I've been told that my turning down the channels on my mixer before turning off my monitors, I can prevent the loud popping noise they make, which will damage the speakers in the long run.
However, I always make sure all levels are down, and the master volume is at 0, and the monitors still make a popping sound.
What can I do to prevent this?
A well as turning the monitors down, I would suggest muting the monitor channels and turning down the volume on the monitor if they are independently powered. I know turning the levels right down should remove any voltage across the output, but some mixers seem to still have a DC offset which will cause a pop when the mixer is powered down.
Muting on some mixers seems to either cut the amp stage off from the output entirely, or possibly allow any DC output voltage to dissipate (not sure exactly)
Update as your monitors have volume controls, I would guess they have power amps in them. These should definitely be turned down to zero before powering down the mixer, as they will amplify the electrical pop as the mixer shuts down.
The rule is to always turn your sound system on from "upstream" to "downstream", and to always turn off from "downstream" to "upstream". At its simplest and most basic, "amps are last on, first off". If you are following this rule, and still getting pops when you turn the amps on or off, one of two basic things is happening:
You are turning everything on or off at once. Regardless of their volume setting, audio equipment can send a signal spike down the line when they turn on or off. This will be amplified even with power to the amplifiers cut, because the amplifier stores a charge that it draws from to amplify the signal, and it can take a few seconds after the power is cut for the amp to discharge all of this stored energy.
Consider a power sequencer. It's basically a power strip that can be "timed" to provide power to each of its plugs in some sequence of delays after the power switch is pressed, and conversely to cut power to each plug in a reversed sequence.
Your amplifier is shunting the charge stored in its capacitor banks through the signal circuit, instead of shunting it to neutral or ground. Most newer amplifiers (including the ones in powered speakers) have features that will avoid sending a "pop" though the speakers when they turn off, either by disconnecting the speaker output circuit before discharging, or by discharging onto an isolated "rail" that leads straight to ground.
Consider upgrading to a newer or better-quality power amplifier. Also, if you know what you're doing, it's possible to set up a system that will allow you to switch the output of the amplifier from your speakers to a simple high-wattage 8- to 16-ohm resistor bank, basically allowing you to "turn off" unpowered speakers before you turn off the power amp that feeds those speakers, letting the resistor load silently handle the power discharge. You should ONLY do this if you are SURE you know how to do it properly; not having some sort of impedance load connected to the amp's output can permanently damage the amp, and the amount of power we're talking about can easily kill you. DO NOT do this with powered speakers; in addition to all the dangers, you will likely void your warranty, and the circuitry between a powered speaker's amp and its speakers can be much more complex, increasing the likelihood you'll get it wrong.
It's still not clear to me what exactly you mean, but here all reasonable possibilities:
