Some effects loops let you attenuate the signal level heading to the input of the effects device.
Effects loops: What are they and why
would I want one? Back in the day,
I went down to the local music store and
bought an Echoplex tape delay, plugged
it into my amp, and had a lot of fun.
(Ironically, I also dreamed of the day when
technology would bring us the “perfect
delay”—one without warble and with perfect
copies of the input signal. Live and learn.)
After a while, I noticed that something
wasn’t quite what I was looking for, and it
didn’t have to do with the recording limitations
of the tape. I was using a decent
amount of distortion in my tube amp
and as the repeats trailed away, they got
cleaner. In nature, echoes are copies of the
original, and they get quieter and darker,
but never less distorted. A trip down to
my local amp-mod guy yielded an effects
loop, version I. The idea was that you plug
straight into the amp, and then break the
signal chain after the preamp but before
the power amp section, thus creating a
loop in which you can insert effects. Using
this loop, the input signal to the Echoplex
now contained preamp distortion and the
repeats didn’t clean up as they decayed.
Problem 1 solved.
On to problem 2. In general terms, the
internal voltage levels in effects are 1V to
2V; in tube amps, they’re 100V to 200V. In
order not to “smoke” the input stage of the
Echoplex, the signal from the preamp stage
of the tube amp needs to be attenuated
by about a factor of 10 (20 dB). But now
when the signal goes back in to the power
amp, we have lost 20 dB of gain and can’t
drive the amp to full volume.
Okay, back to the amp guy for more
mods and effects loop, version II. The first
mod is a send level control so we can adjust
the amount of attenuation of the tube preamp
stage to match the expected level of
the effects input stage. The second mod is a
return level control and an additional tube
to add back the gain we lost sending the
signal out. The biggest problem with effects
loops is getting these two different animals
to play nice with each other—and to do
that we need someone in the middle.
So far we can see that the advantages of
an effects loop are that we get to use the
tube amp’s preamp distortion and plug the
guitar straight in to the amp. But the level
matching can be problematic without send
and return level controls. Short of hacking
up the insides of the amp, what can you
do if you have an effects loop with no send
and return controls? There are companies
that sell level shifters that are basically transformers
in a box, and these devices provide
a fixed gain or attenuation of the signal.
Unlike electronics, there is no defined input
and output with these level-shifting boxes.
The loop we have been considering so
far is a series loop. At the time I got my
amp modded, parallel loops, if in existence,
were not well known. In a series loop, the
dry signal goes out and is mixed with the
wet signal within the effect. In a parallel
loop, the dry signal stays in the amp and
the effect’s mix control is set to 100 percent
wet. Now we can control the effect amount
with the loop’s return level and thus preserve
the dry path.
Some delay effects have an effects loop. A
loop in a delay effect is not the same as what
we have been describing here. The simplest of
delays would record the input signal and play
it back some time later. This would give you
one repeat. I used to do this with a 3-head
cassette deck. If we take the output of the
delay and send it back to the input we will
get more repeats—this is the feedback loop of
the delay. (This control is labeled feedback on
some delays and repeats on others.) Increasing
the level of the feedback signal increases the
number of repeats. If we break the loop, we
can insert effects that will only act on the
repeats. For example, if we insert an overdrive,
the repeats will get distorted each time they
pass through the loop. Or a low-pass filter will
make the repeats successively darker.
What about an effect loop for a looper?
That’s a topic for another day. Happy
is Senior Analog Guru
and Engineer at Strymon.