Why Some Headphone Amplifiers Have A Noticeable Noise Floor

In most cases, the hiss you hear between tracks is the amplifier’s own heat noise, pushed up by too many gain stages for a sensitive pair of headphones. Quick check: disconnect the source, turn the volume up, and listen. If a steady hiss fills the silence, the amp is the cause.

High gain multiplies the noise floor. Every resistor, op-amp, and transistor in the signal path generates its own tiny bit of thermal hiss, and the total gain of the amplifier pushes that hiss into the audible range.

Sensitive in-ear headphones force the amp into a high-gain setting. Because these drivers need very little power, the amplifier must work harder, lifting its own noise along with the music.

A hum or buzz layered on the hiss points to power-supply ripple or a ground loop, not to the thermal noise itself.

For low-sensitivity over-ear headphones, a simpler, lower-gain amplifier keeps the noise floor lower because the signal needs less amplification and the noise is multiplied by a smaller factor.

Does This Sound Familiar?

You hear a steady, soft hiss when no music is playing, especially when the volume is turned past the halfway mark.
The hiss is clearly louder with your in-ear or IEM headphones than with your over-ear cans.
Switching to a different pair of over-ear headphones makes the background noise drop below the level where you can hear it.
You notice a low hum or buzz under the hiss, and it gets worse when the amp is plugged into the same power strip as a laptop charger or a monitor.
The noise is present on both channels at the same level, so it is not a bad cable or a loose connection.
Turning the volume down makes the hiss quieter, but the music gets quieter too, so the ratio between them stays roughly the same.
The hiss is more noticeable in a quiet room than in a room where a fan or air conditioner is running.

Most Likely Causes First

Cause	Why it produces this symptom	How common
Excess gain stages matched to a high-sensitivity driver	Each gain stage adds its own thermal noise, and the total gain multiplies all of it up to the headphone output	Very common with IEMs and sensitive in-ears
Power-supply ripple bleeding into the audio path	A switched or under-filtered supply lets 50 or 60 Hz mains frequency leak through, adding a hum on top of the hiss	Common with USB-powered and budget desktop amps
Ground-loop coupling between two devices on different circuits	Two ground references at slightly different voltages create a small circulating current that the amp picks up and boosts	Common in multi-device desk setups
A single high-gain topology where two lower-gain stages would do	One big gain block produces more output noise than two moderate stages stacked in series	Less common, but seen in compact portable amps
An under-specified op-amp or transistor at the output stage	Cheaper active devices generate more intrinsic noise at the same gain	Moderate, more frequent in entry-level gear

What Is Physically Going On

The top cause is the first row in the table: too many gain stages for the driver you are using. Here is the physical picture.

Think of the signal path like a series of water pumps in a pipe. Each pump adds a little turbulence to the water as it pushes it forward. A one-pump system keeps the water relatively smooth. A four-pump system stirs up more turbulence with each hand-off. By the time the water reaches the far end, the combined turbulence is easy to feel. In a headphone amplifier, each gain stage (a block of electronics that raises the voltage level of the signal) adds its own thermal noise (the random electron movement inside every resistor and semiconductor, present even at room temperature). The total noise at the output is the sum of all those small contributions, multiplied by the total gain (the factor by which the amplifier boosts the input signal).

Early three-stage tube amplifiers from the 1910s reached a total voltage gain of about 125 across three hand-offs, and each stage contributed a share of the noise. Modern solid-state designs follow the same principle. The more stages you stack to reach a target gain, the more thermal noise rides along.

With a sensitive in-ear monitor that needs only a few milliwatts to sound loud, the amplifier must be set to a high gain to reach listening volume. That high-gain setting multiplies the amplifier’s own thermal noise right up into the range your ears can pick up. With a low-sensitivity over-ear headphone that needs more power, the gain can stay lower, so the noise stays below the threshold.

Johnson-Nyquist noise (the heat-generated electrical jitter in any resistor or semiconductor) is the raw ingredient. Output-referred noise is what you actually hear: the same jitter, scaled up by the total gain of every stage before the signal reaches the driver. Audio amplifiers are designed to work with signals below 20 kHz, and the thermal noise spreads across that entire band, which is why it sounds like a smooth “shhh” rather than a tonal hum.

