Audio Output Transformer Design Considerations


Audio transformers are magnetic elements inserted into the signal carrying portion of sound equipment, and are governed by same laws of magnetism and power transfer as other transformers. Although audio transformers may look similar to power transformers in appearance, performance-wise they are very unlike power devices and exhibit special characteristics. Unlike power transformers that are designed to pass a fundamental fixed frequency, the demands on audio transformers include the ability to pass wide band signals within the audio bandwidth generally considered from <20 Hz to > 20 KHz.

The modern generation of power electrics delivers effortlessly delivers large amounts of clean audio, and a good audio transformer should neither add nor detract from the signal. Audio devices are expected to operate in a linear manner over the entire frequency spectrum; and to operate at low or negligible distortion levels to maintain the clean integrity of the source signal.


Toroids are the Ideal Choice for Audio Transformation

AudioRespnseCurve-300x107Toroidal geometry renders them ideally suited to the requirements of audio applications. The ability to design and deliver these properties is largely a function of the transformer engineers’ ability and experience. There is always a trade between good low-end response, good high-end response, and low distortion.

Plitron Can Design for Wide Band and Low Distortion Plitron engineers are experienced to combine all three elements into a single transformer. Plitron engineers have successfully designed and manufactured transformers which operate down to 10 Hz and can remain flat about 70 KHz.

Design Parameters

Low Frequency Performance – Low End Performance is a direct function of high primary inductance. The gapless High Permeability Low loss Hi-B Silicone Steel Core is excellently suited, with all grains oriented in the preferred direction is easily capable to deliver low-end response to 10 – 20 Hz (or less) at full power.

High End Performance – High End Performance is a function of magnetic coupling, or the ability to control interwinding capacitances. Toroids can achieve tight coupling in many ways, including their long winding path (much more so that other geometries) which reduces the number of layers, ability to precision wind, hi induction levels reduce the turns while still offering linear performance, interleave as necessary, and other application specific properties and design techniques.

Low Distortion – Low Distortion throughout the audio bandwidth can be incorporated into the design by working on the long linear portion of the magnetizing curve, which is an inherent property of toroidal cores.

Comments and Design Notes

Avoid Primary Taps – The illustration above illustrates a tapped primary winding, which is shown for example only. Best performance however is achieved with a dedicated primary winding with a start and finish, and no taps. High-end frequency response is a direct function of winding coupling and symmetry, and thus a tap at a location other than 50% (turns, not impedance) results in a turns inequity per winding layer. It is a physical phenomena that reduces coupling, increased leakage inductance, and causes a subsequent decrease in high-end response.
Bandwidth is Variable and Selectable – Toroidal output transformers provide an economical solution in any audio application, but their value is not limited to ultra wide band installations. They are application specific, and can offer excellent value in lesser demanding applications, such as power reinforcement and PA work, where 100-8 KHz (or any reduced) bandwidth is acceptable.
Signal Transformers – There is a class of transformer that meets all of the criteria explained above, with the exception of output power ratings. Although the principles are similar, the above section refers to output devices and not transformers intended to carry low-level signals (such as in preamps, amplifier input or inter-stages magnetics, or microphone transformers).

Specifying Audio Transformers For Common Applications

Plitron Toroidal TransformersPlitron can design audio output transformers offering wide bandwidth low distortion for any application. The information below assumes there is no (or negligible) DC content in the output signal.

The design becomes more complex in the presence of DC, so let our engineers know at the time of design if this is a concern.

Vacuum Tube Outut

Plitron maintains a standard product line on toroidal audio output transformers.  This information can be used as reference when specifying a custom design.  We supply singled ended or push pull for a variety of output tubes, plus more. Check the listing for information about our capabilities and standard products.

Take Control of Multi-Speaker Installations

In situations where multiple speakers are driven from a single power source, a toroidal distribution system allows control of all speakers individually, without the need for separate volume controls, and is compatible for future growth if additional speakers are added. For example, restoration of large theatre may require 188 X 8 inch speakers installed and hidden away in the décor. It is likely drivers installed on the high ceiling will need to be at higher volume levels than those installed in the lobby. Plitron engineers can deliver quality sound to the source, so it will not sound like a PA system.

Audio Distribution Toroids

Situated at the amplifier output, a step-up wideband distribution transformer can output a higher voltage such as 70 or 100 VAC, which is well suited to transmit a high-powered audio signal over long distances to a multitude of small satellite speakers. This transformer is intended to carry the full system power. Toroids can deliver lots of power, with low distortion, and excellent bandwidth to the load, with negligible degradation to the sound. These devices can be specified as simple as Power W RMS, Bandwidth + / – db, Input usually specified in Watts RMS, Output specified as 70 V, Size limitations if any, plus any notes for our engineers. We would work on the assumption the output is purely AC, and if there is a DC component please let us know at the time of design. It is common and wise to specify an autoformer in this application, where no further output isolation is needed.

Line Matching Toroids

At the transducer end, small multi-tapped satellite transformers capable of the same bandwidth and low distortion as other upfront output devices can be mounted on each individual speaker for proximity control. For example, the primary winding can be 70 (or 68 VAC allowing for line drop), and then the installer can select an appropriate output tap (such as 2, 4, 8, 10 watts) to ensure the output volume is correct. As indicated in the paragraph above, where appropriated, and autoformer will offer best value.

Impedance Matching Audio Toroids

To ensure maximum power transfer and best performance between load and source, an impedance matching transformer is needed. This is an ideal solution to match the 8-ohm amp power out to your old bass-reflex Wharfdale 16 ohm drivers, plus many more practical applications.