Matching a DAC and Headphone Amp to Your Headphones

A desktop audio chain has two jobs that people routinely confuse. The digital to analog converter turns a digital stream into an analog signal. The amplifier takes that low level signal and drives it hard enough to move the drivers in your headphones. They are separate functions even when they share a box, and they fail in different ways.

Start with the headphones, not the gear

The correct question is never which amplifier is best. It is which amplifier suits the headphones you own. Two specifications decide that.

Impedance is the electrical load the headphones present, measured in ohms. Low impedance headphones draw more current for a given voltage; high impedance ones need more voltage to reach the same level. Sensitivity describes how much sound pressure you get for a given input, usually quoted per milliwatt or per volt. Read carefully which of the two conventions a manufacturer used, because the numbers are not interchangeable and comparing across them produces nonsense.

Together these tell you whether a source can drive the headphones at all. Sensitive low impedance models run fine straight out of a phone. High impedance or notably insensitive planar designs need real voltage or real current respectively, and starve on weak outputs. Starving sounds like thin bass and audible strain before you reach a satisfying volume, not simply quiet playback.

The conversion from those two published figures to a required power is arithmetic rather than judgement. Enter the impedance, the sensitivity and a target peak level into the DAC and amp power sizer and it returns the milliwatts and the RMS voltage an amplifier has to supply, which is the number to compare against a product’s rating at the same load. If you have not yet decided whether an external unit is warranted at all, start with the buyer’s guide to whether you need a DAC and amp.

Output impedance and the damping factor rule

An amplifier also has its own output impedance. The usual guidance is that it should be low relative to the headphone impedance, commonly cited as a ratio of at least eight to one.

This matters because headphone impedance is not flat across frequency. It varies, often with a large peak near the driver resonance, and that is especially pronounced in dynamic driver designs. A high output impedance interacting with that varying load produces a frequency response change you can hear as boomy or uneven bass. It is one of the few genuinely audible amplifier effects, and it is entirely predictable from the specifications.

The eight to one figure is a rule of thumb rather than a standard, and it is worth knowing what fixes it there. An amplifier’s output impedance sits in series with the headphone, forming a voltage divider whose ratio tracks the headphone’s impedance curve. Hold the source at an eighth of the load or below and the resulting response deviation stays near one decibel, which is about the smallest level difference most listeners reliably notice, so the interaction becomes inaudible rather than merely small.

Published standards point the other way, which is worth knowing before someone cites one at you. The first edition of IEC 61938, issued in 1996 and withdrawn in 2013, recommended a 120 ohm source impedance for headphone outputs so that a single jack could drive anything from 8 to 600 ohms at a tolerable level. Amplifier designers never adopted it and portable-player makers abandoned it, because 120 ohms in series with a 32 ohm headphone both starves it and reshapes its response. Treat 120 ohms as a historical artefact you may still meet on an older receiver, and the eighth-of-the-load ratio as the working target.

What measurements do and do not tell you

Distortion and noise measurements summarize how faithfully a device reproduces its input. SINAD combines signal relative to the sum of noise and distortion into one figure, and higher means a cleaner output. THD plus N describes the same territory as a percentage.

These numbers are useful as a floor. A device with poor measured performance can be audibly flawed. But above a certain level of competence the differences fall below what anyone can hear, and further improvement on a chart buys nothing in listening. Once a source is transparent, spending more on the source changes nothing; spending on headphones changes everything, because transducers are where the large and clearly audible differences live.

Also note that a measurement taken into a high resistance load says nothing about behavior into a demanding one. Power capability and distortion under load are different questions from distortion at idle.

Practical selection

Work through it in order. Confirm your headphones’ impedance and sensitivity. Check that the amplifier can supply enough voltage and current for that combination with headroom left over. Check that its output impedance is low relative to your headphones. Then pick a converter that measures competently and has the inputs you need.

Features often matter more than the last increment of measured performance: the input types you actually use, a usable volume control with good channel balance at low settings, a balanced output if your headphones use one, and the ability to switch between headphones and speakers.

Common mistakes are buying an expensive converter to feed an inadequate amplifier, assuming more power is always better when a very powerful amp on sensitive headphones can leave you with all your usable range in the first fraction of the volume knob, and chasing chip model names as if the converter chip determined the sound rather than the implementation around it.

Where to go next

Once the arithmetic says an external device is warranted, the remaining decisions are architectural and practical rather than theoretical.

• Whether the purchase is justified at all: do you need a DAC and amp?
• One box or two, and what each trade actually costs: combo DAC/amp versus separates
• If the reason you are shopping is a noise rather than a lack of volume: headphone amp hiss and hum, causes and fixes
• To turn your own headphones’ figures into a required output: the DAC and amp power sizer