The Fix, Step By Step

Match the gain setting to the headphone. Turn the amplifier’s gain switch to the lowest setting that still gives you comfortable listening volume. This changes the total noise multiplication factor and drops the hiss.
Separate the power path. Move the amplifier to a different wall outlet or a filtered power strip, away from the laptop charger, monitor, or dimmer switch. This changes what mains ripple and 50 or 60 Hz hum reach the audio circuit.
Break the ground loop. If two devices are plugged into different power strips on different outlets, connect them with a single short cable run or use a ground-lift adapter on one of the connectors. This changes the reference voltage difference that was feeding the hum into the amp.
Reduce the number of active stages in the chain. If you are using a DAC, a preamp, and a separate power amp, try feeding the headphone directly from the DAC’s headphone output or from the preamp output. This changes the total number of noise-adding blocks the signal passes through.
Use the low-gain input if one is available. Many desktop amplifiers offer a “low-gain” or “sensitivity” input designed for IEMs. Plugging in here changes the internal gain structure and typically cuts the output noise floor by several decibels.

Prove It Worked

Disconnect the source. Turn off the phone, computer, or streamer so no signal is reaching the amp.
Set the amplifier to the gain level you just chose in the fix section.
Turn the volume to your normal listening level.
Close your eyes and listen for ten seconds of pure silence. You should hear either nothing or a hiss so low it sits below the sound of your own room.
Now reconnect the source and play a track you know well. During the quietest passage (a long rest between verses, a soft instrumental break), the background should be the room, not the amp.
If the hum is gone but a faint hiss remains, confirm it drops below your hearing threshold when you move to the other gain setting. The fix is working if the ratio between music and noise has improved.

Keeping It From Coming Back

Keep the gain at the lowest workable setting. Usually you reach for the high-gain switch, you are rebuilding the noise multiplication. Make it a habit to check the gain position before you start listening. This ties back to the first cause in the table: excess gain stages.
Run one filtered power cable for the whole audio rack. Tuck the amp, the source, and the DAC into the same filtered strip on a single outlet. This stops ripple and ground-loop hum from re-entering the signal path whenever you rearrange the setup. It ties to the second and third causes in the table.
Pair the amp with a driver that matches its gain range. A high-gain portable amp suits a sensitive IEM. A low-gain desktop amp suits a low-impedance over-ear. When the pairing is right, you stay in the low-gain zone and the noise floor stays low.
Route audio cables away from power and mains wiring. A headphone cable running parallel to a wall outlet or a charging cable picks up 50 or 60 Hz fields and adds a faint buzz. A short twist or a few inches of separation stops that coupling from returning.

Troubleshooting Questions

Does The Hiss Change When I Switch To Over-Ear Headphones?

If the noise disappears or drops well below the room sound with over-ears, the gain setting is too high for the IEM you are using. The amplifier is working fine; it is simply multiplying its own thermal noise because the sensitive driver needs a big boost. Drop to the lower gain setting and re-listen. The fix is a switch position, not a new part.

Is There A Low Hum Under The Hiss, And Does It Shift When I Unplug The Source?

A steady 50 or 60 Hz tone that vanishes when you pull the USB cable or move to a different outlet points to ground-loop coupling or power-supply ripple. The thermal hiss stays constant, but the hum layer changes with the connection. Unplug the source, listen for the hum alone, then move the amp to a different outlet. If the hum tracks the outlet, you have a mains coupling problem, not a gain problem.

Does The Noise Get Louder As I Turn The Volume Up?

Thermal noise at the output rises with volume because the gain is fixed but the quiet parts of the signal are smaller relative to the noise floor. If the ratio between music and hiss stays the same no matter where you set the volume, the issue is the gain stage count. If the hiss stays the same while the music gets louder, you are simply in the low-signal part of the track and the noise floor is normal for that amplifier.

Will A Different Amplifier Fix The Hiss, Or Is It The Source?

Disconnect the source and crank the amp volume. If you still hear hiss, the amplifier is the source of the noise. If the silence is clean, the source (a phone, a DAC, a computer) is adding its own noise floor into the chain. In that case, the amp gain is not the problem; the source electronics are. Swapping in a cleaner source or reducing the number of boxes between the file and the amp will lower the floor.

Does The Noise Come From One Channel Only?

If the hiss or hum is clearly louder in the left or right ear, the issue is usually a ground loop on that channel’s cable path, a cracked solder joint, or a faulty jack. Thermal noise from the gain stages is symmetric by design, so a one-sided problem is typically not a gain issue. Inspect the cable, reseat the plug, and swap in a known-good cable. If the problem follows the cable, the cable is the culprit. If it stays on one side even with a new cable, the amplifier’s internal wiring on that channel needs a closer look.

(Original publication: hifireport.com. No part may be republished without consent.)