What Is Aux In on a Speaker and How Does It Work?

Aux In is a wired analog input that feeds a line-level audio signal from a phone, laptop, or player into a powered speaker’s amplifier, most often through a 3.5 mm stereo jack.

One wrong jack does nothing at all — or worse, pushes amplifier power into a line-level input. Understanding what the Aux In port on a speaker actually expects turns that confusion into a simple, reliable connection. Aux In is the oldest and most dependable way to get sound from a device into a speaker: no pairing, no dropouts, no app. This guide covers what the signal is, which plugs fit, the correct hookup order, and the mistakes that quietly wreck audio quality.

What Signal Does Aux In Actually Accept?

Aux In accepts an analog, line-level signal — audio a source device has already converted from digital or read from a disc, sent at a low-voltage level the speaker’s amplifier can then boost.

The port itself is labeled AUX IN, LINE IN, or just AUX. It is not Bluetooth, Wi-Fi, or a digital protocol; it is a physical wire carrying electrical audio. A standard 3.5 mm stereo plug carries the left channel, the right channel, and a shared ground, which is why a single cable delivers both speakers in stereo.

Signal flows in one direction only. The source — phone, computer, CD player, mixer, or a turntable with its own preamp — sends audio out; the speaker receives it, applies gain, tone controls, and any processing, then amplifies it to drive the drivers. Both the source volume and the speaker volume shape the final listening level.

Which Connectors and Devices Work With Aux In?

Most consumer speakers use a 3.5 mm stereo jack, while home and pro audio gear often uses RCA or 1/4-inch inputs — and the right cable bridges almost any source to any of them.

Common setups include:

• 3.5 mm stereo (TRS): Phones, tablets, laptops, portable players, and most Bluetooth speakers.
• RCA left/right: Home audio equipment; a 3.5 mm-to-dual-RCA cable connects a phone or computer to these inputs.
• 1/4-inch TRS or combo XLR/TRS: Powered PA speakers and professional gear, where the input may be set for line, mic, or instrument level.
• USB-C or Lightning: Not Aux inputs by default — a phone without a headphone jack needs a compatible digital-to-analog adapter first.

Two hardware limits matter. Aux In belongs to powered speakers, which contain their own amplifier; a passive speaker cannot take a phone’s signal directly and needs an external amplifier or receiver. And an amplifier’s high-power speaker output must never be plugged into a line-level Aux jack.

How Do You Connect a Device to Aux In Correctly?

Plug the cable into the speaker’s AUX input, plug the other end into the source’s headphone or line output, press play, and then select the Aux mode if the speaker doesn’t switch automatically.

Marshall’s official Kilburn II instructions describe exactly this sequence: connect a 3.5 mm stereo cable to the speaker’s AUX input, connect the other end to the source device’s 3.5 mm audio output, and press play. Many speakers add one more step — hold the Mode button until the indicator shows line-in, then start playback and adjust volume.

A practical setting: start the source around 70–80% volume. That range keeps a healthy signal going into the speaker while leaving headroom, so you raise speaker volume rather than pushing the phone into distortion. If your speaker is portable and you’re weighing options, this roundup of portable speakers with aux input shows which models keep a physical jack. Marshall’s guide to using the speaker with your devices covers the input modes in detail.

The sequence works the same way on nearly any powered speaker: cable in, source playing, Aux mode selected, volume raised gradually. When it succeeds, the speaker switches to the wired source and sound comes through immediately, with no pairing prompt in sight.

Why Won’t a Turntable, Guitar, or Mic Work on Aux In?

Those devices don’t produce the line-level signal Aux In expects, so they need a preamp or interface in between — a turntable outputs phono level, which is far weaker and needs equalization, while guitars and microphones also fall short of line level.

Marshall states plainly that the Kilburn II does not provide preamplification and cannot directly accept phono devices, guitars, or microphones. A turntable must supply its own phono preamp (or run through an external one) before its output reaches the Aux input; an RCA-to-Aux cable can then carry the signal. Guitars, passive pickups, and microphones need an instrument preamp, mic preamp, mixer, or audio interface first. A connector fitting the jack is not proof the electrical levels match — that distinction is the whole game here.

Other frequent stumbles: plugging into AUX OUT instead of AUX IN (outputs send audio onward and receive nothing), using a four-contact TRRS headset plug where a three-contact TRS stereo plug is expected, and forgetting to switch the speaker into Aux mode when it also has Bluetooth or optical inputs.