Adding effects in between the guitar and amp put circuitry into that signal path and removed some of the "Mojo" that a pure signal path created. Well, you say, when the effects units were bypassed they were removed from the circuit and the Mojo should be back. Not true. The way early pedals operated, they did bypass the guitar signal around the output of the effect but they also kept the effect's input circuitry connected to the guitar signal. This presented a load to the guitar signal that served to degrade the tone. And if multiple effects were being used they hung collectively on the guitar signal degrading the tone further. Of course there were thousands of hit records created using this setup. It obviously was not an impairment to Hendrix, so you could say that there were times when these problems became part of the sound and if one was looking to exactly create these sounds, these "problems" become an inherent part of the creation of that sound.

As time went on recording studios became more sophisticated. Multitrack recording arrived and so did studio effects that could be added to the recording. Producers began to add effects to guitar tracks to enhance them and many players found that when they went back out and played live they couldn't approach the sounds of the guitars that were on their records. In order to look into why this was let's put together some basic categories of effects so we can understand how to best apply them. I would divide effects into four broad categories. 1- distortion effects, 2- time based effects(reverbs, delays), 3- modulation based effects (chorus, flange, phase), 4- filter effects(wa, equalizers). To maximize the use of any of these categories requires different treatment and placement in the signal chain. Distortion is the easiest. It is best suited to be at the amps input and helps drive the amp into distortion when playing loud.It also imparts its own set of harmonics that can be adjusted to taste from the amps tone controls. The difference between studio and stage becomes apparent with the next groups of time based effects.

Think about a tape or analog delay playing into the input of a guitar amp. The level of each delay repeat is three DB's lower than the one before it. Therefore as it plays into the input of the amp each repeat is putting less signal into the amp and this means that if the player is driving the amp into distortion each repeat will sound different than the previous one since the amp is receiving less signal and distorting less. This is not to say that this is a bad or unusable sound,as a matter of fact many players love this. But let's get back to our recording studio. The player's guitar is plugged straight into the amp's input and the producer is adding delay at the console. Since the guitar player is sending a consistent signal to his amp his distortion doesn't vary and since the delay unit at the console is not distorting anything subsequent to it, each repeat sounds the same as the last except a little bit lower in level. So when the guitar player goes back out live to try to reproduce his record with a delay going to the input of his amp, he can't duplicate the sound he recorded in the studio.This gets worse with modulation based effects. When a chorus or flange receives a distorted guitar at its input it gets a rich amount of harmonic distortion content to act upon. If instead the chorus or flange is fed directly from the guitar the effect is not as rich sounding because the effect doesn't see anything but the sound of a clean guitar so it doesn't have as much to process.

The fourth set of effects are filters. Wa's work great in front of the amp as it puts a sweepable peak in the frequency response of the guitar and will influence the sound of the distortion. EQ's in front of the amp tend to get lost in the distortion and become ineffective. I'm sure you have noticed when you distort the power amp stage of an amp the on board tone controls are way less effective than when the same amp is played clean. But put an EQ after the distortion and a new world of tones becomes available to you.

So now guitar players needed a solution.