Noise isolation of the car: identification and suppression of noise sources

Encyclopedia of radio electronics and electrical engineering / Audio equipment

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Sometimes, when work on the installation of a car audio system is completed and its “sound portrait” can already be heard, colors and tones are suddenly found in it, which introduce obvious disharmony and undesirable shades into its perception: these are the notorious “system” noises, which are especially noticeable to a trained ear discerning connoisseur of "audio art". Noises kill pure sound and, like layers and "records", spoil the perfect canvas of a great master. To give the sound natural colors and liveliness, you must first determine the nature and sources of noise, which will help to bring them under control.

It is useful to think both "before" and "after"

Installers are familiar with the situation when system noises appear suddenly, like a disaster that was not expected: after twenty "normal" systems, the twenty-first suddenly starts making noise, confusing its creators, like a cranky child demanding to sit next to three, five or more hours until the pain is found and removed.

If we consider the audio system as a means of transmitting reliable information, then we can compare system noise with introduced, false information that interferes with the full perception of the useful signal by the listener: these can be repeated clicks, hiss, squeal and other interference penetrating into the acoustic link of the audio system. In fact, all components of the system bring a certain amount of noise information into the sound reproduction path, and their total level is usually called the noise threshold of the system. If noise occurs in the initial links of the circuit, then the farther you can "move" the useful signal from the noise threshold in the readout and preamplifier devices, the more efficiently the amplifiers will work and the acoustics will sound cleaner. However, the insidiousness of noise lies in the fact that they can penetrate into the system through the link of final amplifiers, and even after it, for example, through the acoustics band pass filter.

Starting to work on a complex audio system, experienced installers think about eliminating noise even at the stage of circuit selection and planning: the best cure for noise is to take care of its prevention through the correct choice of circuit and installation technology. First of all, installers pay attention to interconnects: if this is not fiber optics, then the vulnerability of any electrical conductor to automotive noise sources is difficult to predict (see "Master 12 Volt" N 1/98, pp. 30-38), although you can take known preventive measures. The generally accepted preventive technology is to lay power and signal conductors remotely from each other, for example, along different edges of the car interior floor, but experienced installers manage to avoid noise even when laying conductors of both types together due to the correct selection of system components. According to one of them, up to 90 percent of the noise originates in the installed equipment itself. The imperative of profitability puts another requirement before the master: to flexibly choose technology and means. For example, if a power amplifier is installed in the system that only drives a subwoofer, then the installer may find it justifiable to run the power and signal wiring side by side on the same side of the cabin. When it comes to switching an amplifier that loads broadband acoustics and a high-frequency link, then the preventive laying of power and signal cable lines on different sides is certainly more reliable.

With what they hunt noises

To eliminate noises, it is desirable to understand their nature and find their sources. Today there are technical means, such as, for example, electromagnetic radiation detectors (the so-called audio sniffers), somewhat reminiscent of a medical stethoscope: they can be used to probe various parts of the car with a probe, the signal from which is converted and fed to the installer's headphones, talking about the presence of strong fields. A cheaper home-made version of such a device - a portable audio player with an external speaker or microphone - imitates such a probe to some extent, although its low sensitivity must be compensated by the high qualification of the master.

Capable craftsmen use tools such as frequency counters, ammeters, or low-frequency oscilloscopes to localize noise. However, these means are most often resorted to when the noise has already "leaked" into the acoustics. The need for preventive measures forces the master to think ahead about ways to protect the system from the threat of induced and other noise. Rule No. 1 consists in reliable shielding of interconnections: you should know not only how to choose the right power and signal conductors with good noise immunity, but also how to ensure shielding when laying lines at joints and connecting to intermediate links, remembering that their condition can deteriorate during operation due to vibration, thermal, atmospheric and other influences.

Shielding helps to protect against inductive interference propagating in the air, but wiring can also pick up electromagnetic interference that occurs as a result of the interference of signals transmitted through conductors. They can also manifest themselves in the form of slightly noticeable clicks, itching or squealing, and the likelihood of their occurrence in cars of recent years of production, stuffed with all sorts of standard electronics, is no less than in older ones. Often in new models, a wide variety of conductors stretch from the trunk compartment under the hood in the form of a thick bundle that carries a lot of signals of various properties. This makes it difficult to route signal cables for the audio system.

