Audio and radio interference

Eliminating radio transmissions interference from your stereo or audio equipment!

Audio interference caused by nearby radio transmitters, such as used by Citizens Band (CB) operators can be annoying and frustrating. Ever since the introduction of CB in Australia and the meteoric rise in the number of sets in use there has been a massive increase in the number of interference cases reported to the Australian Communications and Media Authority.

Consequently it is hoped that the information presented in this brochure will assist in dispelling some of the misconceptions surrounding interference causes and provide an understanding of the possible remedies that may be employed.

WARNING – Some of the techniques referred to on this page involve the removal of the protective covers from the equipment that may contain high voltages and thus expose the operator to danger. The assistance of a qualified technician should be sought if the operator is not himself suitably qualified to safely install the modifications. Equipment warranty may also be affected if the required changes are made by untrained personnel.

How does CB or radio interference occur?

The puzzling thing about interference for most people is “how can a radio transmitter be heard by a tape recorder or an amplifier when audio frequencies are so far removed from radio frequencies?”

To state it simply some portion of the audio system will have the ability to detect strong signals and reproduce the intelligence (ie: the operator’s voice, or whatever) carried by the transmitter’s signal regardless of the frequency on which the transmitter operates. This is not a fault in the basic design or manufacture of the audio equipment – but rather a lack of its adequate ability to reject the strong local radio signals, nor is it a fault in the transmitter causing the interference. So how is it that if something is obviously wrong nobody is at fault?

Assuming that everything else is equal that both the audio and radio installations are properly set up and that the radio transmitter is operating within the power limits set down in the licence issued for it the interference is basically an unavoidable shortcoming of the transistors and integrated circuits used in the audio amplifiers and other hi-fi equipment.

Transistors and integrated circuits used in modern stereo equipment may respond to signals as high as 200 MHz and beyond. This simply happens to be one of the basic characteristics of the semi-conductor devices used.

Manufacturers can reduce or even eliminate the ability of stereo equipment to pick up radio frequency signals but many avoid doing so largely for cost reasons and also because the extra components involved do not add to the performance of the hi-fi equipment. (Also, there is no legislation requiring the inclusion of such filtering in force so, why include them?!)

Preliminary steps

The first step is to consult the equipment manufacturer, or his agent (or distributor). You may have to do this through the store where you purchased the equipment. If the store has a service department, they may be able to offer you advice. There is no harm in outlining your problem to them, and asking for their assistance.

If you know or can find the location of the transmitter that appears to be interfering with your equipment, approach the operator and seek his/her co-operation. A courteous approach is much more likely to result in a quick and amicable solution to the problem, rather than a belligerent one.

A good starting point is to simply say that you are experiencing some radio interference on your hi-fi which may be coming from his/her station, and ask would they mind conducting a few tests with you, to confirm it “yay-or-nay” so that the problem may be dealt with.

If the radio operator is using approved equipment and is operating the equipment correctly you will most likely enlist their co-operation even sympathy immediately.

In the case of a properly set-up and operated station the onus is on YOU to affect a solution NOT the Authority or the radio station operator.

Having located the suspected source of interference the next step is to run a very simple test to ascertain whether or not your suspicions were correct they may not be.

Ask the operator to transmit a test message of about five to ten seconds duration, at intervals of (say) fifteen to twenty seconds. The interference to your set should come on and off in synchronisation with the test sequence of the transmissions.

Do this carefully, several times. Then, have the operator present with you while somebody else transmits the test message. If his station is the source of interference he can confirm it for himself. Now that you have established the source of interference the cure is going to require a technical solution in the majority of cases. If you are unable to do this yourself, then arrange for it to be done by a technical friend you trust or by the service department of the store where you purchased the equipment. If this is not possible then most TV and radio repair shops should be able to assist you.

If you can’t establish the source of the interference, and this is sometimes the case (mobile transceivers can interfere with your hi-fi system when transmitting in your immediate vicinity. This is transitory but may be frequent if you live near a main thoroughfare. You are going to have to seek some sort of cure for the effect without really knowing the source.

