The hook
The acoustic guitar has no sound. Not quiet. Nothing. The guitarist is looking at you. You have maybe ninety seconds before the countdown video ends.
The volunteer who panics starts turning knobs at random. The engineer does something completely different: they walk the chain. Guitar working? Cable in? DI light on? Channel showing signal? Here's the secret: the signal travels one road with a fixed set of stops, and the problem is always at exactly one of them. Learn the road and you'll never randomly turn knobs again.
The lesson
In Level 0 you learned signal flow as a story. Now we do it properly, with every stop named and one new idea: the signal is at a different strength at each stop, and each piece of gear expects the strength it was designed for. Most "mystery" problems are a strength mismatch.
Here's the road:
Source → mic or DI → cable → preamp → console processing → mix bus → output processing → amplifier → speaker → room.
Walk it stop by stop.
The source. A voice, a drum, a guitar amp, a keyboard, a laptop playing tracks. Everything starts as either a pressure wave in air or an electrical output from an instrument.
Mic or DI. Air sources get a microphone. Electrical sources (keyboard, bass, laptop, an acoustic guitar's pickup) get a DI box, short for direct injection. It converts the instrument's fragile, unbalanced output into the same kind of signal a mic sends: balanced, rugged, ready for a long cable run. Rule of thumb: if it has a speaker or a mouth, mic it; if it has a quarter-inch or headphone output, DI it.
The cable. Carries the signal to the board, usually via the stage box or snake. At this point the signal is at mic level: a few thousandths of a volt. It's the weakest, most vulnerable form the signal will ever take, which is why the cabling standard for it (balanced XLR, lesson 2.3) is built like a tank.
The preamp. The gain knob from lesson 0.1. It lifts mic level up to line level: roughly a volt, about a thousand times stronger, and the standard working strength for all processing. Getting this lift right is the whole subject of the next lesson. It's the setting that matters most on the entire road.
Console processing. EQ, dynamics, effects: everything the channel strip does. All of it happens at line level, where the signal is strong enough that the processing adds virtually no noise.
The mix bus. All channels sum into the main mix (plus the separate monitor mixes from lesson 0.1). Still line level.
Output processing. Between the console and the amps, most systems have a system processor doing overall EQ for the room, crossovers (splitting lows to subs, highs to tops), and limiters protecting the speakers. In a small church this may be hidden inside powered speakers; in a pro rig it's a dedicated box. Either way it exists, and it's line level too.
The amplifier. Takes the line-level signal and scales it up to speaker level: tens of volts, enough real power to physically shove cones. This is the last electrical stop.
The speaker and the room. Voltage becomes motion becomes pressure wave. Then the room itself (Level 6) gets the final word on what people actually hear.
Three strengths, then: mic level (tiny, fragile), line level (the working standard, where everything happens), speaker level (huge, dangerous to anything that isn't a speaker). Every connector on stage carries one of the three. Nearly every ugly surprise comes from feeding a stop the wrong strength. Plug a line-level keyboard into a mic input with the gain up, and it screams distortion: too strong. Plug a mic into a line input: barely a whisper, because nothing lifted it. The gear didn't fail. The road was mislabeled.
Now the payoff: troubleshooting is walking the road. No sound on the guitar channel? Start at one end and check each stop: Is the source making sound? Cable seated? DI power light on? Preamp showing signal on the channel meter? Channel unmuted, fader up, assigned to the mix? Meters moving on the main bus? Amp on? Every check eliminates half the road. Ninety seconds is plenty when you know where the stops are. This exact method, formalized, is Level 7's troubleshooting lesson.
Go deeperoptional
The three levels have real numbers. Mic level is nominally around −40 to −60 dBu (a dynamic mic on a moderate voice delivers on the order of 2 mV). Pro line level is +4 dBu ≈ 1.23 V RMS; consumer line level is −10 dBV ≈ 0.316 V. The two differ by about 12 dB, which is why a consumer CD player into a pro system runs quiet, and pro output into a consumer input runs hot. Speaker level is tens of volts: a 500 W amp into 8 Ω swings about 63 V RMS, ±89 V peak. That's three orders of magnitude of voltage across one road, which is why the connectors are deliberately incompatible (XLR for mic/line, speakON for speaker) wherever the industry could manage it.
Instrument level (a passive guitar or bass pickup) is its own animal, and the DI's real job is only half about level. A passive pickup has a very high output impedance (tens to hundreds of kΩ) and wants to see a load of ~1 MΩ; a console mic input offers 1–2 kΩ. Plug it in directly and the impedance mismatch loads the pickup down, rolling off the top end and thinning the tone before any electronics get a say. The DI presents the megohm input the pickup wants, then hands the console the low-impedance balanced source it wants. The general rule underneath: modern audio uses impedance bridging — each output has low impedance, each input roughly ten times higher or more, maximizing voltage transfer and minimizing interaction. (Matched 600 Ω interfaces died with the telephone era.)
Why lift to line level so early and stay there? Noise. Every stage adds some noise, but the noise contributed by later stages competes with the signal at the level it arrives. Boost a tiny signal to line level in one clean jump at the preamp, and all downstream noise is ~60 dB below the signal from the start; run the signal weak through three devices and boost at the end, and you've amplified every stage's noise floor along with the music. This is the audio-domain version of the Friis cascade-noise principle from RF engineering: the first stage's gain and noise performance dominate the whole chain's noise figure. It is the theoretical spine under the next lesson's practical rule: get the level right at the preamp, first, once.
Active DIs vs passive: a passive DI is a transformer. It isolates galvanically, shrugs off phantom power, needs no battery, and its winding ratio trades voltage down; ideal for hot active sources (keyboards). An active DI is a buffer amplifier (phantom- or battery-powered) with near-infinite input impedance; better for weak passive pickups. The transformer's isolation is also the classic ground-loop breaker, which is a lesson-2.3 story.
Remember
- One road: source → mic/DI → cable → preamp → console → mix bus → output processing → amp → speaker → room.
- Three strengths: mic level (tiny), line level (the working standard), speaker level (huge). Mismatches are the source of most mystery hiss and distortion.
- If it makes sound in air, mic it. If it makes an electrical output, DI it.
- The amp exists even when it's hidden inside a powered speaker, and its input knob is part of your chain.
- Troubleshooting is walking the road stop by stop. The fault is always at exactly one of them.
Your turn
- Recite the chain from memory, both directions.
- Physically trace one vocal channel from mic capsule to speaker, touching each link.
- Count the DIs in your rig and name what each one is protecting you from (long run? impedance? isolation?).
- Next "no sound" moment, narrate your walk out loud ("source good, cable good, DI light on...") and time yourself. Beat ninety seconds.