Mic, line & speaker level
The three signal levels, 70 V systems, and what happens when you mix them up.
An audio signal on a wire can be a few thousandths of a volt or nearly a hundred volts, depending on where it is in the system. Gear is built for one of three broad levels — mic, line and speaker — and most “the audio sounds wrong” calls come down to a signal arriving at an input built for a different one.
The three levels
| Level | Typical voltage | In dBu | Where you find it |
|---|---|---|---|
| Mic | ~1–30 mV | about −60 to −30 | Microphone outputs |
| Line (consumer) | 0.316 V | ≈ −7.8 (−10 dBV) | TVs, media players, PCs |
| Line (pro) | 1.228 V | +4 | Mixers, DSPs, codecs, switchers |
| Speaker | ~10–100 V | about +20 to +40 | Amplifier outputs |
- Mic
- Typical voltage~1–30 mV
- In dBuabout −60 to −30
- Where you find itMicrophone outputs
- Line (consumer)
- Typical voltage0.316 V
- In dBu≈ −7.8 (−10 dBV)
- Where you find itTVs, media players, PCs
- Line (pro)
- Typical voltage1.228 V
- In dBu+4
- Where you find itMixers, DSPs, codecs, switchers
- Speaker
- Typical voltage~10–100 V
- In dBuabout +20 to +40
- Where you find itAmplifier outputs
The decibel scale from chapter 1 keeps these manageable. Using dBu (0 dBu = 0.775 V), the whole ladder spans roughly 100 dB — a voltage ratio of about 100,000 to 1.
Mic level
A microphone turns tiny pressure changes into a tiny voltage. Its sensitivity is quoted as output at 94 dB SPL (a pressure of 1 pascal — a loud voice close to the mic). Dynamic mics typically produce a couple of millivolts; condensers, which have built-in electronics, often several times more. Either way it needs a mic preamp — an input stage with a lot of clean gain, often 40–60 dB — to reach line level.
Line level
Line level is the working level between devices. There are two conventions: +4 dBu for professional equipment and −10 dBV for consumer equipment. The difference is about 12 dB — enough that a consumer source in a pro DSP sounds noticeably quiet until its input gain is raised.
Speaker level
An amplifier raises line level to enough voltage and current to move a loudspeaker. Power, voltage and impedance are linked by P = V² ÷ R, so 100 W into an 8 Ω speaker is about 28 V. This is the only level that carries real power — and it can damage inputs designed for the other two.
Too quiet
It needs about 56 dB of gain to reach +4 dBu, but a line input only offers around 15 dB. Turned all the way up it will be quiet and noisy — use a mic input.
Nominal levels in dBu (0 dBu = 0.775 V). Mic figures assume a loud voice close to the mic (94 dB SPL); gain ranges and the +24 dBu ceiling are typical, not universal — check the spec sheet of the actual device.
Low-impedance vs. 70 V / 100 V speaker lines
Home and studio systems drive speakers directly: one amplifier channel, one or two speakers, short cables. Commercial buildings often need dozens of ceiling speakers on one channel, spread over long runs. For that they use constant-voltage (distributed) systems — 70 V in North America, 100 V in much of Europe and Asia.
- The amplifier delivers its rated power at 70.7 V (or 100 V) on the line.
- Each speaker has a small step-down transformer with taps — for example 1, 2, 4 or 8 W — that set how much power it draws.
- Add up the taps. The total must stay below the amplifier's rating; a common practice is to leave around 20% headroom.
- Higher voltage means lower current, so long runs lose less power and can use thinner cable.
What mismatches sound like
| Mistake | Result | Fix |
|---|---|---|
| Mic into a line input | Very quiet; noise when turned up | Use a mic input or an external preamp |
| Line into a mic input | Little headroom, easy to distort | Use a line input, or engage the pad |
| Consumer line into pro gear | About 12 dB too quiet | Raise input gain |
| Speaker level into any input | Severe distortion, possible damage | Never — use a line-level output |
- Mic into a line input
- ResultVery quiet; noise when turned up
- FixUse a mic input or an external preamp
- Line into a mic input
- ResultLittle headroom, easy to distort
- FixUse a line input, or engage the pad
- Consumer line into pro gear
- ResultAbout 12 dB too quiet
- FixRaise input gain
- Speaker level into any input
- ResultSevere distortion, possible damage
- FixNever — use a line-level output
Key takeaways
- 01Audio gear works at mic level (millivolts), line level (around a volt) or speaker level (tens of volts).
- 02Pro line level is +4 dBu; consumer line level is −10 dBV — about 12 dB lower.
- 03Microphones need a preamp with substantial gain, often 40–60 dB, to reach line level.
- 0470 V / 100 V systems run many speakers on one line; the sum of the transformer taps must stay under the amplifier's rating.
- 05Speaker-level signals carry real power and must never go into a mic or line input.
Check your understanding
Q1A media player's output is noticeably quiet in a professional DSP. What's the most likely reason?
Q2A 70 V amplifier channel is rated at 120 W. Which load fits with sensible headroom?
Q3Why must an amplifier's speaker output never be connected to a line input?