Hi-fi connections explained: what plugs into what?

Start with the labels at both ends

Use this three-step check for every link in your system’s signal path:

1. Identify direction. Write down the source’s output and the next device’s input. A socket labelled OUT is not interchangeable with one labelled IN.
2. Match the signal. Establish whether it is analog line, phono, speaker-level or digital. For digital audio, also check the receiving device’s supported format.
3. Choose the documented connection. Use the appropriate cable, any required converter, and the input/output settings described by both manuals.

A useful connection note looks like this: “Player ANALOG OUT L/R → amplifier LINE IN L/R.” “RCA cable” alone leaves out the information that makes the connection work.

Analog connections: the level and job matter

Line level is the analog signal used between source and control/amplifier components. PHONO identifies a cartridge signal needing the appropriate phono stage. Speaker-level is the power-amplifier output used to drive a loudspeaker. A headphone output has its own drive function and volume behaviour; it should not automatically be treated as a fixed line output.

RCA labelled ANALOG OUT or LINE OUT

Route: analog output L/R → compatible LINE IN or AUX IN L/R. Match the channel labels. Check the output level and whether volume controls it.

RCA labelled PHONO

Route: cartridge/phono output → compatible PHONO input. Check cartridge/stage compatibility and any turntable PHONO/LINE switch.

XLR labelled analog audio

Route: analog output → compatible analog input. Check the wiring, input/output specifications and balanced-interface support.

3.5 mm or 6.35 mm jack

Route: HEADPHONES output → headphones, or a documented LINE output → line input. Confirm the socket’s actual job; diameter and contact count do not settle it.

Speaker terminals and speaker-wire plugs

Route: documented power-amplifier output → intended speaker connection. Check load requirements, polarity and terminations. Establish whether the terminals instead link a supplied powered pair.

The same RCA shape can also carry digital audio. Likewise, Esoteric’s connection manual, pages 10–11, documents separate analog and digital uses of XLR. Follow the full label, not a connector’s appearance or colour.

Do not connect a power amplifier’s speaker output to an ordinary line input: the speaker-level output can damage that input. Focusrite’s line- and speaker-level explanation directly documents this distinction. Inputs explicitly designed and documented for speaker-level signals are a different case; follow their stated compatibility and connection instructions.

A balanced connection depends on the interfaces and wiring. An XLR-to-RCA adapter does not create balanced source electronics. Rane’s interconnection note explains why the circuit arrangements matter; there is no single beginner wiring recipe for every output.

A TRS jack has tip, ring and sleeve contacts. The same contact arrangement can carry balanced mono line audio or unbalanced stereo headphone audio; Focusrite’s connector explanation gives both uses. Contact count alone cannot identify which signal a socket expects.

Speaker-wire terminations such as banana plugs and spades are described in Klipsch’s connector reference. Their physical fit does not identify a socket’s role. In the documented powered-pair example, the terminals link the companion speaker, and the instructions prohibit connecting them to a receiver’s or amplifier’s speaker outputs. Use the exact system’s instructions for any proposed high-level connection.

Digital connections: the receiver must understand the data

Digital connections deliver encoded audio to equipment that handles that digital interface and format. A matching socket is only part of the route.

RCA labelled COAXIAL DIGITAL

Route: digital OUT → compatible coaxial digital IN. Check supported formats and use the specified digital cable. This is not an analog RCA input.

XLR labelled DIGITAL

Route: digital OUT → compatible digital IN. Use the documented digital interface and cable, rather than analog XLR assumptions.

Optical digital

Route: optical OUT → optical IN. Check supported audio formats and output settings.

USB AUDIO

Route: compatible computer/device → documented USB audio input. Check device roles, the data connection, software requirements and selected audio output.

USB MEDIA

Route: storage device → documented media-reading port. Check supported storage and file formats. Do not assume the port accepts computer audio.

HDMI ARC/eARC

Route: TV audio → compatible audio equipment through the return-channel ports. Check the required function at both ends, the correct ports, supported formats and settings.

Cambridge Audio’s manual provides a concrete distinction between USB Audio and USB Media. Those are different tasks even when the ports share a connector family. The HDMI Licensing Administrator’s eARC overview explains TV audio return; an ordinary HDMI input elsewhere on the equipment does not establish that route.

A digital input may accept only certain audio formats. For example, the Denon playback instructions specifically exclude non-PCM input. PCM is a digital audio representation; a compatible stereo PCM setting can matter when connecting a TV. Check the actual receiver or converter instead of assuming it decodes every format the TV can send.

Two cases where matching plugs are not enough

Optical TV output and an analog-only amplifier

The TV sends digital audio; the amplifier’s LINE IN expects analog audio. A passive optical-to-RCA plug adapter cannot do that conversion.

TV optical OUT → compatible digital-to-analog converter → analog LINE OUT L/R → amplifier LINE IN L/R

The converter must accept the TV’s selected format and provide the needed analog output. Check where listening volume will be controlled. This is an illustrative route, not a compatibility claim about an arbitrary TV or converter. See connecting a TV to a stereo system for the complete task.

Turntable with a PHONO/LINE switch

The plugs can stay the same while the signal changes. With internal phono processing enabled, the line output goes to a compatible line input. With it bypassed, the cartridge output needs the appropriate phono stage downstream. Audio-Technica’s preamp-selector instructions, page 5, document both routes.

Cartridge → one appropriate phono stage → line-level input → downstream control and amplification

Account for phono processing once. Do not feed an already processed LINE output into PHONO as a generic fix. Phono versus line explains this distinction; the turntable connection guide owns the setup sequence.

Before the first playback

Follow each device’s connection and power instructions, match left/right and speaker polarity labels, and identify the intended input and everyday volume control. Preserve factory safety grounding; Rane’s introduction explicitly warns against compromising it to solve an audio problem.

If a label or function remains unknown, leave that connection unmade until the manual resolves it. For a completed system with a fault, use the no-sound checks or the hum and buzz guide instead of trying unrelated adapters.