By Steve Weiss

The Effects Loop - (Part Two)

Every solution in electronics creates two more problems.

This reminds me of something my High School electronics teacher said. Every solution in electronics creates two more problems. Humorous but true. I think you can see the trend here. There are many variables to interfacing effects and guitar amps so it is very easy to get bad results.

As time went on preamps were designed to create usable distortion without the need to push the power amp stage into distortion,so now a post preamp Effects Loop finally gave players the chance to interface their effects and sound more like what was being produced in the studio. Having delays post distortion yielded a more even distortion sound, choruses were more lush and EQ either in the loop or the amps on board tone controls,if they were placed post distortion allowed the distortion to be equalized into different timbres.

Now that we have looked at the development of the loop its attendant problems let's take a closer look at how the Effects Loop operates. An effects loop is actually two independent circuits. We have a send circuit and a return circuit. The send circuit takes the guitar signal from inside the amp and outputs it through the send jack to an external effect. The recovery circuit takes the output of the external effect and returns it to the signal flow inside the amp. There are two types of loops. The first is the series loop. In this type of loop the signal path of the guitar is completely interrupted and outputted from the send jack to an external effect. The effect unit must be able to pass through the dry guitar signal and add the effect to it. The second type of loop is the Parallel Loop. This loop taps off some of the signal and sends it out the send jack but it does not interrupt the dry signal flow inside the amp. The Parallel Loop return jack takes the output of the effect which must be set to 100% effect (no dry signal passes through) and mixes it back in with the guitar signal in the amp.

Let's move on to looking at how the circuitry design of a loop affects how well it performs. As we stated earlier, the most basic of loops inserts a series effects loop between the preamp and power amp by breaking the signal flow and hooking it to send and return jacks without adding any other circuitry. As we mentioned before this is what was done on the earliest amps with loops. This loop most of the time does not work well and here are the reasons why. The signal levels in a tube amp amp at this point in the circuit are very high and will almost certainly overload the input to an effects pedal(stomp box) creating a bad sounding distortion that you cannot get rid of. And the output of a stomp box is optimized to output a level similar to a guitar's pickup going into the input to the amp, so it will be lacking enough signal to drive the power stage to full output. The other problem is impedance.If proper impedance matching between an effect unit and an effect loop is not observed you may experience degradation in the quality of the signal. The basic rule of impedance matching is that an input impedance needs to be at least ten times larger than the impedance of the output of the unit sending signal to it or loading can occur. This loading will cause the signal to lose lows and highs and be reduced in level. Therefore the effect send must have a low impedance and the return a high impedance. In most amps with the type of loop we are discussing the send is at a high impedance point,so right away this is a problem.

So is there a solution to all of this?

Yes. What is needed is to add a dedicated effects buffer circuit for the send and return circuits. This additional circuitry can clear up a lot of problems.How does this work?. If instead of just breaking the signal flow between the preamp and power amp and installing jacks,we add a tube or integrated circuit set up as a buffer. The buffer does two things. First it has a very high input impedance which means it will not load the signal it is receiving from the amp and the buffer's output is a low impedance which is what is needed to drive the input of an effect. We do the same thing with the return circuit. We take the signal coming back in from the return jack and buffer it before sending it to the power amp. Now we have a loop that works well with effects from an impedance standpoint and if the buffer circuits are designed carefully should present no degradation to the tone of the guitar. We still need to accommodate for level mismatches that can occur. As we stated before a stomp box is not a good fit level wise at this point in the circuit. What can be done is to introduce send and return level controls. Since the send jack already has plenty of level at this point in the circuit we set up its level control as a variable attenuator. Now we can correctly drive the input to any effect from a stomp box to a rack processor. For the return circuit we can configure its circuit to add gain so we can recover the level we lost going through a stomp box and bring it back up to the level the power amp's input needs to see.

Of course the designs of buffered loops vary from amp to amp so your results may be better on some amps than others,but it goes a long way from the effects in front of the amp scenario. This loop will work well placed at various points in the amp's circuitry but for a guitar amp positioning it post distortion is optimal.

Let's move on to the actual use of the effects with the amp and see what happens using an example of interfacing a delay into a series loop.The first thing you want to do is calibrate levels.Set your amp to sound the way you like and at the volume you play at before hooking up the delay. Now connect the cables from the delay. Let's look first at the Send signal going into the input of the delay. If the pedal has an input control set it so any clip lights are not flashing or so the pedal itself is not audibly distorting if you have no clip indicators. You then can calibrate the output of the effect into the loop return jack while playing the guitar and listening to the difference in volume that occurs when you pull the plug from the effect return cable in and out of the effect return jack.

When the plug is out, the loop is disconnected and this is the level of your amp you previously set up before hooking in the delay. Put the plug back in and use the loop return level control to match the level you get when the plug is pulled out. The loop is now calibrated to unity gain (this means the signal level coming out of the loop is the same as the level going into it. Moving on let's consider what else is happening.This is a series loop so the entire guitar sound is sent outside the amp to the delay. Inside the delay itself it passes through your dry sound and adds delay to it. Think about this. You paid big dollars for a high end tube amp and now you are depending on the quality of circuitry in a $50.00 pedal to not degrade your sound. Remember the analogy of the guy with the expensive stereo system that has a $5.00 needle on his turntable. You are only as good as your weakest link. Additionally, if the pedal varies in volume when it is bypassed you guitar sound will change in volume every time you do this.

