DIY Output Transformer






  Heavy storm blown away my network connection month ago, and I'm not be able to update my page, and I need to save this page in CD-RW and borrow my friend's connection to update my page.

  One of my customer ask me to wind a pair of output transformers for him, his required specs are 2.7Kohm : 8 ohm SE, 2 watts Max, 45mA primary working current, He want to experiments with  6C45pi, 5842,  6EM7 etc. to drive his Coral full range speaker.

 Here's some photos of how I wind the c core transformer.

    I choose M5 grade  C core for more efficient, and most of EI type transformer require more iron than copper, but c core require more copper than iron. I can choose thicker gauge to reduce DC resistance.

   Because of customer's request, I need to build the paper bobbin. The center bobbin former was PCB that cut into size and glued together, wrapped with tape for smoother surface. Before wrap several layers of brawn paper to form a paper bobbin, I need to wind a layer of 0.5mm copper wire, then wrap it with several layers of  brown paper on top of the copper as bobbin, the purpose of the copper layer is to remove the finished coil out from the bobbin  former by pull the copper wire out from the former, and there's 0.5mm gap in between former and paper bobbin.




   Every layer of copper coil must add a layer of wax paper ( I used 0.02mm wax paper, same material as used in old Japanese Sansui transformer)  to ensure the next layer will closely and tidily align. Some manufacturer used mylar film, but again, my customer prefer the classic all-paper type.



  It is important that every turn of copper wire must be closely wound, to reduce unwanted leakage inductance. I used manual winder machine for best result because I'm using all- paper method. To further reduced leakage inductance, primary and secondary must be split into several portion, interleaved winding.

 I used 2 layer of thicker tracing paper (0.09mm) for isolation in between primary and secondary. For higher B+ supply, more layer or thicker layer is needed.



  After finished the winding job, need to measure the DC resistance of primary and secondary to ensure the inner connection is correct. Then solder wires and tape into place.


   Wrap few layer of brown paper at the final stage, to protect those fragile windings. I heat it in my home made jig to remove moisture, just a wooden box with 2 pcs of 100 watts bulb inside, with dimmer connected. The temperature is monitor by a cheap electronic thermometer and control to about nearly 100 degree celcious. I burned my transformer once  without temperature control.


  After few  hours of heat up, the transformer is soak into transformer varnish to avoid moisture and fungus build up inside the windings. Hang it to dip out accessive varnish and again, bake it for several hours to dry it.   Some might say that the varnish will increase the stray capacitance and reduce high frequency performance. Its true but my "SIFU" ( means teacher ) told  me long time ago, "a good hand crafted transformer will still perform perfectly even with varnish,  and a transformer that is not moisture-free is consider a bad transformer, it won't last too long especially in Asia countries."



  Here is a photo of my previous made SE transformer that follow the winding method of the 60's Philips V30M 3 watts SE amplifier that using c core. Who will say c core OPT is a new technology?.


Now the transformer already finished. PLEASE READ THE FINAL TEST AND COMPARISON PAGE HERE.