Sound Transformer

The Audio Transformer is designed for use in audio amplifier applications for connecting amplifiers and speakers and impedance pairing.

In addition to increasing or reducing a voltage, transformers have another useful feature: insulation .Since there is no direct electrical connection between the primary and secondary windings, transformers provide complete electrical insulation between the input and output circuits, and this insulation feature can also be used between amplifiers and speakers.

About transformers In this section about transformers, a transformer is an electric device that allows a sinusoidal input signal (such as an audio signal or voltage) to produce an output signal or voltage without the input side and output side being physically connected to each other. other.This connection is achieved by two (or more) wire coils (called windings) from ins isolated copper wire wrapped around the magnetic iron core.

Sound Transformer
Sound Transformer

When an AC signal is applied to the primary input winding, a corresponding AC signal appears in the output secondary winding due to the inductive coupling of the iron core.The rotational ratio between the input and output wire coils allows the signal to increase or decrease when passing through the transformer.

Sound transformers can then be considered as an upgrade or drop type, but instead of hugging to produce a certain voltage output, sound transformers are mainly designed for impedance compatibility.In addition, a transformer with a rotational ratio of 1:1 does not change voltage or current levels, instead insulating the primary circuit from the secondary side.This type of transformer is commonly known as the Isolation Transformer.

Transformers are not smart devices, but they can be used as bidirectional devices so that the normal primary input winding can turn into an output winding and the normal secondary output winding can be an input, and this can provide a signal gain when used due to its two-way nature. When used in reverse to help match signal or voltage levels between different devices, there may be losses in one direction or signal.

Also note that a single transformer can have multiple primary or secondary windings, and there may be multiple electrical connections or "taps" along the lengths of these windings.The advantage of multi-stage audio transformers is that they offer different electrical impedances as well as different gain or loss ratios, making them useful for impedance matching of amplifiers and speaker loads.

As its name suggests, sound transformers are designed to work within the sound band of frequencies, and therefore can have applications in the input stage (microphones), the output stage (speakers), the inter-stage connection, as well as the impedance matching of amplifiers.In any case, frequency response, primary and secondary impedances and all power characteristics should be taken into account.

Sound and impedance harmonization transformers are similar in design to low frequency voltage and power transformers, but operate in a much wider frequency range.For example, the audio range from 20Hz to 20kHz.Audio transformers can also transmit DC in one or more of their windings for use in digital audio applications, as well as convert voltage and current levels at high frequency.

Sound Transformer Impedance Matching

One of the main applications of sound frequency transformers is impedance compatibility.Sound transformers are ideal for balancing amplifiers and loads together with different input/output impedances to achieve maximum power transfer.

For example, a typical speaker impedance ranges from 4 to 16 ohms, while the impedance of the output stage of a transistor amplifier can be several hundred ohms.A classic example of this is the LT700 Audio Transformer, which can be used during the output phase of an amplifier to drive a speaker.

For a transformer, the ratio between the number of coil rotations (N P) in the primary winding and the number of coil rotations (N S) in the secondary winding is called the "rotation rate".Since the same amount of voltages is induced in each coil of both windings in turn, the second voltage ratio (V primary P/V S), therefore, will be the same value as the winding ratio.

Impedance-compatible sound transformers always give the impedance ratio value from one winding to another with a square of rotational ratios.That is, impedance ratios are equal to the square of the rotation rate, as well as the square of the primary to secondary voltage ratio, as shown.

Sound Transformer Impedance Rate

Sound Transformer

When Z P is the primary winding impedance, Z S is the secondary winding impedance, (N P /N S) is the transformer rotation rate and (V P /V S) is the transformer voltage ratio.

For example, an impedance-compatible sound transformer with a rotational rate of 2:1 (or voltage ratio) will have an impedance rate of 4:1.