In a great many cases interference can be tracked down and cured without expending large amounts of time or money.

A number of simple remedies can often be applied to good effect, without getting inside “the works” of your equipment making the services of a qualified technician unnecessary. No special tools or electronic expertise is really required.

Tracking it down

Before proceeding with a cure it is most wise to first find where the interference is being picked up within the system.

Figure 1Figure 1 – A typical hi-fi unit showing the many possible places that interference can enter. Note that every single input and output (including the power cords) is susceptible. RF interference can also be picked up inside any of the units.

A block diagram of a typical hi-fi installation is illustrated in Figure 1. This shows some of the many possible points in the system where interference can enter, or be “picked up”. The interference may be picked up by only one or two components in the system such as the tape deck, turn-table pick-up, and/or the amplifier, rather than at all the points indicated.

The aim of any technique used to cure Radio Frequency Interference (RFI) in hi-fi equipment is to prevent the unwanted radio frequency signal from reaching the point where it is detected and demodulated or reaching a point where it can cause other problems.

There are three basic techniques for curing RFI problems. These methods are dealt with in the following sections and are:

  • Earthing
  • Shielding
  • Filtering

A combination of these techniques may be necessary in severe or stubborn cases.

Starting with the amplifier the first step is to turn the volume control down and note the effect it has on the interference.

If adjusting the volume control has no effect on the interference level the radio signal is likely to be entering through one of the output stages via the loudspeaker leads or the power leads.

If adjusting the volume control alters the interference level the radio signal is likely to be entering through one of the program sources via the connecting signal leads or the input stage/s. Turn the program selector switch to each source to determine if the interference can be narrowed down to one input (say the PHONO or the TAPE etc…) or if it is common to all sources.

The next step is to isolate each of the program sources or inputs (one at a time). Test each item in turn by switching off the power to it and removing the plug from its socket. Note if the interference ceases. If not re-connect the power and turn it back on. Then disconnect the input lead from the amplifier. Has the interference disappeared or not?

By this “process of elimination” method you should be able to determine which item of equipment is picking up the (unwanted) radio signal. Having found where the signal is getting into the audio equipment you can select the technique, or combination of techniques, required to keep the RFI out of your system.

Earthing

It is important that ALL components in a system (turn-table, tuner, cassette deck, compact disc player, or anything else) be properly earthed. Earth loops should be avoided at all costs, and remedied where found. Any hum will probably be reduced in the process also! You win both ways. The correct method for earthing of all equipment in a hi-fi system is illustrated in Figure 2.

The earth connection between the external equipment and the amplifier should be via a heavy flexible lead or copper braid. This applies equally well to the pre-amp and the programme source equipment. The shield braid stripped from coax, or shielded cable, is quite useful for this purpose.

The chassis or cabinet of any piece of equipment acts as a shield, but it won’t be very effective if it’s not earthed properly.

Don’t link all the equipment earth connections to one another. Connect each item of equipment individually to the common earth on the main amplifier. A solid earth connected to a (metal) water pipe, or other earth, should be made from this point only. The mains earth connection can be used and may be sufficient, but only at the main amplifier having the common equipment earth connection. The mains earth connection to each item of external equipment should be disconnected to prevent earth loops.

Figure 2a

Figure 2 a

  • Incorrect method of connecting equipment to an amplifier. Note the potential for the presence of earth loops that may be causes of interference.
  • Incorrect method of connecting equipment to an amplifier. Note the potential for the presence of earth loops that may be causes of interference.

Figure 2b

Figure 2 b – A better method of connecting equipment to an amplifier. Only the main amplifier with the common equipment earth terminal should be externally earthed.

Cassette tape decks, for instance, are often prone to interference because, in many cases, there is no direct connection between the electronic circuit board earth terminal and the metal chassis of the tape deck. A 0.001µF (100V ceramic) capacitor fitted between the two, in many instances removes the interference. (This provides a short-circuit to earth for RF only.) If a capacitor is fitted keep the leads as short as possible.

