If there’s one question that I get more than any others, it’s “Are you Tom Green?” But the second most popular question is about effects (fx) loops in amplifiers. What is an effects loop? What does it do? Can it hurt me? Am I still a Christian if I use one? These are all great questions.
Here’s the general idea of an effects loop. I would argue that the better amps in the world are tube/valve based amplifiers. Amplifiers are comprised of two internal sections: the preamp section and the power amp section. (*I’m oversimplifying an amp, but if you want to know specific details about how amps work, there are a good deal of other sites out there.) The preamp section is where the grindy distortion and overdrive is created and the power amp section is there just to create a louder version of whatever character the preamp is creating. When people talk about a “master volume” amplifier, they are talking about an amp that has a volume for both the preamp section and the power amp section. A master volume allows you to turn up the preamp section and get lots of cool overdrive and turn the master volume, or overall output volume, down to a comfortable listening level.
The need for an effects loop comes when you want to use effects like delay and modulation (chorus, tremelo, phaser, flanger) but you’re also using an amp that has some onboard distortion or overdrive. The order of your effects will make a huge difference on how your guitar tone sounds. Delay and Modulation effects sound splatty and uncontrollable when they are in front of the overdrive section. An effects loop allows you to insert some effects in between those two internal parts of the amplifier (preamp and power amp). Now you can run volume pedals, fuzz, overdrive, distortion, and compressors up front (or into the front end/ input of the amp) and then you can run the more refined and sophisticated effects through that effects loop so that they come after your distortion and overdrive.
Also, you’ll notice that people will refer to two different kinds of fx loops, a “serial” fx loop and a “parallel” fx loop. Many companies will offer a serial loop while some of the more elaborate amps out there offer the ability to switch between serial and parallel or they’ll over both kinds of loop in the same amp. The difference lies in how the amp routes the guitar tone. Serial fx loops will run the effects in line, as if everything is in a row. Guitar tone goes into the input, through the preamp section, through the delays and modulations, through the power amp section and then out the speaker in the form of facemelting rawk. A parallel loop splits the guitar tone off into two signals in the loop section so that you’re mixing in the delayed and modulated sound with the pure unaffected tone from the amp itself.
Not every amp has an effects loop and not every player requires the sonic benefits that an fx loop provides. The Edge from U2 has been running delay pedals into the front of a Vox AC30 amp for much of his career with great results. It’s important to note that there are some guidelines for guitar tones- but never any hard and fast rules. I particularly like the sound of some modulation effects going into the preamp section because they sound goopy and chocolatey. As always, experiment on your own and figure out what your ears want to eat.