digital audio signal converter

Preface

This site explains how to build a circuit that converts a digital audio signal (SPDIF) coming from a CD-player with coaxial digital output or coming from a CD-ROM drive (TTL) to a optical signal used by MiniDisc Recorders.
I know that there are already other plans that show how to do this, but I made this site because this solution is different and it is technically correct and not a thing that works, but you don't really know why. As you will see it is also a very simple and cheap circuit.

Circuit

Take a look at the scheme.
Here is the list of what you need:

• Resistors: 110, 500, 2x 1k, 2x 3.9k, 17k Ohm
• Capacitors: 47pF, 220pF, 15nF
• Transistor: I used a 2N4124 but many other NPN bipolar transistors would do it.
• LED: a red one (maybe you'll have to try different types)

You see, this is probably the cheapest way to get your CD's on MD (...or whatever) without raping your CD-Player. With raping I mean for example directly connecting a LED to the digital out of your CDROM - it would also work, but not with coax signals.

Theory

The digital audio signal coming from a coax (for example the digital output of a CD-player) has+-500mV. (If you take the signal from a CDROM-drive that has 5V you need the part marked with "TTL SIGNAL" and you can leave away the connection of a coax cable "W1")
The signal frequency is about 10MHz. What you see is just a simple transistor amp, but to manage 10MHz you need all those capacitors. The amplified signal is inverted, but this doesn't matter because the signal is symmetric.
The circuit needs something between 4-15 Volts (connect to VCC), but with higher voltage you have to be careful because of the LED. Maybe it would also work with 3V, but I use 3x 1.5V batteries. Maybe that's not enough information. If this is the case, please consult other pages on this topic as well. The links on www.minidisc.org helped me a lot in developing my converter.
At the bottom of this page you'll find my email. Please contact me, if you have questions.

last update: 14. Mar 21

Author: Alain Brenzikofer