Article · Published 2026-10-29
An external master clock for a DAC: what it does and when you need one
Every digital source and every DAC keeps time with a clock: an oscillator that tells the circuit when to read, send and convert each sample. An external master clock is a separate unit that supplies this timing reference to the rest of the system over a cable. This guide explains what jitter is, how an external clock connects, which equipment can use one, and when a system does not need it. It ends with the facts about Charon, our master clock.
What jitter is
Digital audio is a series of samples, 44,100 or more per second for each channel. Each sample is a number, and the number is only half of the information: the other half is the moment at which it is turned into a voltage. The clock sets those moments. If the edges of the clock arrive slightly early or late, the samples are converted at slightly wrong moments, and the analogue signal carries small errors that were not in the recording. These timing variations are called jitter; our glossary of hi-fi terms gives the short definition.
Three points help to keep the subject in proportion:
- The numbers are not changed. Jitter does not alter the data; a file copied with a jittery clock is identical. It matters at the moment of conversion, inside the DAC.
- The clock nearest the converter counts most. Timing errors earlier in the chain matter to the extent that they reach the converter’s clock.
- Jitter is measured in time. Specifications give it in picoseconds or femtoseconds, or as phase noise at given distances from the clock frequency. The figures are only comparable when they are measured the same way.
Where the clock sits in a digital system
How the DAC gets its timing depends on the connection from the source:
- S/PDIF (coaxial or optical) and AES/EBU. The clock travels inside the data stream. The DAC recovers it with a phase-locked loop (PLL), and the quality of that recovery decides how much of the source’s timing errors reach the converter.
- Asynchronous USB. The DAC uses its own oscillator and asks the computer for data as it needs it, so the computer’s clock does not set the conversion timing.
- I2S. The clock signals travel on separate wires next to the data, as they do inside a DAC. Over an external I2S link, often on an HDMI-type connector, the source and the DAC must agree on which side provides the clock.
What an external master clock does
A master clock is an oscillator in its own box, with its own power supply, that sends a reference signal to one or more devices. A device that accepts it locks its own timing to that reference, so that all devices connected to the clock share one time base.
Two kinds of reference are common:
| Reference | Frequency | How the device uses it |
|---|---|---|
| Word clock | The sample rate or a multiple of it, such as 44.1 or 48 kHz | Directly as the sample timing, or as the reference for its own PLL |
| 10 MHz reference | 10 MHz, whatever the sample rate | As the reference from which its PLL derives the audio clocks |
Both travel on coaxial cable with BNC connectors: word clock usually at 75 Ω, a 10 MHz reference at 50 or 75 Ω, so check the manuals of both devices. Which frequency, signal level and connector a device accepts is set by its maker, which is why the device’s manual comes first.
In a studio, a master clock has a second job: keeping recorders, converters and mixers running in step, so that digital signals pass between them without clicks or drift. In a home system with one source and one DAC that job is smaller, and the question becomes whether the reference reaches the converter with less jitter than the device’s own clock.
When an external clock is useful, and when it is not
The answer depends on the receiving device more than on the clock:
- The device needs a clock input. A DAC or transport without a word clock or 10 MHz input cannot use an external clock. Look for a BNC socket marked “Word clock in”, “Clock in” or “10 MHz”.
- The input design decides the outcome. An external reference passes through a cable and usually through the device’s PLL. A good PLL passes the stability of the reference on to the converter; a poor one can add its own jitter. Makers who design for an external clock describe how their input works.
- Several digital devices. When a transport, a digital processor and a DAC must run in step, a common clock is the usual way to do it.
- One source into an asynchronous USB DAC with no clock input: there is nothing for an external clock to connect to.
Listeners describe changes after adding a master clock in very different terms, and some hear none. A clock that suits the receiving device, and a careful comparison at matched levels, are the fair way to judge it in your own system.
Charon: the facts
Charon is our external master clock. From its model page:
- Frequency. The clock frequency is not a fixed catalogue value: it is set for the transport or DAC the Charon will drive, so every unit is configured for its system.
- Circuit. Solid-state as standard. Tube versions use 6Zh4, EF14, EL8 or ML6 tubes, and a tube-switching module lets you compare the two.
- Power. An isolated linear power supply with several transformers, and galvanic isolation between the supply and the clock circuit.
- Outputs. Heavy-duty BNC connectors. One additional digital input or output can be added to order.
- Mains. Built for 100, 120 or 230 V with the matching plug, fixed when you order.
- Size. 300 × 200 × 150 mm, from 2 kg depending on the configuration.
- Documentation. Each unit ships with its own measurement report.
Among our other digital models, the Ulanor music computer can be ordered with a custom independent master clock by Demograf Audio. Our converters, Prometheus, Reference DAC and Hyperion, list their digital inputs on their pages. If you are considering a Charon, name your DAC and transport when you write, and we will say how they would be connected. Our guides to NOS multibit DAC chips and to the tube output stage of a DAC describe the rest of the converter.
What to check before you order a clock
- Find the clock input. Check the back panel and the manual of your transport and DAC for a word clock or 10 MHz input, and note the frequencies it accepts.
- Decide which devices share the clock. One device or several; each needs its own output and cable.
- Measure the cable runs. Keep the coaxial leads short, and match the cable to the inputs: 75 Ω for word clock, 50 or 75 Ω for a 10 MHz reference, as the manuals state.
- Send us the models. Charon’s frequency and outputs are set for the equipment you name, so we need the exact models before the build.
Questions
Can a master clock be used with any DAC?
No. The DAC, or the transport feeding it, must have a clock input that accepts the reference the clock sends. Without one, an external clock has nothing to connect to.
Word clock or 10 MHz: which should I use?
Neither has an advantage in itself. Use the one your equipment accepts; a device built for a 10 MHz reference will not lock to a word clock, and the reverse.
Does a USB DAC need an external clock?
An asynchronous USB DAC already times the conversion with its own oscillator. An external clock is relevant only if the DAC has a clock input and its maker designed it to use one.
Write to us by e-mail or WhatsApp with the models of your transport and DAC and a photo of their back panels, and we will tell you plainly whether a Charon can be connected and how it would be set.