Connection of audio systems (II)

We publish in this edition the second part of this interesting article in which the author highlights the leading role played by connections in an audio and video solution.

By: Alejandro Bidondo*

A cable resonates (becomes an antenna) for frequencies whose quarter wavelength is equal to the physical length of it, that is, the 50m will resonate, become "open circuit", at a frequency of 900KHz.

Referring to cables for connections between systems, with one or two live, mesh and insulating materials, the electrical equivalents are more complex (parameters concentrated or distributed depending on the lengths of the same) and will also present magnitudes of their impedances variable with the frequency.

Note: A term whose concept should not be overlooked is impedance: "it is the apparent alternating current resistance of a circuit containing capacitors and/or inductors, in addition to pure resistive components."

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The goal of any grounding is to electrically connect conductive objects (e.g. equipment cabinets), to minimize voltage (voltage) differences between them. A good definition would read: "Earth is only a return path for electric current." In addition, it is worth mentioning that the electric current always returns to the source, through projected or accidental physical paths (the latter in reference to electrical contacts or "leaks" not foreseen or not taken into account in the projection stage of the system). Figure 2 shows a simplified scheme of the typical electrical connection of the household energy of 3 conductors.

One of the most common cases of failure in electrical and electronic equipment is the loss of insulation, so the electric current will most likely circulate in significant quantities into the metal cabinet. If the electrical/electronic equipment is connected to the home power piping network by means of a connector commonly called "three-legged" or three-contact, one of them is the line signal, the other is the neutral and the third is the earth (or "ground"). Some examples of these can be seen in Figure 3, courtesy of Brocade (http://www.foundrynet.com).

The ground contact must be electrically connected to the neutral on the main board of the building in order for it to function properly.

It is worth mentioning that it is not advisable to deliberately disconnect the third electrical contact corresponding to the earth from any equipment that has it, since it puts at risk of electrocution the users of the equipment to which it corresponds.

In the case of installations without electrical connection of the cabinets to earth (eg: 3rd non-existent contact), it is advisable to use a differential switch at the entrance of the home circuit. It permanently compares the current of the living or line and that of the neutral. If the difference between them exceeds a certain magnitude, the differential switch opens the circuit as a safety since it would be deriving through unforeseen paths, such as through a person, electrocuting him.

Note: A current of approximately 1mA passing through a human being will produce only a tingle. One of about 10mA will cause involuntary muscle contractions. One of more than about 50mA can lead to death.

If the case of a short circuit due to the loss of electrical insulation of some component is presented, the equivalent circuit would be that of Figure 4, in which the grounding is not involved.

Parasitic capacities and noise-generating circuits

In all electronic or electrical equipment there are parasitic capacities (so called because they have not been designed or intended) between the power line and the cabinet or chassis. These exist between the windings of the primary and secondary transformer (CP) of the power supply and are never shown in the circuit diagrams. In addition, other parasitic capabilities appear if the equipment under analysis has RF (CF) filters. These parasitic capabilities cause a leakage current to circulate between the power line, the mass conductors of the equipment under analysis, and its cabinet.

Knowing this, the origin of the noise type "hum" and "buzz" are not due to bad ground or mass systems but to the parasitic capacities (CP and CF) of any power supply.

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According to the US safety regulation UL (Underwriters Laboratories), for equipment without ground connector a maximum of 0.75mA of leakage current from the power sources to the cabinets is allowed. For equipment with ground connector, this current is limited to a maximum of 5mA.

It is not a minor fact to understand the power distribution line (220VAC or 110VAC) as a poorly loaded transmission line (this means that the impedance of the transmission line is not equal to the load impedance of the same), which generates standing waves and reflections between the load and output impedances of the power generator. That is, the power line not only carries the pure signal of 50 or 60Hz but also the infinity of transient events that are added on it, as can be seen in figures 6 and 7.

Figure 7 shows the spectrum of a leakage current through a 3nF capacitor powered by a 110VAC, 60HZ system (Courtesy of Jensen Transformers and Bill Whitlock, http://www.jensen-transformers.com).

Briefly exposed the reasons that introduce noise in the interconnection of systems, it would only be necessary to know the methodologies to detect the established circuits to eliminate them efficiently.