Amplifier gain: what high and low gain change

What a gain figure means in decibels

An amplifier's output is 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. At 0 dB, called unity gain, what goes in comes out at the same voltage. +6 dB doubles it, +12 dB multiplies it by four, +20 dB by ten, and the gains of stages in series add up.

The range in real equipment is wide. The Denafrips Hades 12th preamp runs at unity gain by design, so it passes the signal on at the level it arrives and only ever turns it down. The Tonewinner AD-1PA+ power amplifier has 29.5 dB of gain, which multiplies its input voltage by about 30. A headphone amp's gain switch usually picks between two or three fixed ratios by changing the resistors that set the feedback around its gain stage.

Why the gain switch isn't a power switch

Gain, volume and power are three separate things. The volume control 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 as much power into 30 Ω as into 300 Ω. What an amplifier can deliver is capped by the voltage its supply rails allow and the current its output stage can supply. Gain raises neither limit. It changes how early in the knob's travel you reach them, which is why a knob position means nothing when you compare settings or amplifiers.

Choosing between high and low gain

Low gain suits efficient headphones, IEMs above all, because it keeps noise down and puts normal listening in the middle of the knob. High gain is for headphones that need a lot of voltage for loud peaks, typically low-sensitivity planar magnetics and high-impedance dynamic designs, where low gain can run the knob to its end stop with the music still too quiet. The HIFIMAN EF499 review pairs its low setting with sensitive IEMs and its high setting with demanding over-ear headphones.

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. The loudness is the same either way, but high gain puts the knob near the bottom of its travel and brings more hiss. A demanding headphone that needs 3 Vrms for the same peaks can't get there on low gain, which stops 3.5 dB short at 2 Vrms. On high gain those 3 Vrms sit 8.5 dB below the ceiling, with travel to spare.

What gain does to noise and channel balance

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 spare feedback to correct its own errors, so distortion can rise slightly, though in a good design it stays far below audibility. What people hear are side effects.

Noise comes first. The gain stage multiplies its own noise along with the music, and in the common layout, with the volume control ahead of that stage, the knob can't take that noise back out. Turn down to a comfortable level and the hiss stays where the gain switch put it, which a sensitive IEM will reveal. The noise floor explainer covers why sensitivity decides it.

Channel balance is second. An analog volume knob is two potentiometer tracks on one shaft, and they match worst in the first part of their rotation. High gain with an efficient headphone forces the knob into that stretch, and the stereo image can drift off center. A stepped attenuator avoids it: the Hades switches a 60-step relay ladder, and its review lists perfect channel balance at any volume among its strengths. The third side effect is level. Compare settings without rematching the volume and the louder one tends to win.

Headroom, and why gain adds none

Headroom is the margin, in decibels, between the loudest peaks your music asks for and the level at which the amplifier clips. You set the volume by the average level, and the peaks sit well above it, about 10 dB on a heavily compressed master and 20 dB or more on a dynamic acoustic recording. 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. An amplifier that clips at 5 Vrms clips at 5 Vrms on every gain setting. Gain can release swing the amplifier already had, as with the demanding headphone above, but it can't add any. More headroom takes more swing or more current, and doubling the swing adds 6 dB, which is why a balanced output of the same design usually has more.

Setting gain through a whole chain

Gain staging means setting levels so that no stage clips before the volume control has had its say. In software, an EQ boost can push a full-scale track past digital full scale and clip it before the DAC converts anything, so set a negative preamp at least as large as the biggest boost. At the DAC, 2 Vrms from RCA and about twice that from XLR are common. The SMSL RAW-MDA1 goes further, with 5.2 Vrms on XLR, 2.5 Vrms on RCA and 8.2 Vrms from its headphone output, and its review warns that some amplifiers can be clipped if you aren't careful with levels.

On the headphone amp itself, start on the lowest setting. Play your loudest track at the loudest level you 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. Keep the computer's volume at or near full so the analog control does the attenuating. If you're choosing an amplifier, the headphone power calculator works out the voltage your headphone needs, and the headphone amplifier guide ranks the amps reviewed here.