On this page I have collected some formulas for calculating transformers. So if anybody wants to "roll their own", here are some recipes. I have never tried to make output transformers myself, so I can't comment on the quality. I have, however, used the power transformer formulas. They work well.

Single-Ended Transformers

Power Transformers


Push-Pull Transformers

Originally by E.A.Schulze, Funkschau 1966, No.1, p.27f.

I will translate the most important parts of the original article.
I will use a 40W transformer with a primary imp. of 6k6 as an example.

Here we go:

First the sectional area of the core.



The original formula given in the magazine is:

Fe = 200P / min. frequency in Hz.

Fe is the area in square cm. P is the max. power.

However this formula gives an Fe than is way to big, and it must contain an error. I believe, judging from the example in the article, that the 200 should have been 20. If we use the 40W and 20Hz as an example, the Fe will be:

Fe = 20 * 40 / 20 = 40cm²

This is also a very big figure, but if you can afford such a big core it will guarantee that there is no phase shift at 20Hz, and it will give you a -3dB point of 5Hz (see Bild2.gif)

Using a rule of thumb that is mentioned in the text gives a smaller value. That formula is as follows:

Fe = Square root of (P * 20) = sqrt (40 * 20) = 28.3cm²

A third rule I know of simply states that the area should be twice the area of a mains transformer of the same power, for a 20Hz transformer. That gives the following formula:

Fe = (Square root of P) * 2 = (sqrt 40) * 2 = 12.65cm²

So you have three different numbers to work with. Which one you should choose depends on the space and the money available. But perhaps the 28.3 is the best compromise.

The next thing you do, is find the standard core that suits the calculated value best. You should use the effective area of the core, both for choosing the core and for the further calculations, and not simply the area. The core should be a standard EI core. The laminations should be made of grain-oriented silicon steel, and should be between 0.2 and 0.35mm thick.

Primary windings are calculated from voltage at max. power.
That gives the formula:

U = (Square root of P * Zp) = (sqrt of 40 * 6600) = 513.8V

The number of windings in the primary winding should be:

Np = 115 * U / Fe = 115 * 513.8 / 28.3 = 2088

The number 115 depends on the lower frequency limit. It should be 230 for 10Hz, 115 for 20Hz and 57.5 for 40Hz.

The secondary windings are calculated using the impedance relation:

Ns = Np / (Square root of Zp / Zs) = 2088 / (sqrt 6600 / 8) = 73

If you need outputs for more than one impedance it is not a good idea to make a tapped secondary. It is better to make four secondaries that can be connected in series or parallel. That way the entire transformer is always active, and not only half, as it is when you connect a 4 Ohm speaker to a transformer with 4, 8 and 16 Ohm taps. If you use four 0.8 Ohm secondaries, you can obtain the following outputs:

-->->->->     -->->     -->->     -->
  > > > >       > >       > >       >
  > > > >       > >       > >       >
-->->->->       >->       >->       >
                 |        |         |
                >->       >         >
                > >       >         >
                > >       >         >
              -->->       >         >
                          |         |
                          >         >
                          >         >
                          >         >
                        -->         >
                                    |
                                    >
                                    >
                                    >
                                  -->

  0.8 Ohm   3.2 Ohm   7.4 Ohm   12.5 Ohm