Digital coaxial to RCA converter: the DAC inside

The AV receiver is gone, replaced by an active speaker pair that only accepts analog RCA. The streamer still has its coax digital output, and the drawer still has an RCA cable that fits both ends. Plugging it in gives hiss, a faint buzz, or nothing at all, because the speaker is being fed a pulse train it was never designed to read. Something in the chain has to decode the stream first, and that something needs power. The part is easy to find once you stop looking for an adapter and start looking for a converter.

What the converter has to do internally

The coax input presents a 75 ohm load and feeds a receiver that recovers the bit clock and the frames. From there the samples go to a converter chip, which turns each 16 or 24 bit word into a voltage. An analog stage then filters the steps out of the waveform, sets the output level to roughly two volts, and drives the RCA jacks through a low impedance buffer. Every one of those steps needs power, which is why the box has a USB port or a DC jack and why a passive plug cannot substitute for it. The whole chain is short but real, and each stage has to be built rather than wired. That is the real work the cable is only carrying to the box.

The converter chip and the output stage

Chip specifications matter less than the analog stage in practice. A modest converter with a well-designed output buffer will sound clean and quiet, while a better chip behind a poor buffer can hum or roll off the top end. Look for a stated output level and a signal to noise figure rather than a long list of formats that the box claims to support.

The analog output and its level

After the chip turns each sample into a voltage, an analog stage drives the two RCA jacks at roughly two volts through a low impedance buffer. The level is usually fixed or only mildly adjustable, and that buffer is what keeps the output quiet and stable. A weak or noisy output stage is the first place to look if the converter works but the sound is thin or buzzy.

Power, ground, and the hum people hear

A converter powered from a USB charger shares a ground with the charger, the source, and the amplifier. When those three sit on different outlets, small currents flow through the shield of the analog cable and appear as a steady buzz at 60 Hz. The first test is simple: unplug the coax cable and listen. If the hum disappears, the noise is entering on the digital side. If it stays, the loop involves the analog connection or the power supplies. Fixes run from using one outlet for the whole system to inserting an isolator on the analog side. A quick way to confirm the diagnosis is to touch the converter case while the hum plays: if the level changes, the ground reference is the problem, not the signal.

Breaking a ground loop without lifting mains ground

Never remove the ground pin from a mains plug, and never use a cheater adapter to float a chassis. Instead, plug the converter, the source, and the amp into the same outlet strip, try a different USB supply, or add a transformer isolator between the converter and the amp. An isolator can shave a little of the lowest bass, so test with familiar material before you commit to one.

Keeping the power supply quiet

The power brick is a common source of the hum, more often than the audio path. Use the supplied adapter, keep it off the same desk spot as the audio cable, and avoid powering the box from a cheap multi port charger. A noisy supply adds a buzz that no input setting downstream can remove, so get the power right before you chase the rest of the chain.

Sample rates the box will accept

S/PDIF over coax carries two-channel PCM to about 96 kHz and 24 bit in normal use, and many converters accept 192 kHz on paper while the source or the cable limits it in practice. Compressed 5.1 bitstreams usually pass through the coax input, but the converter outputs only two channels, so a surround stream may arrive as noise or silence depending on the box. Set the source to stereo PCM at 48 or 96 kHz before you judge the converter, and change one setting at a time. Write the setting down, because menu names differ between players and a later change is easier to reverse when you know what worked. A short readout check confirms the rate the source is actually sending, not just what is selected.

Wiring into a powered speaker or amp

Run a 75 ohm coax cable from the source to the converter input, then a short RCA pair from the converter to the speaker or amp. Keep the analog pair as short as the layout allows and keep it away from power bricks, because an unbalanced line picks up noise easily. If the speakers are self-powered with their own volume control, set the converter output fixed and use the speaker control. If the amp has no volume at all, the converter needs a level control, so check before buying. Label the two RCA plugs left and right at both ends, since a swapped pair is easy to miss on speakers that sit close together. That labeling habit saves you a long hunt the first time the volume ends up wrong.

How this differs from a passive plug

A passive RCA adapter contains no electronics, so it cannot decode anything. Feed it a digital stream and the receiving input sees a fast square wave, which it reproduces as a click, a buzz, or nothing at all. The distinction matters when shopping, because both parts look like a small shell with RCA jacks. The converter has a power inlet, a chip inside, and usually a small indicator light; the passive adapter has none of those, and no listing should be able to confuse the two. Price is a rough guide as well, because the active part costs more to build than a molded shell. Look for the power inlet and a small chip on the part before you buy the wrong one.

Inputs, outputs and power for converter boxes

Real-World Application Scenarios

Three installations show the pattern. A bedroom system pairs a streamer with a coax output and a pair of active monitors that accept only RCA, so the converter sits between them on a USB supply. In a cafe, a media player with a coax output feeds a mixer through the converter, which lets staff keep the existing analog snake. In a workshop, an old CD transport with a coax output drives a small amplifier, and the converter keeps the transport in service after the receiver failed. None of these setups needs a receiver, which is the point: a small powered box restores analog output without replacing the rest of the system.

Related Guides on This Site

For the product side, our RCA S/PDIF converter page covers the boxes and their ports. If you already own an adapter and want to know why it fails, the S/PDIF to RCA connector note explains the difference. The S/PDIF coax overview covers formats, and the coax to RCA guide covers the analog wiring.

Frequently Asked Questions (FAQ)