Craig's Digital Audio Circuits page

This is a small collection of information about digital audio. I've also included some circuits I've built and provided some useful references.

Sony/Philips Digital Interconnect Format (S/PDIF) is a common electrical digital audio format that sends information over 75 Ω coax. RCA connectors are used and signaling ranges between 0 and 0.6 V.

Toshiba-Link (TOSLINK) is an optical version of S/PDIF that uses LEDs to transmit the data over plastic fiber using JIS F05 connectors. The LED is illuminated for logical 0 and is off for logical 1.

Example 5 V S/PDIF trace

Building blocks

One approach to designing digital audio circuits for S/PDIF and TOSLINK conversions and distribution is to use building block circuits. 5 V is a good choice for an internal signal level; 5 V TOSLINK transmitters and receivers are cheap and 5 V power supplies are cheap to buy or build.

S/PDIF input

This is a circuit I found at various places on the internet that does a good job of converting 75 Ω digital audio into 5 V TTL. It uses a pair of TTL inverter gates (e.g. 74HC04N).


S/PDIF output

This is a circuit I found that converts 5 V TTL to 75 Ω digital audio. It uses a buffer that needs to be able to drive about 13 mA. The SN74AS1034AN can be used since its outputs can drive 5 V at 15 mA.


When you're driving more than other TTL gate inputs, it's more efficient to sink the current to ground. Here's a version of the circuit revised to use a CD74ACT04E hex inverter which can sink 24 mA.


These two circuits also feature isolated output using a pulse transformer. Output isolation is not something you find on cheap TOSLINK to S/PDIF convertors and you will sometimes hear audio dropouts when turning older fluorescent lights off.

One good choice for a pulse transformer is the Pulse Electronics PE-65612. Other part number I have not tried but should work are the Newava S22083 and the the Lundahl LL1572.

TOSLINK input/output

TOSLINK is totally easy. Well it was until Toshiba end-of-lifed all of their part numbers! These sometimes come up on ebay, usually shipping from China.

Anyway, the 5 V transmitter part is TOTX177L(F,T) and the receiver part is TORX177L(F,T). I like the 'L' versions because they have integrated spring loaded dust shutters. The data sheet for these parts recommend installing a 0.1 µF capacitor between the VCC and GND pins, no more than 7 mm from the module; I usually just solder them directly to the module.


This is the first digital audio distribution amp circuit I built. It has one TOSLINK input and four isolated S/PDIF outputs and runs on an external 5V power supply.


This is another digital audio distribution amp. This one has one S/PDIF input and four isolated S/PDIF outputs and runs on an external 5V power supply.

S/PDIF to S/PDIF 1x4

This is a 2 X 5 digital audio distribution amp that does auto input sensing. If the layout of inputs and outputs seem strange it's because it's specific to my application; I'm doing TOSLINK splitting, auto input sensing, conversion to S/PDIF and distribution all in a single box. There are two inputs and five outputs that follow these rules:

I use a retriggerable monostable multivibrator to do input selection. Since the typical pulse width for stereo 16 bit, 48 kHz digital audio is about 500 ns, I set the RC circuit to about 100 µs; since even silence has transitions, this insures that it won't switch to the low priority input unless the high priority input has really gone idle.

Auto Input Distribution Amp


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Craig Leres