Audio Converter Question Example 1

A sound transformer with an impedance ratio of 15:1 will be used to pair the output of a power amplifier to a speaker.If the amplifier's output impedance is 120Ω.Calculate the nominal impedance of the speaker required for maximum power transfer.

Sound Transformer

The power amplifier can then efficiently operate an 8 ohm speaker.

100V Line Audio Transformer

Another very common impedance matching application is for 100 volt line transformers for music and audio transmission through general voice over systems.This type of ceiling-based speaker system uses multiple speakers located a little farther away from the power amplifier.

Using line isolation transformers, any number of low impedance speakers can be connected to properly load the amplifier, ensuring impedance matching between the amplifier (source) and speakers (load) for maximum power transfer.

Since the power loss of signals through speaker cables is proportional to the square of the current (P = I 2 R) for a specific cable resistance, the output voltage of an amplifier used for general voiceover (PA) or tannoy systems uses a standard and constant voltage. 100 volt peak output level, (70.7 volt rms).

For example, a 200 watt amplifier running an 8 ohm speaker provides a 5-amp current, while a 200-watt amplifier using a 100-volt line at full power provides only 2 amps, allowing smaller measured cables to be used.However, keep in mind that this 100 volt only exists when the power amplifier running the line is running at full nominal power, otherwise it will have low power (low volume) and line voltage.

The line transformer for the 100V (70.7V rms) line speaker system increases the audio output signal voltage to 100 volts, so that the transmission line current for a given power output is relatively low, reducing signal losses and allowing smaller diameter or measured cables.

Since the impedance of a typical speaker is usually low, an impedance-compatible drop transformer (often called a line-sound coil transformer) is used for each speaker connected to the 100V line, as shown.

100V Transmission Line Transformers

Sound Transformer

Here, the amplifier uses an amplifier transformer to provide a constant transmission line voltage of 100 volts at reduced current for a specific power output.The speakers connect parallel to each speaker, which has its own impedance harmonization drop transformer to reduce secondary voltage and increase current, thus pairing the 100V line with the low impedances of the speakers.

The advantage of using this type of audio transmission line is that many individual speakers, tannoys or other such audio actuators can be connected to a single line, even if they have different impedances and power processing capabilities.For example, 4 ohm at 5 watts or 8 ohm at 20 watts.

Sound Transformer

In general, transmission line matching transformers have multiple connections called ports on the primary winding, which allow the selection of appropriate power levels (and therefore volume) for each speaker.In addition, the secondary winding has similar guide tow points that offer different impedances to match that of the connected speakers.

In this simple example, the 100V line-speaker transformer can operate 4, 8 or 16 Ohm speaker loads on the secondary side with amplifier power ratings of 4, 8 and 16 watts on the primary side, depending on the ports selected.In reality, PA system line transformers can be selected for any combination of serial and parallel connected speaker loads with power processing capabilities of up to several kilowatts.

However, in addition to stationary voltage impedance line transformers, it can be used to connect low impedance or low signal input devices such as sound transformers, microphones, rotary moving coil receivers, line inputs, etc. to an amplifier or pre-amplifier.

Since input audio transformers need to operate in a wide frequency range, usually the internal capacitance of their windings is designed to resonate with the inductance to improve the operating frequency range by allowing a smaller transformer core size.


In this tutorial about sound transformers, we found that audio transformers are used to match impedances between different audio devices, for example, between an amplifier and speaker as a line driver, or between a microphone and an amplifier for impedance pairing.

Unlike power transformers operating at low frequencies such as 50 or 60 Hz, sound transformers are designed to operate in the sound frequency range of about 20 Hz to 20 kHz or much higher for radio frequency transformers.

Due to this wide frequency band, the core of sound transformers is made of special quality steels such as silicon steel or special iron alloys with very low hysteresis loss.

One of the main drawbacks of sound transformers is that they can be somewhat cumbersome and expensive, but the use of special core materials allows for a smaller design.This is due, as a general rule, the increased core size of a transformer as the feeding frequency decreases.