The Hidden Reason Some Audio Devices Need Isolation

Consider a scenario where you set up a microphone for use in a small conference room. It appears that everything has been set in place. However, as soon as a person begins to speak, there is a low buzzing sound coming through. Even after checking the cables and adjusting the settings, the hum will not go away.

Such problems may seem challenging since it may not be clear what the problem is. The cause of such difficulties may be as a result of grounding issues or the interaction of the components. At other times, there is the need for a circuit design that allows for the transmission of the audio signal without establishing an undesired connection between two parts of the circuit.
This is the main reason why an audio transformer is employed in some electronic appliances. The reason is that the audio transformer transmits the audio signal via magnetic coupling and not via electrical coupling between the windings. This is how isolation of the signals can be achieved and the effects of noise from specific paths of the current be reduced. However, not all electronic devices require an audio transformer.

Microphone Preamplifiers and Recording Equipment

It might be helpful to use an example from a recording studio to learn about the importance of signal isolation. It would be a situation where the singer stands in front of the microphone, while the recording apparatus is located several metres away. Additionally, other equipment such as the monitors and computer could be connected to the system.
Microphones are designed to generate fairly weak signals that require careful treatment. Any additional noise which is generated along the way can become apparent after amplification.

Audio transformer could be included in the microphone input section to provide isolation or balance in the signal connection. Transformer characteristics can play a role in defining impedance matching and frequency response. Both properties can become significant when a recording system requires preserving the fidelity of a voice or musical instrument.
However, transformers do not have to be present in all microphone preamplifiers. Modern electronics can achieve good performance without using transformer coupling.

Professional Audio Mixers and Sound Consoles

Enter a concert hall before a show begins and you will notice a mixing console accompanied by various microphones, amplifiers, computers, and other pieces of audio equipment. All of these have a specific function but must work in tandem without creating any additional unwanted noises.

If there is a potential difference between any connected devices, it may lead to a hum. Audio transformers can help isolate a connection within the circuit if it is designed to do so. It can be especially useful if the audio is passing between equipment that is getting power from separate sources.

For instance, a transformer isolated connection can help in preventing some particular ground-loop currents from moving through the audio cable. In this way, the signal can pass through a magnetic coupling but will be completely electrically separated.
Nevertheless, a transformer is not a cure-all solution to the mixing board noise problem. Improper cable shielding, faulty connection, and equipment issues can also create troubles. A properly designed mixing console utilizes a proper balance of these components.

Audio Amplifiers and Home Stereo Systems

A home stereo system might look relatively simple from the outside: you hook up your source, set the volume and enjoy your music. Inside your system there are several stages that take charge of reception and signal processing prior to it reaching the speaker stage.

There are amplifier systems where a transformer is used to pass the signal between the stages or to achieve isolation for a certain connection point. There are also amplifiers that use transformers in their outputs to achieve electrical loading or isolation. This technique is less common among ordinary home audio gear and more popular in specialized and vintage designs.
The advantage, in this case, may depend on the application. If there is a need for isolation, then the transformer will allow the isolation of circuits that otherwise have a direct electrical connection. There are designs where the isolation and signal passing is performed by electronics without the help of a transformer.

One should not forget about the danger of introducing distortions into the audio signal. It can be done unintentionally by a mis-match of the transformer with the system.
Public Address Systems in Schools and Commercial Buildings

Now imagine a school where the announcement has to be made inside the classrooms, corridors and the assembly hall. In this case, a single amplifier might drive many speakers distributed around the building. However, running such a setup will require much more than just selecting speakers that can provide enough volume.
Commercial PA systems often employ audio distribution lines with 70 volts or even 100 volts. Suitable speakers will have transformers to enable them to take the selected power from the audio distribution line. This design is more convenient in the case of cables of significant length and multiple speakers.

In the above mentioned example the role of the audio transformer is the adaptation of the speaker to the line as well as the selection of suitable power tap for each particular speaker.

However, the total load of all speakers has to stay within the capacity of the amplifier. The line voltage is part of the specially designed audio distribution system and is not to be mistaken for the mains electricity.
The example above proves that isolation is not the only function of the transformers in audio devices.

Intercom Systems and Communication Equipment

In office buildings, factories and apartments, an intercom provides communication that does not require a person-to-person connection. In such a system, a microphone and loudspeaker unit at one end is interfaced through electronic circuits at another location.

Depending on the design of the system, an audio transformer can play an important role in isolating the audio circuit or providing the proper impedance relationship between two circuits connected to each other. This would be necessary in an environment where there could be electrical interference or long wiring distances.

It is also important that the equipment is capable of reproducing a clear voice communication. In case the voice becomes weak or distorted, this is due to any of the following; the microphone, the amplifier, the wiring or the transformer itself.
The correct choice of interface equipment depends on many factors including cost, isolation and environmental factors.

Telephones and Other Communication Devices

Conventional telephone systems have used transformers for various purposes like audio coupling and electric isolation between different segments of the circuits. Certain types of telephone systems use specific transformers to connect voice signals with the telephone lines.

The principle of operation is quite simple. Voice is encoded into an electrical signal which will be passed through the communication system. There is a need for handling this signal in the desired electrical parameters with the desired quality of voice.
An audio transformer may be useful in this case if its winding ratio, frequency range and isolation features match the requirements of the task.

New generation mobile telephones and other digital communication devices do not function in the same way. Such communication systems use electronic signal processing and integrated circuits instead of conventional audio transformers in their voice path circuits.

Medical and Industrial Monitoring Equipment

Some medical and industrial devices use audio signals for alerting, feedback or communications between the device and the operator. Here electrical isolation might play an essential role in the overall design of the device.
An audio transformer may help achieve isolation in the correct signal path. Still, the requirements are particularly critical when the device is used close to patients or in conjunction with other electrical devices. A transformer itself doesn't make a device medically safe. It is necessary for the whole design to meet the required safety and isolation standards.

Industrial monitoring devices may be operated next to motors, switching circuits and other sources of electromagnetic interference. Electrical isolation may help avoid certain types of unwanted current from impacting the signal connections. Proper grounding, shielding and circuit layout are still necessary.

For manufacturers of such devices it is important to select the transformer taking into account all the electrical requirements. At CET Technology, we manufacture standard and customized magnetic components for our OEM customers.

How to Know Whether a Device Needs an Audio Transformer

The first issue is not whether a transformer may be used but whether there is a need in the circuit for it.
If there is a problem with the grounding of equipment then the isolation transformer may help after the reason of this problem is established. If there is a need in impedance transformation between two circuit levels then a particular ratio may be needed. If there is sufficient isolation in the circuit through other means then the transformer will be useless.

The frequency response, distortion, signal level, loading conditions and physical environment all should be taken into account prior to the choice.
It will be helpful to start from examining the circuit schematic and equipment specifications. In case when the reason for isolation is not clear a design engineer can help with the decision about the transformer coupling.

The Right Component for the Right Purpose

Audio transformers are used in different types of electronic appliances such as professional audio equipment or selected telecommunication and industrial appliances. The function of transformers varies depending on their use. Some of them provide isolation while others are used for impedance matching or distributing audio signals across different speakers.
What is crucial in using audio transformers is understanding the problem the transformer should address. Improving isolation may eliminate some problems associated with noises but it won't compensate all mistakes in circuit design. Selection of a proper component that fits the circuit becomes a key to success.

With knowledge of signal flow and electrical requirements, the engineer can easily determine when transformer isolation becomes appropriate and when there is a need to use something else. It will lead to creation of reliable and efficient equipment.

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