The SignalPath

The plug is the package shape, the signal is what is written inside, the level is how strongly it is delivered, and a converter is the person who rewrites it for the recipient.

Imagine sending the same message on two receipts, with one printed as a photographic negative. Rain leaves the same watermarks on both. At the destination, one receipt is flipped back and the pair is combined: the opposite messages reinforce each other, while the matching watermarks cancel. The two opposite messages are the balanced hot and cold signals; the shared watermark is picked-up noise.

Picture the signal as water in a pipe. Voltage is the pressure behind it — how hard the water is pushed, not how much arrives. Current, in amps, is the flow: how much water actually moves. Impedance, in ohms (Ω), is how much the pipe fights that flow — and because audio wiggles rather than runs steadily, the fight changes with how fast you wiggle it. That is why it is impedance and not simply resistance. Power, in watts, is pressure times flow: the work actually done at the far end.

Read the chain that way and it stops being mysterious. A microphone is a trickle at low pressure — almost no power at all. A preamp raises the pressure. Line level is decent pressure but still barely any flow, which is plenty, because the next box only has to read it. A power amplifier is the stage that finally adds volume of flow, and that is what moves a cone. Pressure first, flow second — and a cable supplies neither.

Impedance is the same picture from the other side. A guitar pickup is a very delicate pressure source: connect it to a pipe that is too wide and open, and the pressure drains away, taking the top end of the tone with it. A Hi-Z input is a deliberately narrow pipe that barely draws anything — which is why the same plug into the wrong input sounds dull rather than broken.

Decibels are not a quantity. They are a ratio wearing a number's clothes: a step means times something, never plus something — the way stops on a camera lens or points on the Richter scale work. Doubling the voltage is +6 dB whether you began at one millivolt or at ten volts. Ears need this. The quietest sound you can hear and the loudest you can bear sit about a trillion to one apart, and nobody wants to read thirteen digits.

A ratio needs something to be a ratio of. Altitude means nothing without a sea level, and a decibel means nothing without a reference — which is exactly what the trailing letter names. dBV takes 1 volt as its sea level. dBu takes 0.7746 volts, the voltage that puts one milliwatt into the old 600 Ω telephone standard. So −10 dBV and +4 dBu are two towns quoting their height from two different sea levels. They are 2.2 dB apart by definition, about 12 dB apart in practice — which is why consumer gear arrives quiet in a pro input, and pro gear arrives hot in a consumer one.

A microphone or turntable cartridge whispers. A preamp brings that whisper up to ordinary conversational line level. A power amplifier then turns the instructions into the physical force needed to move a speaker. A cable can carry any one of these jobs only if it is built for that job; it cannot supply the missing gain or power.

Seeing a familiar wall outlet does not tell you its voltage, frequency or whether the circuit is live. Likewise, seeing RCA, XLR or USB-C does not tell you whether the port carries analog audio, digital data, power—or anything at all. The shape tells you what can plug in; the device specifications tell you what can happen next.

Analog is like carrying the finished cake: heat, bumps and dust can alter the thing itself along the way. Digital is like carrying an exact numbered recipe. If every instruction arrives correctly, the destination can recreate the same result; if too many instructions go missing, you do not get a subtly “warmer” cake—you get an obvious failure. Digital bandwidth is the courier’s capacity: a bicycle can carry one envelope, while a truck can carry a full kitchen’s instructions at once.