DIY air core toroid inductor for audio amplifier output stages

Abstract

Every audio power amplifier needs an output inductor to maintain stability with real world loads. Andrew C. Russel wrote a good article about the background of this requirement. You may want to read the article about amplifier output inductors on his webiste Hifisonix.

Aprat from properly sizing the output inductor, the damping resistor and Zobel networks at the amplifier output, the main challenge is to deal with the magnetic fiel generated by the output inductor. Usually the output inductor features an air core to avoid saturation and hysteresis related distortion. The downside of air core inductors is that the magnetic field is not contained within a ferrous core and therefore, shielding or toroid inductor design to lower stray field are not possible. The magnetic field generated may couple into adjacent audio circuitry, which is highly undesirable.

The best solution would be an air core toroid inductor. I did a lot of research and found that such parts are not offered commercially anywhere. They are also difficult to build due to the lack of suitable cores. While it is possible to build an air core toriod inductor without any coil former, it is difficult to do so. One method would be to wind a cylindrical inductor and then to remove the cylindrical coil former and bend the inductor into a toroid shape. The resulting inductor is a bit delicate and will lose its toroid shape if handled improperly. My preference is clearly a coil former that stays inside the finished inductor and maintains the correct geometry.

I did a lot of research looking for toroid with suitable shape and material to use as toroid inductor coil former. Finally I succeeded to find some standoffs that can be modified to suit as toriod inductor coil former. In this article I present my DIY air core toroid inductor design.

DIY air core toroid inductor core design

DIY air core toroid inductor core
This is the plastic standoff made from polyethylene with the chamfers added. The standoff is designed to be used for M12 screws and is available in different outer diameters and thickness. I plan to add chamfers using a pipe reamer hand tool. In case the modification using the hand tool fails, I need to use a drilling machine and / or lathe to add the chamfers.

DIY air core toroid inductor core manufacturing

The initial plan for making the cores was to use a pipe reamer hand tool at home. The tool turned out to be worthless and I changed my plan. The best way to machine any rotational part is on a lathe and this is what I did in the end. It was my first time working with a lathe and my friend helped me to set it up so that I can safely and quckly machine my cores. The chamfers at the inner bore were done on a standard drilling machine after finishing the outer chamfers.

DIY air core toroid inductor design

The easiest way to calculate inductors is to use online calculators. The best calculator I found is the ferrite core toroid inductor calculator from coil32.net. By setting µr to 1, this should work for calculating air core toroids as well as long as the core coverare stays reasonably high. The calculator takes the chamfer into account and for such a small toroid geometry, this has considerable effect.

single layer toroid inductor maximum number of turns formula
I drafted my DIY inductors in FreeCAD for checking feasibility and getting an impression how the finished inductor may look like. It is not required to use CAD in order to calculate the maximum number of turns for a single layer toroid inductor. There is also a formula for the maximum number of turns. I plan to build three different inductors using the maximum possible wire gauge for each to minimize resistance. The CAD preview shows that the shunt capacitance of a single layer toroid inductor is much lower than the capacitance of a cylindrical inductor, which makes the inductor perform better at high frequency.

470nH

1000nH

2000nH

DIY air core toroid inductor realized

No plan survives first contact with reality. This is true for most projects and especially for this one. While manufacturing the inductor cores worked pretty well, winding the inductors according to the plan made in CAD was difficult. Actually I did not even stay close to the plan. Winding such a heavy wire around such a small core was very challenging and I ended up just puttting as many turns on the core as I could and had a significanly lower number of turns in the end. I did not even attempt to build an inductor with the heaviest wire I designed for initially.

DIY air core toroid inductor using 1.5mm diameter wire

This inductor is made from 1.5mm diameter wire and has 19.5 turns. I estimate the inductance to be 600nH roughly. The wire is roughly 72cm long and this results in 7mΩ DC resistance.

DIY air core toroid inductor using 1mm diameter wire

This inductor is made from 1mm diameter wire and has 29.5 turns. I estimate the inductance to be 1.3µH roughly. The wire is roughly 109cm long and this results in 23mΩ DC resistance.

Measurement

No measurements yet...

Conclusion

No conclusion yet...