The Guitarist’s Guide to Electric Guitar Amps – Part 3: Effects Loops

A Detour, Quite Literally — FX Loops

Greetings. In Part 2 of this series, we looked at the preamp section of an electric guitar amp. This installment is dedicated to Effects Loops — a.k.a. FX loops. Why are we taking this diversion, instead of “logically” moving straight from the preamp to the power amp? There are two reasons, my friend.

FX loops literally sit between the preamp and power amp. So in terms of signal path, FX loops being the next topic is really not a detour at all. That said, as the title of this piece states, FX loops are quite literally a detour for the signal path — provided they’re used, of course!
FX loops are a given in the vast majority of modern guitar amps, so they should (IMHO) be covered.
Now that we know where the FX loops lurk, let’s quickly examine the whatand why.

What Is an FX Loop?

As its name suggests, an FX loop is literally a loop where you can put effects: if you choose to, of course. The use of an FX loop is not mandatory — just because your amp has one (or more), doesn’t mean you have to use it!

Why an FX Loop?

According to many (but certainly not all) people’s ears, tastes, and opinions, certain effects (e.g., modulations and time-based reverbs and delays) sound better when they’re used in an FX loop — namely, added to your signal after the preamp has worked its magic. I’ll get into the nitty-gritty of this rationale shortly.

Types of FX Loops

There are two types of FX loops in a guitar amp — Series (sometimes referred to as Serial, not to be confused with “cereal,” a la Cornflakes or Lucky Charms!) and Parallel. Both names (series and parallel) make perfect sense in terms of their definition/use in electric circuitry lingo — which is 110% logical as we are dealing with electric guitar amps here! Let’s deal with Series FX loops first, as that’s the most common type.

FX Loop Type 1: Series

The definition of a series electrical circuit is this: a circuit where the current follows one path — and one path only.


If your amp boasts a Series FX loop, it will exist as a pair of jacks labeled Send and Return (see below diagram) that, in most cases (but not all), will be found on the rear panel.

To use it, all you have to do is:

Run a cable from the FX loop Send jack to the input of the effect(s) you’re putting in the loop. This takes 100% of the preamp signal and sends it to whatever gear you insert in the loop.
At the end of the FX chain you’re using, you run a cable from the Output of the last effect unit back into the Return jack of the FX loop. This returns the effected signal back to the amp so the power stage can work its speaker-moving, tonal magic.
As a result, your guitar signal will follow the below path when you employ a Series FX loop:

Guitar -> Preamp -> FX Loop (Pedals/Processors) -> Power Amp -> Speaker(s)

So in a nutshell, by using a Series FX loop, you detour the entire preamp signal out of the amp and then return the processed signal back to the amp (and on to the power amp stage) via the FX loop.

Series FX loop signal routing — the direct path to the power amp is now closed!

Important Note: If you only put a plug in the Send of the FX loop, you will mute your amp. Ditto if you onlyput a plug into the Return. Why? Because you’re breaking the circuit. Makes sense. That’s why, on some amps — like Marshall’s JVM line — the Send is often labeled Preamp Out, and the Return as Power Amp In as shown below.

FX Loop Type 2: Parallel

The definition of a parallel electrical circuit is this: a circuit that is divided into two (or more) paths.

In a Parallel FX loop, there are two paths — we’ll call them Path 1 and Path 2 (brilliant).

Path 1: This path remains in the amp and goes straight from the preamp to the power amp.
Path 2: This is the Parallel loop that leaves the amp via the Send jack and comes back via the Return.

What determines how your guitar signal navigates this pair of paths? A Mix knob (see below) that you control…

If we go back to the detour analogy — the Mix control is effectively a traffic cop that decides how many vehicles take the Parallel FX loop detour route and how many stay in the amp and drive straight to the power amp.

Parallel FX loop signal routing: the setting of the Mix control determines how much of your guitar’s signal leaves the amp!

For example, these Mix control settings do the following:

Mix set to Dry (or 0): no signal takes the loop detour — it all stays within the amp and goes straight from the preamp to the power amp. The “loop” is quite literally out of the loop in this case!
Mix set to 12 o’clock (or 5): 50% of the signal stays in the amp, and 50% takes the FX loop detour.
Mix set to Wet (or 10): the entire signal travels the loop path — this setting means the Parallel loop is now acting as a Series loop since 100% of the signal is going through the FX loop. Make sense?
 

Quick Summary: The Story So Far

Simply put, a Series FX loop involves 100% of the preamp signal; a Parallel loop only involves the amount of signal you decide to divert to the loop via the Mix control — literally from 0% to 100% or any point in between.

Now, let’s deal with three common FX loop-related questions.

Question 1: How can I tell whether the FX loop on my amp is Series or Parallel, if it just says “FX Loop” or “Effects Loop” on the panel above the Send and Return jacks?
The giveaway is the presence of a Mix control. A Parallel FX loop will have a Mix control, but a Series loop will not. Also, do not confuse Send Level (found on certain Mesa amps) or Return Level controls with being Mix — some Series effects loops will have those, but they will not be labeled Mix.

