Extensive compressor reviews and FAQ

Power conditioners:
This is a very contentious subject. Do not believe everything they say in the ads and packaging of power conditioners--those technical claims are willfully meant to snow you with scientific-sounding mumbo-jumbo that means nothing and does nothing. Even the word "conditioner" is misleading, it implies more correction of the power than most of these devices can actually perform. This is true even of reputable brands like Furman, unfortunately. Monster is the worst of the liars, do not believe anything they say.
These are the usually-true benefits of a typical power conditioner:
1) A row of outlets that you can mount in your rack. Personally I like to just bolt a non-racked power strip into the back of my rack, rather than take up 1 RU of front panel space.
2) Surge suppression. This means if there is a power spike due to a local electrical incident or fault, that spike will be blocked from harming your gear--up to a point. Note that the suppression component (called a MOV) loses some of its protective ability every time it takes a voltage spike, and eventually it will not provide any protection anymore. There is no warning light or way to know that the MOV is failing. So it is not good to buy or rely on an older surge suppressor that may have taken a few spikes already. The same type and quality of suppression sold in a typical power conditioner can be found in many good-quality non-rack outlet strips.
3) Noise filtering. With most modern amps and gear, there is already adequate noise filtering built in. Even though it is very tempting to believe in cleaning up the "dirty power" from your wall outlet, there is nothing fancy or special about the noise filter in most power conditioners, compared to what is commonly already part of most decent gear these days. Where the noise filter in the conditioner can help is if you are using a very old amp, or quirky/custom/cheapo gear, that may not have been designed with adequate filtering inside. While the noise filtering in some power conditioners can be quite good, this is the area where I see the most lies in ads. Be skeptical, and read this: Rane Note 110 to learn about factual and reliable methods of reducing noise. Ground loops are the most common cause of noise and hum, and noise filtering from the incoming AC line will not prevent ground loops.
What about correcting high or low voltage? Power "conditioners" do not regulate the voltage, apart from surge suppression. Some will shut down the power entirely in case of low voltage, basically ending your gig, but many don't even have that feature. There are products called "power regulators" that do correct the voltage from high or low, but these cost at least twice as much as a "conditioner".
Won't cleaning the dirty power make my amp perform better? No, in fact these conditioners can limit the current draw available to your amp, so in the case of an amplifier that draws a lot of current for peak transients, a power conditioner can actually make the amp perform WORSE.
What about grounding? A power conditioner does not provide grounding on its own, and will not "fix" or help with bad grounding at your house or venue. Also, most of them do not isolate the grounds of their outlets, so in those cases they cannot fix ground loops or interference between gear. If you are in an old building without grounded outlets, you can run a very long wire from the metal housing of your rig's outlet strip to some metal plumbing (water or gas) in the basement, and this should provide a ground, a path to earth. Note that even if there is metal plumbing in a nearby kitchen or bathroom upstairs, almost all modern homes have replaced the metal pipes under the floor with non-conductive plastic, which means the upstairs metal will not conduct to earth. Some outlet boxes and conditioners have a screw on the back that is specifically meant for the purpose of attaching an external wire; this screw is often dyed green.
Some units have extendable lights to illuminate your rack; those could be useful on a dark stage, if you tend to adjust your amp or rack effects often. But most people don't want to fuss with their rack gear once it is set up, and I don't think I've ever seen anyone actually use those lights during a concert. Some conditioners include a wiring diagnostic feature, to let you know if there is a problem with the wiring/grounding of the wall outlet; this is a very good and valuable feature, but you can do the exact same thing with a small outlet-tester plug bought cheaply from any hardware store. I strongly recommend you buy one of those and keep it in your gig bag! Another feature you may see, less commonly, is power sequencing. This means it powers up each outlet one at a time, rather than all at once, to avoid a surge that may make a loud pop noise in the speakers. However the only sequencing you need is to remember to turn the power amp on last when firing up your rig, and turn the power amp off first when shutting down, so only the smallest amount of surge reaches the speakers.
Note I am not saying all power conditioners are worthless and you shouldn't buy one. Instead my real point is that you need to be very skeptical and very specific. Identify which features and functions you really need, and then double-check to be sure that any power conditioner you might consider buying actually provides those benefits. Again, do not rely on the claims made by the manufacturers themselves; look for independent reviews from people who actually install or repair electronics for a living (not guitarists). You generally do have to pay more to get the better functions, but it doesn't follow that a higher price means you get those functions--some units are just more expensive for no reason other than marketing.
Two brands that seem to provide real features and quality without lying about it are Tripp Lite and SurgeX. I have been using Tripp Lite Isobar Ultras for years now, because they have filters that block interference between the outlets, stopping most ground loops. They also make the "Isolator" series if you need true transformer isolation in more extreme cases. The SurgeX Power Titan series uses a more robust non-sacrificial surge blocking system, if surges and spikes are your biggest issue. So check them both out and compare your local prices.
There are more intense solutions, such as "balanced power" systems, UPS (uninterruptible power supplies), and even bringing your own generator (for outdoor events); but generally these are a huge step up in size, weight, and cost for anyone who had just been in the market for a modest-priced rackable power conditioner. Also, the larger power devices often make some mechanical hum/buzz noise, like from a large toroidal transformer vibrating; so you have to figure out a way to keep that ambient noise from being an annoyance, especially while recording.

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