Eliminating interference in stereo or audio equipment | ACMA

Eliminating interference in stereo or audio equipment

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Eliminating amateur and CB radio transmission pick-up in your stereo or audio equipment

Disturbance can occur when audio equipment fails to reject strong local radio signals. However, there are steps you can take to eliminate the interference:

  • Consult the equipment manufacturer, outline your problem to them, and ask for their assistance.
  • Contact operator of the CB transmitter. If the radio operator is using approved equipment and is operating the equipment correctly you will most likely get their help.

A simple test

Ask the operator to transmit a test message of about five to 10 seconds duration, at intervals of 15 to 20 seconds. The interference to your set should come on and off in synchronisation with the test sequence of the transmissions.

If you and the CB operator can confirm the station is the source of the problem, the solution is likely to be a technical one.

If the station is set-up and operated correctly it is up to you to find a solution - not the ACMA or the radio station operator.

Tracking it down

Find where the interference is being picked up within the system.

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Figure 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.

This diagram shows some of the many possible points in the hi-fi 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 pickup, 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 a point where it can cause other problems.

There are three basic techniques for curing RFI problems:

  1. Earthing

  2. Shielding

  3. Filtering.

A combination of these techniques may be necessary.

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.

By this 'process of elimination' method you should be able to determine which item of equipment is picking up the (unwanted) radio signal. Then you can select the technique, or combination of techniques, needed to block the interference.

Earthing

It is important that all components in a system (turn-table, tuner, cassette deck, compact disc player, or anything else) be properly earthed. 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 co-ax, 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.

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Figure 2 a

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

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

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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 earths (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)

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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).

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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.

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Figure 5—Fitting a ferrite bead inside an RCA phono 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.

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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 radio frequency interference. It really is a matter of 'trial and error' to find a solution for your particular interference.

Keeping the connectors in your system clean also helps. Iso-propyl Alcohol is a good cleaner. It evaporates completely, leaving no residue behind. Take care to avoid plastic components.

Telephones

Interference to telephones results from the inability of the telephone instrument to reject Electromagnetic Interference such as Radio Frequency Interference.

It is illegal to tamper with telephones. Contact either the supplier of the telephone or the dealer from whom it was bought.

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.



Last updated: 27 February 2017