Shielding

Linking all of the equipment together through the signal shields is another source of potential to pick-up unwanted radio signals. Disconnect the signal earth’s (shields) at the programme source while leaving the shields of each item of equipment connected at the input to the main amplifier. This is similar in principle to the technique used to eliminate mains-borne interference, except that it is at the signal level now. It has an added bonus in that it also eliminates another source of possible earth loops. (See section on EARTHING)

Figure 3

Figure 3 – Disconnecting the signal shield at the programme source. This may be necessary for all inputs.

One method of substantially reducing or even eliminating the interference picked up by the loudspeaker leads is to use shielded speaker cable in which the outer shield is connected to the metal chassis of the amplifier only (in much the same way that the incoming signal shield was connected only at the amplifier).

Figure 4

Figure 4 – Method of connecting shielded loud speaker cables to the amplifier. The shield is not connected to the loudspeakers.

CAUTION – DO NOT use low voltage signal or microphone cable for this purpose! It probably won’t last long because of the much higher voltages and currents it is expected to handle and it may cause serious damage to your amplifier if it melts or breaks down.

Filtering

Ferrite beads can be installed in a number of different places. Ideally, they should be inside the amplifier mounted on one of the leads of the input transistor/s. Obviously this would involve the services of a technician. The next best place to install them is in the connector that plugs into the amplifier.

Figure 5

Figure 5 – Fitting a ferrite bead inside an RCA phone plug for FFI suppression of input or output leads.

RF “chokes” (or filters) can be formed on the leads that pick up interference in a hi-fi system. Various forms of ferrite material can be used to increase the inductance of a lead at the radio frequencies thus effectively “choking off” (or filtering) the offending RF interference without having any effect on the audio signal at all.

Figure 6

Figure 6 – Speaker cables wound on a ferrite rod (left) and on a toroidal former (right).

This form of suppression is particularly effective in persistent cases. For removing interference picked up on the loudspeaker leads winding the leads around a suitably sized ferrite former can be quite effective. Either a ferrite rod or a toroid can be pressed into service as illustrated in Figure 6. Ferrite rods, such as used for transistor radio antenna coils are inexpensive and easily obtainable. A toroid is more effective in really bad cases, as it has no external magnetic field. The ferrite rod or toroid should be installed as close as possible to the amplifier’s output terminals.

In removing interference coming in through the mains power lead the easiest method is to purchase an in-line mains filter unit. Any other method involves the assistance of a technician to fit the filter components inside the amplifier. This requires fitting of a 0.022µF high voltage ceramic (or film type) by-pass capacitors from “active” to “earth” and “neutral” to “earth” at a point right where the mains cord is terminated or at the transformer input winding. The leads of the capacitors should be as short as possible. Any capacitors used for mains suppression should have a minimum rating of 1,000 Vac (preferably 1,500 Vac).

The final step

There is no simple solution to the age-old problem of radio frequency interference. It really is a matter of “trial and error” to find a suitable remedy for your particular interference.

Certainly it wouldn’t hurt to occasionally clean all the connectors in your system. Judicious use of Methylated Spirits would clean the contacts but it must be wiped thoroughly afterwards as it leaves a residue which would pick up airborne dust particles and in the long run may make things worse. A far better cleaner is Iso-propyl Alcohol because it evaporates completely leaving no residue at all. (This is available from chemists.)

A word of warning though these cleaners are solvents which might attack the plastics used as insulating material in the wires or connectors — you could end up with a sticky mess of dissolved plastic and loose wires!

By keeping your audio equipment clean you remove one potential source of interference plus there is the added bonus of protecting your expensive investment.

Telephones

As with audio equipment, interference to telephones results from the inability of the telephone instrument to reject Electromagnetic Interference such as Radio Frequency Interference.

Caution: It is illegal to tamper with telephones

Contact either the supplier of the telephone or the dealer from whom it was bought.

 

Last update 16/05/2013

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