There is an effects processor now (December 1998) that brags about permitting you to put some of its stages before your guitar amp's preamp, and some other stages in the amp's effects loop. Their ad banner poses this as a question: "Which is the best position for your processor: before the amp, or in the amp's effects loop? Answer: both!" But they don't get it. Both positions are equally dissonant, as far as the placement of heavy time-based effects relative to heavy power tube saturation. The correct answer is, the best position for your processor is *neither* before the amp nor in the amp's effects loop; the best position is after the amp, and before the amp -- but not in the amp's effects loop. This is the way they do it in professional recording studios and stage setups, when you take into account the total signal path from guitar to tape:
If I could eliminate any of the 3 positions for the processor, I would eliminate the one in the amp's effects loop. You need a processor before the amp's preamp, because that is the only way you can control the distortion voicing, via pre-distortion filtering and compression. And you need a processor after the amp, because that's the only place you can safely put heavy time effects and equalize the speaker cabinet (and with MIDI, control the overall volume of the amp's completed sound). But you don't really need processing between the amp's preamp distortion and saturating power tubes; the amp already has an equalizer (tone controls) placed there, so the one important processing stage at that point is already provided by the amp. If your amp has poor tone controls, then I would recommend placing an EQ in the amp's effects loop, but that is really the only thing needed there. You *can* safely put filtering there, without introducing intermodulation distortion, such as:
power stage saturation
So, I disapprove of effects loops in amps, given that the amp has decent tone controls. All they do is encourage people to put time effects before power tube saturation -- resulting in garbled tone, which would be avoided by adding time effects through the mixer, after the amp tone has been securely established all the way through the guitar speakers.
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