How audio transformers work

Output transformers are key parts in tube amplifiers and are very complex. This page will expose only basic knowledge of transformers to help understand the importance of having a good output transformer.

The need for a transformer

Transformers can be used in many ways. The most common using is to transform electricity from one form to another. In tubes amplifiers, they are used to transform the high-voltage and low current produced by tubes to a low-voltage high current needed by the loudspeaker. Tubes typically produces 20 times the speaker voltage and one 20th of the speaker current.

It also provides an insulation between bobbins to enhance safety, because of the high voltage of tubes. This insulation is also mandatory for power transformers as they are connected on the mains.

 

The main physics effects

Transformers are based on two mains effects : electromagnetism which if the process of making a magnetic field using electrical current, and electromagnetic induction which is the process of making an electrical current by using magnetic fields.

 

The illustration below shows a basic view of a transformer. The core (1) is composed of thin plates of soft metal and conduces the magnetic flux (6). The main bobbin (4) is a wire making an helix with windings. When a variable current (5) flows through the bobbin, it creates a variable magnetic flux (6), this is the electromagnetism process.

Secondary bobbin (2) is influenced by the variable magnetic flux (6) and creates a current (3), this is the electromagnetic induction process.

TR Phy 1 legend

The core material is made of multiple layers of metal. These layers helps reduce losses and are insulated each other.

The wire size depends on how much power is needed by the bobbin to creates the magnetic flux. The more the windings, the more the voltage, but the less the current.

 In the illustration, the main bobbin (4) has many windings indicating a higher voltage than that of the secondary bobbin (2). The size of the wire is smaller, then the current is lower. This main bobbin could be the one connected to tubes, and the secondary bobbin could be connected to loudspeakers.

 

Bandwidth limitation of transformer

The transformers are not a perfect elements. They cannot work with steady voltage : the input voltage must be changing, this is due to the electromagnetic induction. The lower frequency at which the transformer can work is known as low frequency cutoff, and the higher is the high frequency cutoff. The two cutoff frequencies makes the transformer bandwidth, which is the main limitation on tube amplifier bandwidth.

The bandwidth ranges from 15Hz-50Khz for normal transformers, and above 10Hz-80Khz for high-end transformers.

Below is an example of transformer response curve showing bandwidth : the low frequency cutoff (1), the pass frequencies (2) and the high-frequency cutoff (3).

TR Bandwidth legend

 The low frequency performance of transformer is primary determined by the ratio of the transformer's primary bobbin inductance (which is determined by the number of windings, the core size and material, and other complex parameters) and the primary resistance (determined by the internal tube resistance paralleled to the reflected load impedance on primary).

Push-Pull transformers has then a better lower frequency response than single-ended transformers, since their inductance is higher. The lower inductance of single-ended transformers is due to the air gap in the core needed to overcome saturation that would result from the constant magnetic flux. There are no constant magnetic flux in Push-Pull transformers.

The high frequency performance is determined by the leakage inductance (which is caused by the magnetic field that manages to escape from the core) and the parasitic capacitance between bobbin's layers. One solution to limit the leakage inductance is to wind wire very close together and on a common axis. Another technique is to use layering where sections of different windings are wound in a sequence over each other to interleave them. To limit capacitance between windings, a shield, which is a thin sheet of copper foil, is placed between each bobbin section. All shields are connected together.

 

Saturation of transformers

The core is responsible of carrying all power from the primary winding to the secondary winding by guiding the magnetic flux.

When the flux is too much high, the core cannot handle it, and the excess power is not transfered. The primary winding dissipates the excessive energy by heating his wire. Beside drastic sound distortion, this can cause serious problems as the primary current rises quickly and damage to tubes or other electronics can occur.

This is why the bias precision is important in Push-Pull section, it will be detailed in the following pages.

 

 Single ended-transformers uses only one tube. Then, a permanent magnetic flux exists in the core. A small air-gap is made on the core to avoid saturation. But this will come at expense of reduced performances. The bias current value precision and stability is also importance, as exposed to the section The importance of Bias.

 Push-Pull transformers use two tubes, and the windings are made in such a way that each permanent magnetic flux produced by tubes cancels each other, no air-gap is then required.

 

Impedance matching

Audio transformers are generally made with output impedance ranging from 4 to 16 ohms. This permits the adaptation of transformer impedance to loudspeaker's impedance : this is the impedance matching. This is an important process because it affects distortion and maximum output power : for the maximum transfered power from tubes to loudspeaker, the transformer output impedance must match the loudspeaker's impedance.

The following curve shows transfered output power for a 8 ohms transformer, regarding to the loudspeaker's impedance. The maximum output power of 35 W is for a loudspeaker of 8 ohms.

ImpMatching legend

 

 

Audio transformer construction

 The process of making an audio transformer is highly complex, and is a trade secret.

 A simplified winding plan for a Push-Pull transformer is shown below. The winding sections (2), external connexions (1) and shields (3) are represented. The insulations layers are not represented since they are between each winding sections (3). All bobbins are wound around the core (4).

 

TR Bobinages legend

 Transformers are oftenly molded in resin to provide mechanical protection and lower vibrations.

 

Conclusion

 Transformers are very complex parts, their building is a trade secret. Their performances are of prime importance for high-end audio as they can be a real limitation.

Monange's amplifiers uses over-sized audio transformers of best quality to give you the best sound.