This page describes how to add to a device with has already an optical spdif output a coaxial spdif output as well.

The modification described on this page was adapted to a Sony MDS-JE520 MiniDisc deck. Although it contained both an optical and coaxial spdif input, it only contained an optical spdif output. The lack of a coaxial spdif output is inconvenient and this modification mitigates that.

S/PDIF signals are defined in IEC 60958-3 (2003)¹⁾. The output shall have a substantial resistive impedance of $(75\ \pm\ 15) (\Omega)$ when measured at frequencies from 0.1 MHz to 128 times the maximum frame rate, which equals for a sample rate of 44.1 kHz to about 5.6 MHz. The signal amplitude shall be $(0.5\ \pm\ 0.1 ) {\text V}$ peak-to-peak, when measured across a $(75\ \pm\ 0.75) (\Omega)$ resistor connected to the output terminals, without any interconnecting cable present.

[schematic]

Giving the following circuit:

Vin > ---o
         |
        .-.
        | | Rx
        | |
        '-'
         |
         o-- > Vout
         |
        .-.
        | | Ry
        | |
        '-'
         |
    0v --0-- 0v

Taking into consideration that the impedance of source Vin is substantially low, following relationships can be established:

$$\frac {1}{x} + \frac {1}{y} = \frac {1}{a}$$

and

$$\frac {y}{y + x}=\frac{1}{b}$$

Where

$a$ = needed output impedance

$\frac {1}{b}$ = ratio Vout relative to Vin

(For example, with a given Vin of 3v and output voltage Vout of 1v, $\frac {1}{b}$ would equal to $\frac {1}{3}$)

Expressing x and y in $a$ and $b$ gives:

$x = a \cdot b$

$y = \frac {a \cdot b}{b-1}$

For example: $a=75\ \Omega$ and $b=3.3\ {\text v}$ gives:

$x = 247.5\ (\Omega)$

$y = \frac {247.5}{2.3}\approx 107{.}6\ (\Omega)$

Please note that under load, when the impedance is connected to Vout, the voltage measured at Vout will reduce to exactly half the voltage in comparison to when it is open.