The master is required to exclude possible intersections with such standard wiring in order to reduce the likelihood of interference. As already noted, for this they try to conduct wiring on different sides of the passenger compartment floor, but sometimes, in especially "complex" cars, it is necessary to pull both types of conductors along its center. When the choice is limited, experts recommend maintaining a distance between both lines of at least 25-30 cm, and if the signal conductor cannot but cross the power wire, then it should be done at an angle of 90, which will minimize the induction induced around the shield (when laying in parallel the vulnerability of the signal conductor to noise of this kind is maximized).

What gives the design of signal conductors

To increase the noise immunity of shielded conductors, in particular when laying them in pairs, it is recommended to use a twisted pair, in which both cables are rigidly fastened, for example, with insulating tape, in order to better protect against stray electromagnetic fields. Not so long ago, designers of signal conductors began to use twisted pair (signal and ground wires are twisted in them and enclosed in an additional external shield) as a means of shielding in the interconnect cables themselves. This solution is inferior to symmetrical ("balanced") pair conductors in terms of efficiency of inductive interference suppression, but it is much simpler and cheaper.

As for the balanced pair conductors themselves, their use is primarily justified when the system units (for example, the head unit, processor and power amplifier) ​​are already prepared for their use. As a version of such a conductor, high-quality balanced microphone cables are sometimes used, which, with comparable characteristics, can have a more attractive price. It should be noted here that signal transmission over balanced lines cannot eliminate all the noise that occurs in a car audio system. If noise arose before a balanced pair (for example, in the head unit), this noise component will persist even after the complex signal is transmitted over balanced lines (see "12 Volt Master" N 3/97, pp. 26-28).

Formation of parasitic circuits

Even if the installer has used the highest quality materials and proven cable and wire laying techniques, there is still the possibility of noise in the system, since there is another danger - the formation of a parasitic loop. This phenomenon occurs mainly in audio systems with two (or more) ground points per chassis, in which the impedances are unequal: in this case, the electric current flows towards the ground point with less electrical resistance, regardless of the length of this section of the circuit.

As a rule, an audio system with power amplifiers installed in the trunk compartment has two ground points: one of them is near the head unit, the other is near the amplifier. As experts explain, all modern autoelectronic products powered by the car's on-board network are equipped with one or another built-in filter, the purpose of which is to "collect" noise resulting from pickups and transmitted through power wires or generated in the device itself, and "reset" them along the earth conductor to the body. So what happens when the head unit ground is better than the amplifier ground? Noise from the headunit can be carried along the shield of the signal interconnect to the internal signal conductor and appear as general system noise in the signal amplification link. Although parasitic loops have been known to installers for a long time, they can still occur when this important part of audio system installation technology is neglected.

Automotive power amplifier manufacturers have taken different paths to combat spurious noise, with a primary focus on reducing noise at the preamplifier input. For example, Blaupunkt amplifiers are known for their HUSA noise cancellation system, Philips amplifiers use a CMR (common-mode rejection) circuit, and LA Sound products use an active noise cancellation circuit.

Some installers recommend using a single chassis ground point, such as near a power amp in the trunk, and then running an additional ground wire to the head unit. In this case, even over the years, the "balance" of the system's grounding reliability will not change in any way. The main rule is to choose a thick metal part of the housing, drill a hole and use a bolt and nut with a lock washer to press the conductor to the cleaned metal surface. Even better is to use a stock hole or mounting bolt, some installers use one of the rear seat belt mounting screws for this purpose.

A few additions to this basic rule: use conductors of sufficiently large gauge to provide low resistance in the ground circuit. The level of system noise will also increase when one of the elements of the car's battery "sits down" and the resistance on it increases. The size of the power cables used is also important: if the audio system, for example, selects 100 A, and the power conductor - only 10 gauge, then at full volume, the high resistance of the conductor will degrade the electrical parameters of the system and can raise the noise level up to component damage.

Little champions in the fight against noise

Some products on the market today can somehow affect the system noise in the car. However, experienced experts say that these "drugs" should be used very carefully. For example, noise filters installed in the circuit of a power conductor (in particular, feeding a power amplifier) ​​are considered by many as "temporary dressings" that do not remove the problem, but only hide their source. As far as head units are concerned, they usually use power supplies that do not provide adequate noise filtering, so it is sometimes useful to supplement their low-amp supply circuit with a small filter.

