DACs, external clocks, and what's what

"... a well designed DAC with a high quality internal clock cannot benefit from an external clock ..."

"Cannot" is a strong word. A well-designed DAC featuring internal clocks (usually SC OXCO) that are galvanically isolated from noise, have their own separate low-noise power supply, and demonstrate very low phase noise, is a fairly expensive DAC. It may not deliver clock performance that approaches a Mutec REF10 SE120 external clock (6500 USD), but designed competently, it pushes what the best external clocks can improve well into the realm of diminishing returns. Conversely, relatively inexpensive DACs with external clock inputs have become fairly popular, and can often be improved to some extent with external clocks <1000 USD that offer strong phase noise performance (e.g. LHY OCK-2S). External clocks aren't limited to what can be fitted into the confines of a DAC enclosure, so there is space to improve hardware and performance ... in theory anyway.

We should note that unless the DAC has an internal PLL (phase locked loop), its clocks are engaged only when the input is via USB or a direct ethernet connection anyway. If the connection type is SPDIF, AES EBU, or I2S, we’d want to start looking at the source component (or DDC) for an external clock port. 

ted_b

"We should note that unless the DAC has an internal PLL (phase locked loop), its clocks are engaged only when the input is via USB or a direct ethernet connection anyway. If the connection type is SPDIF, AES EBU, or I2S, we’d want to start looking at the source component (or DDC) for an external clock port."

I was under the impression that all modern DAC's have buffers, that then use the internal clock.  Older DAC’s without the buffers used the external clock.

I was under the impression that all modern DAC’s have buffers, that then use the internal clock.  Older DAC’s without the buffers used the external clock.

@glennewdick USB audio data is asynchronous, and is fed into a FIFO buffer in the DAC. The release of data from the DAC’s buffer is timed by the DAC’s internal clock. Open a DAC, and its clocks can usually be found on the USB transceiver board.

Data transmitted to the DAC via SPDIF/AES EBU/I2S connections differs in that it is synchronous, containing both audio and clock data from the source. The source holds the master clock, and the receiver (DAC) is the slave. As such, source clock quality is a defining influence of overall audio quality, and expensive streamers put emphasis on clock architecture for this reason. A few upmarket DACs (e.g. Gustard A/X/R-26 and later, Holo Audio May, etc.) have a PLL feature that allows the DAC to slave the source clock to its own.

Popular external DDCs (Digital-to-Digital Converter) virtually all convert USB to a synchronous format (SPDIF/AES EBU/I2S) that makes the DDC the master clock in the chain. Predictably, they also emphasize clock accuracy as a selling point.

Some may find this pro sound take on the issue informative.

Is adding a clock a waste of money?  Maybe, but one thing is for certain is that they do change the sound. Is it the clock or is it the other goodies in the box? You’d think that buying that $30K Refrence DAC would sound incredibly, but then the same company starts selling a matching $30k clock and a $30K power supply and you begin to wonder.