Quick Answer: The socket decides the cable. A line input takes 0.3 to 2 V through a shielded RCA, jack or TRS lead and runs 5 m safely. A speaker-level input takes 10 to 50 V through two-core copper of 16 AWG or heavier. A digital input takes TOSLINK or 75 ohm coax, with 96 kHz and 192 kHz ceilings respectively. Never connect an amplified output to a line input, and never use a digital optical lead in place of a network cable.
Most damage to powered speakers comes from a cable chosen by connector shape rather than signal type. Two plugs can look identical and carry levels that differ by 30 dB, which is the difference between a quiet channel and a burnt input stage.
Three inputs, three cables
Line level is a low-voltage signal that expects a high-impedance input, typically 10 kohm or more. Speaker level is already amplified and expects an 4 to 8 ohm load or a transformer. Digital is a stream of frames on either a plastic fibre or a 75 ohm coaxial pair. Sorting the source into one of those three categories settles the cable, the connector and the maximum length in one step.
Level matching without guessing
Consumer line outputs deliver about 0.32 V for full scale, professional equipment about 1.23 V, and a speaker output can exceed 30 V. If the input sensitivity control must sit below a quarter of its travel to avoid clipping, the source is too hot for the input and an attenuator is the correct fix, not a longer cable.
Input type against cable and length
| Input | Nominal level | Connector | Cable | Max run |
|---|
| Unbalanced line | 0.32 V | RCA, 3.5 mm | Shielded single core | 5 m (16 ft) |
| Balanced line | 1.23 V | XLR, TRS | Two core shielded | 50 m (164 ft) |
| Speaker level | 10 to 50 V | Binding post | 14 AWG copper | 24 m (79 ft) |
| TOSLINK | Digital | Square optical | 1 mm plastic fibre | 5 m (16 ft) |
| S/PDIF coax | Digital | RCA | 75 ohm coaxial | 10 m (33 ft) |
Speaker-level inputs and passive volume
Small powered monitors often include a speaker-level input so they can hang off an amplifier that has no line output. That input contains a high-value resistor network, so it looks like a few hundred ohms to the amplifier and draws almost no current. Use twin speaker cable of 16 AWG or heavier, keep polarity consistent, and never bridge the two channels onto one plug. If the amplifier has no spare output pair, a line-level tap is the wrong workaround, because the pad value depends on the amplifier’s minimum load and a mistake there can overdrive the input.
Digital inputs and their ceilings
TOSLINK carries up to 96 kHz in most implementations, and plastic fibre loses 0.2 dB per metre, so 5 m is the practical length before dropouts begin on 96 kHz material. Coaxial S/PDIF reaches 192 kHz and tolerates 10 m because the impedance is controlled at 75 ohm; an ordinary RCA audio lead of the wrong impedance causes reflections and the receiver mutes.
Distributed lines change the rules
Where many speakers share one amplifier, 70 V and 100 V lines use a transformer at each speaker, so the cable carries a high voltage at low current and 18 AWG reaches 100 m without loss. 70v outdoor speakers with 5 W taps and 100v line ceiling speakers with 3 W taps both follow this pattern, and total tap wattage must stay under the amplifier rating.
Practical scenarios
- Situation: A pair of powered monitors on a desk fed from a laptop. Use a 2 m 3.5 mm to dual RCA lead and set the monitor trim to unity, so the laptop controls level.
- Situation: Ceiling speakers in a shop with a 100 V amplifier and 12 speakers at 6 W each. Total is 72 W, so a 120 W amplifier leaves headroom and 18 AWG carries the run.
- Situation: A powered subwoofer with speaker-level inputs in a stereo system. Run 14 AWG from the amplifier to the sub, then 14 AWG to the main speakers, keeping total loop resistance under 0.4 ohm.
- Situation: A TV with optical output to a soundbar 6 m away. If the TOSLINK run must exceed 5 m, convert to coaxial for the crossing and back to optical at the far end.
FAQ
Can I plug a speaker output into a line input?
No. The line input expects under 2 V and the speaker output can deliver 30 V or more, which will clip and then destroy the input stage. Use a speaker-level input or a resistive attenuator rated for the amplifier power.
Does an optical cable carry surround sound?
It carries compressed 5.1 in the formats the transmitter supports, but not the uncompressed multichannel streams used on disc. Check the source menu for a bitstream setting rather than PCM before blaming the cable.
Why do my speakers hum with a 3.5 mm lead?
The shield is carrying current from an earth loop between the source and the speaker supply. Try a ground-loop isolator on the line input, or power both devices from the same distribution board.