Question 2: Why an FX loop at all? Can I just put all my effects in front of the amp?
To answer the second part first — the answer is: yes, of course, you can put all your effects in front of the amp. That said…

As stated earlier — there are ears and opinions that believe that certain effects sound more realistic/authentic when applied as close to the end of the signal path as possible. And by using an FX loop, while not at the very end of the chain, you’re at least placing said effects after the preamp, where a lot of tone shaping and, if desired, distorting is done. Simply put — you’re adding the effect to the preamp-shaped signal, rather than the preamp shaping the effect. Make sense?

So what are these effects, and why will they possibly sound better? Remember, visual beauty is determined by the eye of the beholder; likewise, sonic beauty is determined by the ear of the listener!

Included on this list are effects such as certain modulations, plus time-based ones such as reverb and echo/delay. Let’s deal with the latter ones first:

Time-based effects: Reverb and echo/delay.

Here, the “whether to put these in an FX loop or not” question is kinda-sorta answered by another question. “Since these effects are essentially emulations of sound being reflected in a room, hall, cathedral, cave, canyon, or whatever, would you rather add reverb or echo/delay to your distorted sound, or distort your sound after reverb and echo/delay have been added to it already?” Arguably, logic and realism would suggest distorting first then adding reverb/delay, rather than vice versa.

Modulation effects such as Chorus, Flange, and Phase, plus Rotary Speaker (Leslie).
Once again, without getting all historical, these boxes all emulate sounds originally created at the end of the signal chain. So the same rationale as above applies. But with that stated, let’s never forget the following “rule.”

When It Comes to Where Effects Should Sit in a Signal Chain — There Are No Rules!! Whatever Sounds Best Is the Right Way!!
Furthermore, for certain legendary tones, the order is “incorrect” (to some). For example, Edward Van Halen’s trademark MXR Phase 90 sound needs the stompbox before the amp. Ditto his famous flange-flavored intro to “Unchained” — flanger first! I rest my case.

Likewise, certain people will tell you that effects units such as distortion, fuzz, and wah should alwaysreside in front of your amp andneverbe placed in an FX loop. If your ears tell you otherwise, then ignore them and do it your own way! Never let other people’s opinions stop you from experimenting — safely, of course!

Question 3: Why are there two types of FX loops? And what are the pros and cons?
There are a few answers/rationales to this pretty important question. Here are a few common thoughts:

Series FX Loop Pro: the entire signal is affected by the effects in the loop. This is great for adding extra post-preamp EQ (e.g., parametric or graphic) or noise reduction, for example.
At this point, you could understandably be thinking, “but surely, all effects should be on all the signal.” The answer to that is, maybe surprisingly, not always. Confused? Fret not, my friend, read on, and all will become clear.

Series FX Loop Con: the entire signal is affected by the effect processor(s).
“Huh?” Do I hear some snort with a possible smidgen of indignant derision? Let me explain.

There are two particularly prevalent instances where not having the whole signal processed is beneficial.
Instance 1. There are certain digital multi-effects processors out there that sound amazing but do not switch between complex patches seamlessly — there’s a tiny but audible delay. In those cases, clever use of a Parallel FX loop comes into play. Let me explain.

If you’re using a multi-effects unit that has an audible switching delay, here’s what you can do:

Put it in the Parallel FX loop of your amp — if it has one, of course!
Set the unit up to produce a 100% wet (effected) output signal.
Set the Mix control on your Parallel FX loop such that the desired effect is still created.
This will mean that only a portion of your signal is subjected to the switching delay, thus minimizing its impact. Make sense?

Other than minimizing switching glitches, there are two other significant benefits of the aforementioned setup when a Parallel loop is used with any FX unit. The first is that setting the FX unit up to produce a 100% wet signal will eliminate any possible nasty out-of-phase issues between the dry signal from the processor and the pure dry signal that stays within the amp.

Instance 2. Whenever a digital effects device (be it an affordable stompbox or expensive rack unit) is used, your signal has to be converted from analog to digital (AD) on the way in, and from digital back to analog (DA) on its way back out. Nowadays, such AD/DA converters are excellent, but to those with a discerning ear, there can be a slight loss in warmth. So for the tube tone purist, it is often considered sacrilegious to subject the 100% pure tube-generated tone of an often extremely expensive, all-tube amp to be “digitally polluted” in this manner! Hence the importance of a Parallel FX loop and the digital device’s output to be set to 100% wet!

Other FX Loop-related Stuff
In addition to the Mix control on Parallel FX loops, the following are fairly common:

FX Level buttons that offer -10dB (stompbox level) and +4dB options. These work for both Series and Parallel. Below is a photo of a Series FX loop with that type of switch.

True Bypass switches for the tonal purist (see above picture).
Return Level or Send Level controls on Series FX loops, to control the loop’s (wait for it!) level. The photo below shows a Series FX loop with a Return Level control and also a Loop Bypass switch.

There are a few amps out there that boast both Series and Parallel FX loops. Marshall’s JVM line is one such example.

Notice the fact that the Brits have chosen to label the Series FX loop (the one on the right) as Power Amp Insert/Serial Loop and also label the Parallel loop (the one on the left) as Serial/Parallel — thus reminding the user that if the Mix control is turned all the way up (to “eleven” or Wet) then the Parallel loop does become a Series (Serial) one, as mentioned earlier.

Below is a picture minus the glorious quartet of EL34 power tubes so you can see the back panel labeling a little better.

Phew, that was another mini-marathon. Hopefully you understand FX loops better now.

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