How a PA System Works: Signal Flow from Microphone to Speaker

In this guide:The chain · Levels · The preamp · Where the amp lives · Monitors · Finding a fault · Picks · FAQ

Almost every problem on a small stage comes down to not knowing where in the chain the sound is. Someone says the mic is not working, and four people start plugging and unplugging at random. If you know the order the signal travels in, you can find the break in under a minute by testing the middle, then the middle of whichever half failed.

Every PA, from a two-speaker function band rig to an arena, is the same five stages. The boxes get bigger, and some of them get combined into one product, but the order never changes.

The chain, in order

Source, preamp, processing, amplification, transducer. In plain terms: something makes a signal, something makes it big enough to work with, something shapes it, something makes it big enough to move a speaker cone, and something turns it back into moving air.

That is the whole thing. A pair of powered speakers on sticks and a full touring rig differ in scale and quality at every stage, not in the order of the stages.

Why level matters more than anything else

The three levels in that table are not adjectives, they are wildly different voltages. Mic level is measured in thousandths of a volt. Line level is a couple of volts. Speaker level, on a big system, is tens of volts into a very low impedance.

This is why the connector alone does not tell you what is safe to plug in. A jack socket might be an instrument input, a line input or a speaker output. Confuse those and you will either hear nothing, hear something horrible, or damage something. Our connectors guide covers which plug carries which signal.

Stage two is where most people go wrong

The preamp is the most misunderstood control on a mixer. It is not a volume knob. Its job is to bring whatever arrives at the input up to the level the rest of the desk expects, and it should be set once, per source, at soundcheck.

Set it too low and you turn the fader up to compensate, which raises the noise floor along with the signal, so the mix hisses. Set it too high and the channel distorts before the fader has done anything, and no amount of pulling the fader down will fix it, because the damage happened upstream.

Getting this right across the whole chain is the discipline called gain staging, and it is worth reading on its own: see gain staging for live sound.

Where the amplifier lives

Stage four is the one that moves around. In an active, or powered, speaker the amplifier is inside the cabinet, matched to the drivers at the factory, and you feed it line level on an XLR. In a passive system the amplifier is a separate box in a rack, and it feeds the speaker over heavy speaker cable.

Neither is better. Active means fewer decisions and less to carry for a mobile rig. Passive keeps weight off the poles, puts the heat and the fans in one rack, and lets you replace the amplifier without replacing the boxes. The full comparison is in active vs passive PA speakers, and if you go passive, read matching amplifiers and speakers before you buy either half.

The second chain nobody mentions

Everything above describes the front of house signal, the one the audience hears. A second chain runs in parallel, taking the same channels and building a different balance for the people on stage. It leaves the desk on aux sends rather than the main output, and it ends in wedges or in-ear packs instead of the main speakers.

It is the same five stages, sharing stages one to three and splitting at the mix. Understanding that the monitor mix is a separate destination fed from the same sources explains most of what confuses people about live sound, including why turning the singer up in the wedges can make the room worse rather than better. There is more in stage monitors, wedges and mixes.

Using the chain to find a fault

Because the stages are in a fixed order, a dead channel can be found by halving. Do not start at the microphone and work forwards, start in the middle.

• Is there signal on the channel meter? If yes, the fault is after the preamp, so look at the fader, the mute, the routing and the mix bus. If no, the fault is at the source, the cable or the preamp.
• Is the channel assigned to the main mix, and is the main fader up? On digital desks this is where most silent channels live.
• Swap the cable before you swap the microphone. Cables fail far more often than microphones do.
• If the whole system is silent, go to the last stage first. Speakers powered on, correct input selected, output cable in the correct socket.

Once the chain is second nature you stop guessing. A crackle that changes when you wiggle a connector is stage one or two. A hum that appears only when the laptop is plugged in is a grounding issue, covered in balanced vs unbalanced audio. Distortion that stays when you pull the fader down is upstream of the fader, every time.

Frequently asked questions

What order do you set up a PA in?

Physically, back to front: speakers and stands, then power, then the desk, then the stage boxes and cables, then sources. Electrically, power the amplifiers or active speakers on last and off first, so that any thumps from the rest of the system are not amplified into the room.

What is the difference between mic level and line level?

Roughly a thousand to one in voltage. A microphone puts out a few thousandths of a volt, and a line output puts out a couple of volts. The preamp exists to bridge that gap, which is why plugging a microphone into a line input gives you a very quiet, very noisy signal.

Do I need a mixer if my speakers are powered?

Only if you have more than one or two sources, or you need any control over tone and balance. A single microphone or a laptop can go straight into a powered speaker. The moment you have a band, a couple of microphones and a backing track, you need somewhere to combine and balance them.

Why does my system hum only when everything is connected?

Almost always a ground loop created when two pieces of mains-powered equipment are connected by an unbalanced cable. Balanced connections and DI boxes exist to break that loop, and the fix is covered in our balanced vs unbalanced guide rather than by lifting an earth pin, which is dangerous and never the answer.