Amplifier Gain: What the Switch Changes, and What It Never Does

What gain actually is

An amplifier hands the headphone a larger copy of the voltage at its input, and gain is the ratio between the two. It is quoted in decibels, where voltage gain is 20 times the base-10 logarithm of output over input.

0 dB is unity: what goes in comes out at the same voltage. +6 dB doubles it, +12 dB quadruples it, +20 dB multiplies it by ten, and gains in series add. A gain switch picks between two or three fixed ratios, usually by switching the resistors that set the stage's feedback.

Gain, volume and power are three different things

The volume control adds nothing. It is an attenuator that can only take signal away, and gain sets the ceiling it works under: the most voltage the knob can ask for, fully up, from a given source.

Power is voltage squared divided by the load's impedance, so one volt delivers ten times the power into 30 ohms that it does into 300. What an amplifier can deliver is capped by the voltage its supply rails allow and the current its output stage can source. Gain raises neither limit; it changes how early in the knob's travel you reach them, which is why a knob position means nothing across settings or amplifiers.

Does gain change the sound?

At the same listening level and short of clipping, a competent amplifier sounds the same on either setting. Higher gain leaves a feedback amplifier less feedback to correct its own errors, so distortion can rise slightly, but in a well-designed circuit it stays far below audibility. What people hear are side effects.

Noise is the first. The gain stage multiplies its own noise along with the music, and in the common layout, where the volume control sits ahead of that stage, the knob cannot take it back out. Turn down to a comfortable level and the hiss stays where the gain switch put it, and a sensitive IEM reveals it.

Channel balance is the second. An analog volume control is two potentiometer tracks on one shaft, and they match each other worst in the first stretch of rotation. High gain with an efficient headphone forces the knob into that stretch, and the image can drift off centre. A stepped attenuator or a resistor-ladder volume control avoids it.

The third is level: compare settings without rematching volume and the louder one wins, as louder almost always does.

High gain or low gain?

Low gain suits anything efficient, IEMs and easy portable headphones above all. It keeps the noise down and puts normal listening mid-knob. High gain is for headphones that need a lot of voltage for loud peaks, typically low-sensitivity planar magnetic designs and high-impedance dynamic drivers, where low gain can run the knob to its end stop with the music still short.

Round numbers make it concrete: a DAC with a 2 Vrms output feeding an amplifier with two settings that can swing 8 Vrms.

An IEM that reaches your loudest peaks at 0.1 Vrms needs the volume control to cut 26 dB on low gain and 38 dB on high. Same loudness, 12 dB more attenuation, a knob near the bottom and more hiss.

A demanding headphone that needs 3 Vrms for the same peaks cannot get there on low gain: fully up, the knob stops at 2 Vrms, about 3.5 dB short. On high gain those 3 Vrms sit about 8.5 dB below the ceiling, with travel to spare.

Gain staging from the DAC to your ears

Gain staging means setting levels through a chain so that no stage clips before the volume control has had its say. There are three places to look.

Digitally, an EQ boost can push a full-scale track past digital full scale, clipping it before the DAC converts anything; a negative preamp at least as large as the curve's highest boost prevents it.

At the DAC's output, roughly 2 Vrms from RCA and about 4 Vrms from a balanced output are the common full-scale levels, so moving between them is itself a 6 dB change. Feed 4 Vrms into an amplifier on +12 dB and full volume asks for 16 Vrms, more than many headphone amplifiers can swing: past that point the knob is adding clipping, not level.

At the amplifier, the output clips at the limits of its rails and output stage, and a design with a gain stage ahead of its volume control can clip on a hot source whatever the knob says.

Headroom, and why more gain is not more of it

Headroom is the margin, in decibels, between the loudest peaks your music asks for and the point where the amplifier clips. Music is not a steady tone: the level you hear is set by the average, and peaks sit well above it, around 10 dB on a heavily compressed master and 20 dB or more on a dynamic acoustic recording. The amplifier has to swing that much further, instantly, on every drum hit. Clip those peaks and their tops are flattened, which usually registers first as hardness and lost impact.

Headroom is fixed by the amplifier's maximum voltage swing and the current it can deliver into your headphone. Gain only decides how far round the knob you travel to reach that limit, so an amplifier that clips at 5 Vrms clips at 5 Vrms on every setting. Gain can release swing the amplifier already had, as with the demanding headphone above, but it cannot add any. More headroom means more swing and more current: doubling the swing adds 6 dB, which is why a balanced output of the same design usually has more.

How to set the gain switch

Start on the lowest setting. Play your loudest, most dynamic track at the loudest level you actually listen at, and look at the knob. If it is near the top with nothing left, step up one setting. If normal listening sits in the first stretch of travel, or you hear hiss between tracks, step down. The target is the lowest gain that reaches your loudest listening with the knob in its comfortable middle range.

Keep software volume at or near full so the analog control does the attenuating. If you are choosing an amplifier rather than setting one, the headphone power calculator works out the voltage and power your headphone needs, which tells you whether you need high gain at all.