As hi-fi equipment becomes more sophisticated and complex, the circuits seem to be getting more susceptible to interference from outside sources. If a radio transmitter is operated near a stereo system, the radio signals can be intercepted by the hi-fi wiring and peripherals, and delivered to the amplifier.

Unshielded interconnecting cables act as radio receiving antennas. This problem is exacerbated if any of the connecting cables happen to resonate at the operating frequency of the radio transmitter.

In the amplifier, the RF currents are rectified, causing changes in the audio gain. Sometimes the signal data can be heard in the speakers or headset.

This is known as electromagnetic interference (EMI).

In most cases when EMI takes place in a hi-fi setup, the fault exists in the stereo system design, not in the radio transmitter. The transmitter system is doing its job: generating and radiating electromagnetic signals.

There are several steps that can be taken when installing a stereo hi-fi system to minimize the likelihood that EMI will occur. These precautions should be followed:

• Connect the stereo amplifier chassis to a good electrical ground.
• Use shielded interconnecting cables as much as possible.
• Use shielded (coaxial) speaker cables.
• Keep all cables as short as possible.

If you have an amateur or citizens’ band (CB) radio station in your house and it causes EMI to your hi-fi system, two more steps might be necessary:
• Locate the radio transmitting antenna as far from the hi-fi equipment as possible.
• Use the lowest possible transmitter output power that will ensure reliable communications.

If EMI problems continue even after all the above steps have been taken, a professional engineer might be able to help. If the radio transmitter is not always in use (a CB or amateur station, for example), efforts should be made to avoid using both the radio and the hi-fi at the same time.

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