Noise, Hum and other … 2
LAB Audio Technology®
Noise, disturbance and other …2
Solutions: Shielding techniques.
To reduce capacitive and inductive couplings, the circuits and cables used to transmit signals are shielded.
The shield consists of a conductive metal surrounding the insulated cable. For reasons of flexibility and construction convenience, the shield is made of the same material as the conductor and braided with a mesh.
It should be specified that in the case of a cable with a single conductor plus shielding (coaxial cable), the shield (i.e. the outer conductor) is not a real shield, as it is the shield itself that acts as a conductor for the return current. In this case the cable geometry minimizes the emitted and / or received fields.
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The careful study of the shielding of the circuits of electric guitar or electric bass can avoid disturbances of various origins.
A fundamental aspect in the transmission of signals is the ground reference. An incorrect ground connection of a circuit can often lead to inductive couplings, called “ground loops“. A ground loop occurs due to closed ground connection paths that allow a current to flow, forming areas in which variable magnetic fields are formed that induce a current within the created ground mesh.
For example, a 4 m long coaxial cable, connected to two devices, which in turn are connected to two different power sockets located in the same room and equipped with earth connections, easily generates a loop with an area of a few square meters.
Noise coupled in this large loop can then be induced on the conductor carrying the signal through a very strong inductive coupling.This highlights the fact that creating ground loops in an uncontrolled home environment is very easy.
The drawing below can better illustrate the idea.
The study of mass connections within electronics is a fundamental aspect of the correct design of any electronic equipment.
The care of the connection between electronics (power supply, signal transfer, etc.) is fundamental in order to minimize inductive couplings and radiation disturbances, and this requires a careful study of materials and geometries, to obtain maximum protection against disturbances of various kinds.
We have seen what the various types of disorder and the various possible causes can be, in this way we can define the field of study.
In our case we deal with the transmission of analog audio signals, low AC voltage power supplies (220V AC) and digital transmissions for the various protocols that are necessary for transmissions for use in the audio field.
We therefore neglect the aspects related to the transmission of signals that exceed 5 Mhz, taking into account only the analog and digital signals included in the range between 1Hz and 5 Mhz.
Let’s start with the simplest case of the 220V / 50Hz power supply of the electronics. It is necessary that there is a certain ground reference and therefore a well-made “grounding” with an adequate earth rod and well connected to the system.
When using multiple sockets, it is highly recommended to have star connections and not “in series” to avoid mutual interference between the connected electronics, as explained in the section dedicated to conducted disturbances.
An excellent multiple socket with phase, neutral and earth star connection and with wiring of adequate section, can solve many problems that often afflict the stages of many musicians and audio services.
In the case of signal transmission with a shielded cable, usually the shield connection must be connected to the reference potential (which is the ground) as in the figure 3 above.
To avoid any ground loops, it is not necessary to connect both ends of the ground shield, because there could be a potential difference between the different masses (ground) which would lead to inducing a current in the shield (see ground loop diagram). Practice confirms that a screen is effective only if a current flows in it and in many cases the solution is to ground the screen with a capacitor.
For low frequencies (in our case no more than 10 Mhz), an excellent solution is to use a pair of twisted or twisted cables with shield connected to ground on one side only, usually on the generator side.
Another common example is the USB 2.0 standard, which makes it possible to reach a speed of 480 Mbps over distances of 5 m with the use of shielded twisted cables. This standard defines that the ground connection of the screen must be performed by the host, then by the computer, while none of the peripherals must perform this task.
But in the audio field, the generator is not normally connected to ground and therefore has a reference defined as “floating” (for example guitar pick-ups or a turntable phono cartridge), and in these cases it is necessary to connect the screen to ground (ground) of the target electronics with ground connection and reference to ground, to have a reliable connection of the reference potential.