Effect loop pitfalls
Many guitarists run time-based effect pedals (i.e. modulation and delay) in the loop of their amp. This is a good thing to do when using a distortion channel. But the attempt is not always successful. Depending upon the gear, there may be complaints of volume loss in a loop, mushiness, "filter" sounds when engaged, etc. Often these complaints are associated with digital effect units, leading to complaints that the effect "sounds digital".
In reality, digital gear doesn't have a distinctive sound. The defects noted above are all result of interaction between the effect and the loop.
There are several potential issues with running a stompbox effect in a loop:
- If the loop's signal level is too high you'll get clipping in the pedal. This causes harsh distortion (which you may not notice as such if you're running your amp's preamp with a lot of distortion). This defect will occur with any overloaded device in the loop, but digital devices tend to sound worse than analog devices when overloaded. Fortunately digital devices tend to have a lower noise floor than analog devices, so you don't lose anything by reducting the levelto avoid clipping.
- If the loop is pushing a hot signal to the pedal, a pedal that's designed for use with guitar-level signals may not be able to push a comparably hot signal from its output. When combined with the clipping that's almost certainly happening you get a perceived volume loss when the pedal is engaged.
- Some amps have what's called a parallel effects loop. Your pedal gets the same signal that goes around the loop (in parallel). Digital processing causes a slight delay (typically a few milliseconds) in the dry sound. Take a look at how a flanger works: dry signal mixed with a slightly-delayed copy of the signal. In technical terms this is called a comb filter. It accounts for - depending upon the actual delay time - various maladies such a "thin" or "cocked wah" sound. If the pedal has any gain (for example, a graphic EQ with some of the sliders set above zero) you may even get uncontrollable feedback depending upon the design of the amp's loop.
What can you do about these problems?
For 1 and 2, see whether your amp has a loop-level switch or adjustable levels for loop send and return. What you want to do is to reduce the level of the signal going to the pedal, and boost the loop return volume to make up for the lower level from the send. Make your adjustments with a clean signal so you can tell whether the engaged pedal is adding distortion.
For 3, you have two choices: Either completely turn off the dry signal in your digital FX unit or get the loop converted to a serial loop.