Comparing different types of effects loops

I’ve come to realize that there is a lot of confusion and misinformation floating around regarding effects loops.  First, what is an effects loop?  Generally speaking, an effects loop (aka FX loop) is a set of connections on an amp that allows the user to connect effect units such as reverb, delay, vibrato, chorus etc. after the preamp and before the power amp.

There have been countless on-line discussions and articles written on how and when to use an FX loop, what kind of effects should be connected to them and so on.  The purpose of this article is to explore the different types of loops and the advantages and disadvantages of each type.

The most basic loop is achieved simply by disconnecting the internal connection between the preamp and power amp and installing jacks on the amp so that effect units can be connected to the signals.  This type of loop is still being used in many amps today because of the ease and low cost to install it.  However, it often provides very disappointing results for a couple of reasons.  #1 A typical power amp needs around 7 volts RMS to drive the power amp (phase inverter) to full power.  That means your effects units need to be able to provide that much signal to drive the amp since the effect’s output now provides the signal to drive the amp.  Most effects run on 9 volts DC and are only capable of outputting around 3 volts RMS (9 volts peak to peak).  Also the effects unit may easily overdriven by the large signal coming out of the amps preamp circuit.  #2 – The impedance of the internal circuits and the effects are usually not well matched.  That means there will be additional looses when long cables are used.  This is clearly the worst type of effects loop.

A better type of loop is one that is buffered.  Buffering means the loop circuit can correct for mismatched impedances will and not create incorrect loads on the circuits of the amp or the effects.  In addition to buffering, signal cutting and boosting can be added.  That means that the loop circuit can reduce the level of the signal coming from the preamp down to a proper level  so the the effect unit is not overloaded and then boost the low signal coming from the effect back up to the proper level for the power amp if necessary.  The best loops have a switch or variable control to accomodate both low level (stomp boxes) and high level (rack mount) effects.  Keep in mind however that some 9 volt units will still be overloaded when playing at high volume.  Even with a buffered loop, best results are achieved with effects units that run on supplies higher than 9 volts.

There are two main types of buffered loops that can be used in tube amps.  They are tube driven loops and solid state loops.  Let’s look at the advantages and disadvantages of each type.

Tube Driven Loops
Advantages
1) Maintains a pure tube signal path for all signals in the the amp?  Remember however that when a loop is used, you will be connecting effects that are solid state devices so you really won’t have a pure tube signal at all.  Most effects that would be connected to your loop have many solid state gain and buffer stages your signal will pass through.  Adding just two more clean solid buffer stages will not change the fact that your effects signal path is already solid state.  Therefore, there is no real benefit to having a tube buffer over a solid state one.

Disadvantages
1) Will cause more noise such as hiss and hum.
2) Will alter the tone somewhat.
3) Will take up space necessitating a larger amp chassis
4) Higher cost
5) More heat

Solid State Loops
Advantages
1) Lower noise
2) Less distortion
3) Can be made extremely “transparent”
4) Smaller footprint
5) Lower cost

Disadvantages
1) No longer a pure tube signal path? See above.  A switch can be added to allow the loop to be completely disconnected from the circuit when not needed.  That switch also allows an easy A/B comparison of the affect the loop circuit has on the tone and noise of the amp. 

I highly recommend buffered solid state effects loops with a level switch and a bypass switch for any application.

Jeff Andrews

 

 

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