passive and active DI-box designs
http://sound.westhost.com/project35.htm
how to build a transformer isolated DI box
http://hem.passagen.se/peodal/db/dibox.html
DI-box with instrument and speaker level inputs
http://www.procosound.com/images/DB1.PDF
unixersal DI-box product which works with guitar and keyboards
http://www.procosound.com/images/CB1.PDF
http://www.jensen-transformers.com/as/as066.pdf
four channel DI-box with keyboard or dum machnine inputs
http://www.procosound.com/images/DB4A.PDF
DI-box with line level and speaker level inputs, with input volume control, data sheet with circuit diagram
http://www.procosound.com/images/AVP-1V.PDF
takes high level line signal and outputs balanced microphone level signal
http://www.jensen-transformers.com/as/as008.pdf