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.
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.
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.
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.
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.
No measurements yet...
No conclusion yet...