There is a solution for this problem.

The Parallel Loop. If we put a delay into a parallel loop the beauty of this is that your dry sound never leaves your amp so it can't get degraded by the effects external circuitry. The most important point here is what many people miss and it defeats the purpose of using a parallel loop. The Delay or any type of effect in a parallel loop must be set to output the effect only, in the case of our delay this would be only the repeats and not the initial note. Now you can mix back the delay only sound with the original signal that has never left the amp and your quality is must higher. Additionally it doesn't matter if the delay varies in volume when bypassed, because when you bypass it there is no signal coming out of the delay.

Remember we are adding the delay to the original signal which never varies. Why does this sound so familiar you say? Because it is exactly what is done at the recording studio when they add effects to a guitar track at the console, so now the guitarist really can duplicate his studio sound. But there are problems that arise when using the parallel loop. You can put time based effects into the loop or you can put modulation based effects in the loop. But what happens when you put time based and modulation based effects in a parallel loop.This is where many people get confused. The answer is you can't use time and modulation based effects together in the Parallel loop. Here is an example. You plug the send into the delay the delay into the chorus and the chorus into the return. Since these effects are set to all wet the chorus only can chorus the delay repeats because that is the only signal it is getting.The dry sound remains without any chorus effect added to it. Reversing the order of the chorus and delay does not change a thing so we are left with having to use the effects one at a time or put one of them back in front of the amp if both need to be used at once. Frustrating isn't it? Now that we have looked historically and technically at the development of the effect loop, I want to cover a few other topics that come up when Effect loops are discussed. The first is that it is not necessary to create an effects loop in a tube amp using tubes.

People feel that inserting a solid state device in a tube audio path will trash the signal. This is not true in a well designed loop. A solid state loop can be completely transparent to the audio path.If it is a series loop the signal is going through the solid state circuitry in the effect unit anyway. Many amps, for example the Fender Hot Rod Deville will switch out the solid state loop circuitry when the loop is not being used reverting to a total tube path.

Many people are lost trying to balance the ratio of effect to dry sound when setting up their effects. I would suggest as a general starting point to mix the effect in and then back it down to the point where you think it is not on. Then bypass it and you will hear it drop out. This is a great general starting point especially for modulation based effects to keep your sound from becoming watery sounding.

I often see people with multi- effects pedals plugged into the input or Effects Loop of their amp. If you are using a pedal such as the Line 6 Pod XT Live and want to derive all your sounds (including distortion) plugging it into the front of your amp or putting it in the loop is not the way to go. You are subject to the gain and EQ of the amp's preamp in addition to what is provided in the pedal. Why not plug your guitar into the multi effect box's input and the output of the unit it into the Effects Return and use the amps power amp section only making the multi-effects unit your preamp. It will make your sound more solid and controllable.

Confused by the terminology?

People are often confused by the terminology they see on effects and loops as to level. You will possibly see switches for Instrument and Line Level or -10/+4. Instrument is generally -10 and Line can be +4. These are level conventions that are used to interface audio gear. As a general rule Stomp boxes are more in the -10/ Instrument range range and a rack processor may be +4/line level. Set switches appropriately for the type of unit in use.

The last thing I would like to cover is how a pedals circuitry bypasses the signal. If you remember earlier we talked about the vintage pedal bypassing the pedals output, but the pedal's input was still connected to the guitar signal causing a loading effect. One solution to this was to modify the pedal using a different bypass switch to completely remove the pedal from the signal path when the effect was not in use. This was a big help, especially if you had a long chain of effects in front of the amps input. Now only the effects in use were loading the guitar signal.

Of course as I said before about every electronics solution causing two more problems you now had tone and level shifts when it was switched in and out and don't forget about the popping when you stepped on the switch. An expensive and professional solution to this is to create a series of buffered effects stations strung together in a loop and controlled from a remote switching system. This is what Bob Bradshaw developed in the eighties and it allows the optimization of the effects levels and impedances.

A mention should also be made of the Boss style pedals. These pedals have the circuitry passing the dry and delay signals at all times even in bypass so if you are looking to have your signal not go through solid state circuitry when the unit is in bypass this is not a way to go. You can tell if your pedal is set up like this by disconnecting its battery and or power adapter. If you can't get any sound through the pedal when it is un powered it is one of this type.

Well there you have it, what I have come to call my Effects Manifesto. Hoped you enjoyed it.
Feel free to email in suggestions for topics you would like to see in future columns and see you next month.

See Part One in Previous Issue...

 

Steve Weiss is the owner and main technician of Steve Weiss Electronics Inc. He is experienced in the repair of analog and digital musical equipment. This includes everything from Vintage Tube Amps and Pro Audio equipment to Digital Keyboards There is also a guitar repair shop staffed by some of the areas top guitar repair techs. He is authorized for warranty work on most major brands. Steve Weiss Electronics is located inside of Sam Ash Music at 5460 West Sample Road Margate, FL 33073 954-975-3390 Ext 272. Steve has also spent 25 years on the road as a performing guitarist and is the designer of Primal Guitar amps that can be seen at Primal Audio.com Steve can also be reached at stevew@metromusicmayhem.com

 
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