forward error correction. This correction system is an important subset of the patented THX-AAA™ (Achromatic Audio Amplifier) technology. It is one of the systems that keeps these Benchmark amplifiers virtually distortion free when driving heavy loads. It is also the reason that these amplifiers can support 500 kHz bandwidths without risk of instability when driving reactive loads.
This paper explains the differences between feedback and feed-forward systems. As you read this paper, you will discover that you already understand the benefits of feed-forward correction because you use it instinctively to improve a feedback system commonly found in your automobile. If feed-forward correction can improve your driving experience, it may also improve your listening experience!
- John Siau