Monitoring level

A fixed, recallable monitoring level#

The second you turn the monitors up or down, you change how you perceive the tonality of the music. That's the Fletcher–Munson curves. If you keep fiddling with the monitor gain, you can chase your tail and never finish a mix. I've been shouting this from the rooftops the whole time I've been making content.

It applies to headphones just as much. The point of a headphone check is to assess the mix with the room and other variables removed. If you listen at a random, uncalibrated level, you've just put a massive new variable back in. You don't always have to listen at the reference level. But you need to be able to return to it exactly, every time you critically assess a mix or a master.

Recalling a level by number#

I set my headphones by typing a number into the computer, not by turning a level pot.

A dot of tippex on the knob isn't accurate enough. Cheap pots also often have channel-balance tracking errors that grow as you turn them down. I measured one wandering by about 7 dB at low settings, which is like panning your headphones off to one side, and sensitive headphones are often used at exactly those settings. Better pots track much better, but you still can't recall them. You need digital or relay-based level control. That's why a pot on an audio interface headphone output is just a bad design.

Output impedance is the headphone amp metric that matters#

Every competent headphone amp is ruler flat from 20 Hz to 20 kHz. One that isn't is broken or junk. Audible differences between amps come from output impedance, which should be near zero. A high output impedance forms a voltage divider with the headphone driver and shifts the response by 2 to 3 dB, mostly in the bass and lower mids. How much depends on the headphones, which makes it unpredictable.

Expensive headphone amps#

Slew rate, intermodulation, noise floor, crosstalk and power supply are solved problems in anything that isn't junk, and they distract from what matters. Output voltage matters mainly for high-impedance headphones.

An expensive amp can be genuinely excellent engineering: the Benchmark HPA4 was perfect on every metric I tested. A studio usually doesn't need one. A cheap unit with a recallable level and low output impedance does the job, and so does the headphone jack on a Mac. Its output is under 3 V, so very high-impedance headphones might need more, but it drives HD600s just fine.