Advantages of toroidal vs EI and R-core transformers

Toroidal transformers - best choice for your audio application


As an important part in a chain of electronic components, making a choice of a transformer will probably open a box of questions. The following informations shall support to get basic informations about common used transformer constructions in audio applications, further about their differences. And because we're actually often asked, why we offer toroidal transformers, MPAudio also hopes to answer this question, describing the main advantages of toroidal transformers vs other transformer constructions. In some aspects our transformers differ from "usual" toroidal transformers, as they are designed to meet necessary characteristics for sensitive audio setups. This differences are specifically mentioned in the following description as well:


Electrical efficiency

High efficiency and performance of toroidal transformers are obtained mainly thanks to the use of cores made of silicon steel with oriented grain (CRGO - cold rolled grain oriented) with the right shape. The core is coiled under a large tension - into the shape of a clock spring - with a continuous, unbroken strip of material without an air gap. The effect of setting the molecular structure of the material in magnetic direction is obtained by stress relief annealing. For comparison it is determined that cores composed of EI-cut shaped bodies have about 40% of grains provided in the wrong direction.

Symmetrical distribution of transformer turns around the entire perimeter of the toroidal core without an air gap gives the opportunity to obtain a larger magnetic flux. Typical values ​​of the magnetic flux of the EI-transformer is 12-14 Kg, while for toroidal transformers values ​​of 16-18 Kg are achieved. One of the benefits is, that toroidal transformers offer very stable output voltage, resulting in less voltage drop also during high loads. It should be remembered here, that in a toroidal transformer the magnetic flux is oriented in a direction consistent with the grain system of the material.

Thanks to these unique properties it is possible to obtain extremely high electrical efficiency. The average efficiency of toroidal transformers is 95%, while for high powers of 20kVA, even efficiency up to 98% is achieved. Compared to classic constructions these values are higher by 14% on average.

The graph below presents typical efficiency values ​​of classic and toroidal transformers

as a function of load.

Efficiency vs Load of MPAudio Transformers

Low Noise Level

The most important cause of mechanical noise during transformer operation is the so-called magnetostriction phenomenon. This phenomenon consists of deformations (vibrations) of ferromagnetic materials under the influence of an alternating magnetic field. This undesirable phenomenon can be significantly reduced by using toroidal cores in transformer design: Windings tightly wrapped, no air gap and the use of modern impregnants makes it possible to obtain even eight times lower loise level, when working under load. While one shouldn't perceive noise from our Audio Grade transformers, our Premium Audio Grade transformers are designed to guarantee absolute noise-free operation due to their construction.

Low magnetic field emission

Due to its unique design and efficiency, the toroidal transformer is characterized by

90% reduced magnetic field emission. Transformer winding, especially exactly covering the core, act as a shielding element and focusing the magnetic field there, where it is optimally used to transform primary energy into secondary energy. This feature allows the use of toroidal transformers for exceptionally sensitive electronic devices such as low level signal amplifiers or medical devices - and of course sensitive audio application. Due to the closed magnetic circuit, toroidal transformers are characterized by the lowest available leakage inductance and the minimum stray magnetic field, especially comparing to other transformer constructions. As an additional benefit, MPAudio improves the Premium Audio Grade transformers with an electrostatic and magnetig shield.

Low power loss during unloaded operation

A toroidal transformer requires significantly less electric resources when operating without load. The energy consumed in this state is 16x smaller compared to classic shape transformers. This results in gigantic energy savings for devices with discontinuous performance characteristics - such as transformers for industrial machines.

For example, the equivalent of a classic toroidal with 100W resting power based on a toroidal transformer will consume about 6W only. The chart below shows a schematic comparison of electric energy consumption by both types of transformers.

Energy Consumption vs Power Rate

Flexible dimensions

Another advantage resulting from the design of the toroidal transformer is the ability to adapt the dimensions of the transformer to customer needs. The toroidal core can be modified in three dimensions: outer diameter, inner diameter and height while maintaining the same cross-sectional area of ​​the magnetic core. This makes it possible to use toroids where transformers built in EI shape do not fit.

Minimization of dimensions and material consumption

The easiest observable difference between toroidal and shaped transformers

are their dimensions. Toroids are up to 50% smaller and need 30% less production materials in average. It should be remembered that both CRGO steel and high purity copper are raw and very expensive materials, and their processing requires a lot of energy.


Special: R-core transformers

R-core transformers are a good compromise between EI and toroidal transformers.

Their efficiency is higher than that of EI transformers, but lower than that of toroidal cores (EI - average approx. 80%, R core average approx. 87%, toroidal core - average approx. 95%). Due to this fact, the toroidal transformer always remains the leader regarding off-load vs on-load regulation, which is the smallest in case of toroidal transformers.


At the same time, due to their design, R-core transformers do not have the greatest disadvantage of toroidal cores - the high inrush current.

Often the shape of the transformer also determines the implementation and purchase decision. R-core dimensions are usually narrower - but wider.

Further R-core transformers benefit of lower capacitance between primary and secondary windings and are less sensitive to DC on the mains.


The disadvantages of the R-core transformer are their magnetic field emission, which can likely induce noise into a sensitive circuit, and the possibility of vibrations, especially with higher power rates. That's why R-core transformers are often implemented into external enclosures and outside of the powered electronics. Be aware, that avoiding the mentioned problem, this can cause other issues (due to longer wires, more medium transitions like wires to plugs and vice versa. Further it results in higher costs and more effort.


Addition

Due to their popularity, toroidal transformers are easily available in a big variety of power rates and common secondary voltages, making it easy to handle power requirements. The characteristics of the toroidal transformer make it also easy to realize an application matching power supply, easily avoiding noise or other influences to the sensitive audio application.

Offering individual secondary voltages and a great variation of power rates, heat and power saving power supplies of highest quality can be realized with our transformers and HPULN modules.

Further MPAudio enriches that through individual secondary voltages matching almost all applications.


To handle the high inrush current of a toroidal transformer, an inrush current limiter can be used. If this is necessary, depends on the power rate of the transformer, the power supply that it feeds (the more capacity, the more it effects as a short circuit during the start up) and finally the individual electric installation including the used mains fuses. As a rule of thumb, we recommend to use inrush current limiters above the power rate of 120VA, in most cases the high internal resistance of the smaller transformers make the limiter unnecessary.

However, some (famed) sources on the internet claim, that the high inrush current during the startup stresses the components, such as the transformer itself, the rectification diodes and the buffer capacitors, and that it takes a while, until the components get back to their "normal" operation mode. In combination with the often published opinions in forums, that sound experience changes after time and sounds "better" after a while (which is for some people a reason to keep their audio equipment always powered), this can be a hint, always to use inrush current limiters, even if technically not necessary. Finally it's just a small and cheap module in customer's setup, that can avoid many issues.


Being more sensitive to DC on the mains, a toroidal transformer can cause mechanical noise. Usually toroidal transformers with lower power rate have a higher internal resistance, usually high enough not to cause that noise even with higher DC on the mains. DC on the mains is a seldom and individual issue, that depends on local energy supply, local companies next to one's house etc. This issue can easily be solved, blocking DC using a few diodes and capacitors, as described here. The article includes more informations regarding that issue and is very recommended to read.

Beside that diy-solution, there are also some ready-to-use circuits out there.

And as usual: Caution! You are working with 230 volts here! Absolute danger to life! If you are not familiar with something like that, please keep your hands off it.

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