Coaxial S/PDIF Output

This circuit is an alternative to the optical S/PDIF output described elsewhere in this issue. The quality of the connection provided by this link is usually better than that of an optical link (less jitter). In order to avoid earth loops, a small output transformer is normally used for digital audio signals. The construction of such a transformer has been described several times in Elektor Electronics. It is based on a Philips toroidal core, with 20 turns on the primary and 2 turns on the secondary, both using 0.5-mm varnished copper wire.

Resistors:
R1 = 1MΩ
R2 = 220Ω
R3 = 75Ω
Capacitors:
C1,C3 = 100nF ceramic
C2 = 47nF ceramic
C4 = 47µF 25V radial
Inductor:
L1 = 47µH
Semiconductor:
IC1 = 74HC86
Miscellaneous:
K1 = 2-pin SIL header
K2 = cinch socket, PCB mount (e.g., Monacor/Monarch T-709G)
K3 = 4-pin SIL header
Tr1 = ferrite ring core Philips type TN13/7,5/5-3E25. Primary 20 turns, secondary 2 turns