With a serial effects loop, the signal path between the preamp and the power amp is interrupted, and sent out to whatever is in said loop. The then changed signal is returned to the amp and fed to the power amp. None of the original signal from the preamp reaches the power amp directly without going through the effects units first. Thus, "serial" because the FX loop is in series with the preamp and power amp. The signal path looks something like this:
Code:
preamp----->Effects----->power amp
In a parallel loop, in addition to the signal path of the loop itself, there is an additional one which bypasses the loop and feeds directly to the power amp. This unaffected or "dry" signal is then blended back together with the affected, or "wet" signal. Usually there is a blend control, as is the case in our Nomads and other Boogies with parallel loops. The idea is to preserve the amp's original signal, so as to avoid as much unwanted coloration of tone as possible. The internal bypass and the FX loop are parallel to each other, thus the name "parallel" loop. The signal path looks something like:
Code:
preamp----->Effects----->Blend----->power amp
|_________bypass________|
The problem with a parallel loop comes in when you have digital effects in the loop. Digital effects have an ever so slight delay to them. It is small, usually only a few miliseconds, IIRC, but that's enough to shift the effected signal slightly out of phase with the dry signal in the amp. When the two are blended back together, you'll get a phasing or flanging effect in addition to whatever effects were applied in the loop. Depending on the units in the loop, this can sometimes happen even if the effects are off and bypassed.
"But wait!" you say. "I can just set my blend control to 100% and use all of the effected signal and none of the dry signal, thus effectively getting a serial loop, freeing myself from from the phasing effect caused by parallel loops. Haha! I am clever!" Close, and you're thinking in the right direction, but not quite. In our Nomads (and I suspect in just about any other amp that uses a parallel loop, the blend control is simply a potentiometer. Unless you have some sort of special potentiometer which when turned fully to one side disconnects the other (I don't know if such exists) for your blend control, the bypass part of the parallel loop
is still connected when the blend knob is turned to 100%. Thus, you don't
quite have a fully serial loop, and thus you'll still get an ever so slight phasing or flanging effect through your loop. It might be so subtle that you really have to listen close to notice it, or it might be painfully obvious and render your otherwise favorite digital effect useless in the FX loop. Either way, the only way to completely eliminate the problem, is to convert the parallel loop to serial, so it plays nice with all effects, not just analog ones.
That was probably longer than necessary, but does that clarify at all for you?