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.
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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. |
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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.
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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. |
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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...