Special parasitic coupling insulators have now appeared on the market to deal with the mentioned problem of increasing resistance at ground points to the case, although not all experts recommend their use. The reason is that such an isolator is usually a small block with a pair of transformers inside and external RCA connectors (inputs and outputs).

When a music signal is applied, transformers isolate the bass inputs from the bass outputs, however, by passing through a transformer (usually a wire core type) the AC signal loses the actual electrical connection between the input and output, although it breaks the physical connection between the two ground points to the chassis . The loss of electrical contact in the transformer leads to large losses in the signal broadband: firstly, due to capacitive coupling, the low-frequency component of the useful signal is cut off, and secondly, due to the inductance, its high frequencies are "filtered out".

To get away from the shortcomings inherent in insulators of this kind, auto electronics manufacturers have recently begun to offer motorists the so-called. linear drivers, which are compact signal voltage amplifiers at the linear output (for example, a head unit or a CD changer). Increasing the voltage of the signal transmitted through the interblock conductor allows you to reduce the noise threshold to a level at which noise is absolutely inaudible even at high volumes. In addition, the linear driver makes it possible to decouple signal circuits between devices installed on the dashboard (car radio, etc.) and mounted in the trunk (power amplifier, etc.).

Increasing the input voltage in the signal circuit also allows the amplifier to be operated at lower output power levels and to set the gain control at the amplifier input to a position that provides low sensitivity to externally generated noise. Recently, manufacturers of auto electronics have begun to produce head units with a voltage at their linear output of about 4-6 volts, which allows you to achieve the same effect without additional devices.

System noise has many faces

System noise can be caused by more than poor wiring or poor chassis grounding. Experts are aware of other reasons. For example, in complex car audio systems one cannot do without band-pass filters - crossovers. Inside the passive crossover there are inductors that, in addition to their main function - cutting off frequencies of a certain range, can act as antennas, picking up electromagnetic noise from an electric generator or high-current circuits. When installing component and other acoustics, crossovers often have to be placed within the dashboard, where the likelihood of fields is high.

A modern car may offer another surprise: one user found a noticeable acoustic noise with a frequency of about 1 kHz in the speakers of the new audio system of his BMW, which appeared not only while listening to music, but even when the system was turned off and the engine was running. The reason for the pickups turned out to be a reckless decision by the installer to place passive crossovers in the rear seat area, behind which the battery is located in many BMW models. The power cables coming out of it (and the proximity of the high-current wire supplying the rear window heater) contribute to the formation of sufficiently strong electromagnetic fields, which are well picked up by the coils of the acoustic bandpass filters, which serve as "antennas" for receiving radiation of this kind.

There are also cases when system noise "manifests" some time after the delivery of the system to the customer. To get to the bottom of the cause, the craftsmen first of all "back up", trying to trace the history of the car's operation after it left the installation workshop. It turned out, for example, that the owner changed the distributor cap and removed the special grease on its central shaft. The result is an increase in electrical noise that has entered the audio system through the standard circuit.

If the operating history shows that the generator or battery has recently been replaced, then first of all it is worth making sure that they are properly grounded to the case and that the gauge of the ground wire is sufficient. In some frame construction vehicles, grounding can be performed on the body, which is not always connected to the frame into a single structure with a continuous weld, so when debugging a powerful audio system, the master must pay special attention to choosing the right grounding location.

Corroded battery terminals are a "childish" problem for the installer, however, neglect by the car owner, who did not replace the battery in time or does not follow it, can turn into such an unpleasant situation when after a month or two the system starts to make noise, and the customer tries to accuse the installer of dishonesty or lack of qualifications. If qualifications are really lacking, then the master can make another mistake fraught with noise: installing the power amplifier directly on the metal surface of the case without an insulating gasket.

So, regardless of the already accumulated experience in dealing with noise in car audio systems and pre-designed preventive measures, the installer is still waiting for "noise surprises", because every new system (and every new car) carries something individual. In addition, many traditional rules for dealing with this insidious enemy of pure sound have their exceptions, since often the master is saved by one pattern: the stronger the noise in the car audio system, the easier it is to detect its source.

Publication: cxem.net

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