What is the difference between parallel and serial effects loop in guitar amplifier? When which one is better?
The most common way guitarist chain their effects together is in series. This means that the original output source (the guitar) is plugged in to the input of the first effects pedal and the output from that pedal is plugged into the input of the next one and so on. The last pedal's output is plugged in to your amplifier. This is often called an effects "chain" because all the effects are linked together like the links of a chain.
If you run your effects in parallel, you feed the input source into each effect pedal separately and each pedal then sends the effect directly to the amplifier without first having to pass through the other effect.
One method is not better than the other but they have different uses. So it depends on what you are trying to achieve as to whether you would want to run your effects in parallel or in series (or a combination of both).
If you run your effects in series, then the effect that comes first (closest to the guitar in the chain) will affect the effect that comes next. For example, if your distortion effect comes before your flanger effect, then the flanger effect will be applied to the distortion sound. If the flanger comes first the distortion will be applied to the flanger effect. Each will produce a different sounding end result.
Another thing to consider is that if you want to be able to use a a particular effect (let's use delay for example), and you want that effect to apply to the sound created by applying one or more other effects, you should put it after any of the other effects you want to apply the delay to. So if you want your delay effect to apply to your fuzz effect, the fuzz pedal should come first.
The order of effects will affect the overall end result and changing the order will alter the tone and sound. You can experiment with different arrangements to get the sound you want.
Running effects in parallel will allow you to apply one or the other or both at the same time without either one shaping the other. You could for example apply a chorus effect and a reverb effect at the same time without one effect affecting the other. You could adjust the level of each effect and perhaps even the volume of the output from each of the two effects. Another thing you could do is send both a dry and distorted signal from your guitar to your amp by running the two signals in parallel.
To run effects in parallel you will need to split the output from your guitar in two - so that the output signal can be fed independently to each effects processor. This can be done with a stereo mixer which will separate your mono guitar signal into two identical stereo channels which can be routed in parallel to two different effects units (left output to one and right output to the other).
Another way to split the signal from your guitar is to use something like the Morley ABY Two Button Signal Switcher (pictured below) which can take a single input and split it into two separate outputs which can be combined or selected independently.
If you want to run more than two effects in parallel, you will need a splitter with more than two outputs.
Have fun experimenting with all the different ways you can alter the sound of your instrument!