What is a DAC, and when does an external one help?

From numbers to voltage

A music file is a long list of numbers, each one the height of the waveform at one instant, taken 44,100 times a second for a CD and more often for hi-res files. A DAC reads those numbers at exactly the right pace and produces a voltage for each. That raw output still carries unwanted energy above the audio band, so a low-pass filter removes it and leaves the continuous waveform the numbers describe. An analog stage then brings it to line level for an amplifier or a headphone amp.

The file sets the limits. Its sample rate decides the highest frequency it can hold, and its bit depth decides how far down its noise floor sits. A converter can fall short of those limits, and it can never exceed them. An "upsampling" mode changes the digital filtering; it can't restore anything the recording never contained.

What sits inside the box

Every DAC, from a dongle to a full-size unit, does the same four jobs, and most of its price goes on how well each is done.

• A receiver takes the signal from USB, coaxial, optical, HDMI or Bluetooth and recovers the data.
• A clock sets the moment each sample is converted. Timing error here is jitter.
• A converter turns each sample into a voltage. Nearly all current designs use a delta-sigma chip, and a minority use an R-2R resistor ladder.
• An analog output stage filters, buffers and amplifies the result, powered by a supply that has to stay clean.

The Fosi Audio ZD3 lays those parts out plainly: an XMOS USB interface, an ESS delta-sigma chip, and op-amps in the analog stage that you can swap to change the voicing. The Henry Audio DA 256 spends €1,600 on the parts around a Texas Instruments PCM5142 chip. Its coaxial and optical inputs are buffered and replayed on the DAC's own clocks, and the power feeding the chip is filtered hard. The review calls the chip the least important part of a DAC, and this one takes that idea as far as it goes.

Why you already own several

A phone, a laptop, a TV, a games console and most streamers each contain a DAC, because none of them could make a sound without one. Buying an external DAC is a replacement, so the question to ask is whether the converter you have is the weakest link in your system.

Often it isn't. Converter chips became very good and very cheap, and the speakers, the room and the recording usually outrank them. The site's review of the $29 SMSL PS100 says as much: DACs make less of a difference than amplifiers, and far less than the headphones or speakers, which is why the review frames the PS100 as a way to free up budget for a better amp.

When an external DAC is worth buying

Four problems justify one, roughly in the order you're likely to meet them.

• Your current source is noisy. A converter sharing a crowded circuit board with a processor, a radio and a switching power supply can pick up hiss or a whine that follows screen or disk activity, most audible on sensitive IEMs. A DAC in its own box, on its own supply, moves the conversion away from that.
• Your headphones need more drive than the source has. Many external DACs include a headphone amplifier, and for headphones a phone can't drive, the amplifier is the upgrade. The conversion was never the problem there.
• You need a connection you don't have, such as optical or HDMI ARC from a TV, coaxial from a CD transport, balanced XLR to studio monitors, or two outputs at once. The ZD3 takes HDMI ARC straight from a television.
• You need a different kind of level control. Active speakers with no volume control of their own need a DAC that has one, while a preamp wants a fixed output. The ZD3 offers both through its bypass mode, and the DA 256 has no volume control at all.

How to read a DAC spec sheet

Output level is the first line to read. Around 2 Vrms from RCA is the usual line level, and balanced outputs typically give about twice that: the ZD3 lists 5 V RMS on XLR and 2.5 V RMS on RCA. A hotter output is easy on a low-gain amplifier and can overload a sensitive one. Output impedance matters when it is high. The Denafrips Enyo 15th lists 625 Ω on RCA and 1250 Ω on XLR, so the amplifier after it needs a correspondingly high input impedance.

Signal-to-noise ratio and dynamic range say how far the noise sits below full output, and modern DACs commonly quote 110 dB or more. THD+N sums distortion and noise at one test level and frequency, usually 1 kHz. Once both figures are good, their exact values predict little about what you'll hear, which the harmonic distortion explainer goes into.

Format support lists what a DAC will accept, per input. The PS100 plays 24-bit/192 kHz over optical and coaxial but stops at 48 kHz over USB, which matters if a computer is your only source. Before paying for hi-res support, the hi-res audio checker shows whether your files contain anything above CD bandwidth at all.

What a DAC can't change

A DAC sits at the start of a chain whose later links move air, and those links have far more say over the result. It won't fix a room's bass modes, a harsh recording or a headphone that doesn't suit you. Change it when it causes a problem you can name, such as noise, a missing input, too little output or no volume control. The DAC guide ranks the converters reviewed here by score, from budget boxes to an R-2R ladder design.