Optical to Phono Cable: The DAC Stage in Between

Someone reads optical out on the back of a television, spots a pair of RCA jacks on an older amplifier marked phono, and goes looking for a lead with a square plug at one end and two RCA plugs at the other. No such passive part exists. Orders for it come back canceled, or arrive as a small box with a power port. The mismatch is not a naming problem that a better search would fix. One socket emits light pulses that encode numbers, and the other accepts a voltage that rises and falls with a groove.

Why One Lead Cannot Join These Two Sockets

A Toslink port has no audio voltage on it. It has a light emitter behind a shutter door, and what comes out is a stream of frames: numbers, clocked, with a fixed structure defined by the S/PDIF standard. Nothing in that stream is proportional to the music in a way a phono stage could amplify. An RCA phono input is the opposite kind of port. It expects a small AC voltage, roughly 0.2 to 5 mV depending on the cartridge, and it applies about 40 dB of gain plus the RIAA equalization curve before the signal reaches the volume control. A lead that spliced the two would carry light into a circuit that measures volts. A photodiode at the far end would produce a digital waveform, not audio, and there is no clock recovery, filtering or conversion inside a plug barrel. That is why every working product sold under this name has a box in the middle with a power inlet.

What the Toslink socket sends

The socket sends a framed digital stream over a plastic or glass core, using red light near 650 nm. A stereo pair of PCM channels at 44.1 or 48 kHz is the normal payload, and 96 kHz appears on better gear. Compressed multichannel bitstreams also travel this way, but only a decoder makes sense of them. Nothing in the link is ground referenced, which is why a fiber run cannot hum.

What an RCA phono input expects

The jack is a plain RCA barrel; the label beside it describes the gain stage behind it. That stage expects a moving magnet cartridge at about 3 to 5 mV or a moving coil at 0.2 to 5 mV, loads a moving magnet at roughly 47 kohm, and applies the RIAA curve. Its input capacitance is a few tens of picofarads and it is deliberately sensitive. Line level on that jack overloads the first transistor.

The DAC Stage That Makes the Conversion

Between the light and the RCA plugs there is always a converter, and that stage sets what you hear. The chain inside is short: a photodiode turns light back into a logic level signal, a receiver chip locks to the frame clock and splits the two channels, a DAC turns the samples into voltage, and an output buffer drives the RCA pair. The buffer is not a detail. A converter with a weak output stage will sag into a long cable and a low impedance load, and the symptom is thin bass rather than obvious distortion. Two products do this job. A standalone converter box is the flexible choice: it takes power from USB or a small adapter, offers one optical input and one RCA pair out, and costs little. A receiver or integrated amplifier with an optical input does the same work inside a bigger chassis and adds switching, volume control and often room correction.

Converter box or a receiver input

Use the receiver when the optical port is already there and the amplifier is the last box in the chain. Use a standalone converter when the destination has only analog inputs, or when the television sits far from the amplifier and a short optical lead is easier to hide than a long analog pair. A converter with its own volume knob helps where the destination is a bare power amp.

Power and ground for the converter

Most small converters run from a USB port or a 5 V adapter, and the supply is where noise enters. A switching adapter can leave hash on the analog output, which shows up as a faint whine under quiet passages. Keep the converter and the amplifier on the same outlet strip so their grounds sit at one potential, and keep the adapter away from the RCA leads. The optical side is already isolated; the analog side is not.

Which RCA Input the Analog Output Belongs On

The converter emits line level, about 1 to 2 V RMS, and that number decides the destination. Line level belongs on an aux, tuner, CD or tape input, where the stage expects a volt and attenuates it. A phono input expects millivolts and adds 40 dB of gain plus RIAA, so a line signal arriving there is clipped before the volume control has moved, and the equalization curve tilts whatever survives. The result is a harsh, thin, distorted sound at any usable level. Wiring is the easy part once the destination is right. The left RCA goes to the left jack, the right to the right, and the shield of each is the return. The connection is unbalanced and single ended, with the shield carrying both the return current and any noise it picks up. Keep the pair together, keep it short, and keep it away from mains leads.

Latency, Channel Count and Realistic Expectations

A converter adds delay, usually a few milliseconds for input buffering and frame alignment, which nobody notices on audio alone but which can show as lip sync error when a television picture shares the room. Better boxes trim it; cheap ones do not. Channel count is the harder limit. Toslink carries two PCM channels at full resolution, and while a compressed 5.1 bitstream can travel the same fiber, a two channel converter either ignores the extra channels or refuses the stream outright. Sample rate is the third limit. Most converters lock to 32, 44.1, 48 and 96 kHz, and a few choke on 192 kHz. Tone is not a variable: a working converter on a clean supply is transparent enough that the amplifier and speakers dominate.

Signal path from an optical output to an analog input

Real-World Application Scenarios

A hotel lobby with a wall mounted television and ceiling speakers fed from an older amplifier is the common case. The set has one optical output and the amplifier has RCA inputs only, so a converter sits behind the television on the bracket, powered from a USB port, with a one meter fiber lead and a short RCA pair down to the amplifier. The second case is a clinic waiting room where the set and the amplifier sit in different cabinets; a three meter optical lead reaches the cabinet, the converter sits with the amplifier, and the analog run stays under half a meter. The third is a church overflow room where a disc player feeds an amplifier that predates HDMI; the converter is set once to 48 kHz.

Related Guides on This Site

The powered part that performs the conversion is covered in the optical to RCA adapter note, and the boxed version with its own supply is described on the Toslink to RCA converter page. For the analog side, the phono cable overview explains the pair and the ground, and the coaxial S/PDIF digital audio page covers the copper alternative to fiber.

Frequently Asked Questions (FAQ)