When asked to define pornography,
Justice Potter
Stewart said, in effect, “I can’t
tell you what it is, but I know it
when I see it.” Noise in audio systems
is similar. It may be hard to
describe, but we all know it when
we hear it. Let’s take a look at
noise in guitar rigs and see how
we can reduce and control it.
We can classify noise into
two categories:
random and
periodic. Hiss is a common
term for random noise. Periodic
noise has one or more frequencies
or tones that stand out.
Hum from AC power lines is
a periodic noise source. If we
effect designers design systems
carefully, we can eliminate
periodic noise because we know
something about it in advance.
Hiss is basically built into the
components we use to create
electronic products, and thus it
is here to stay. Like many things
that irritate us, we can’t eliminate
it, but we can manage it.
Before we can apply this
knowledge to our rig, it will be
useful to discuss one more bit
of tech stuff. (Don’t worry if the
nerd level of this section is out of
control—practical examples
are
ahead.) Let’s look at a pedalboard
as an example. At the output
of each pedal, the noise from
the previous pedal is added to
the internal noise of the current
pedal. The never-ending addition
of noise would be worse
if not for the curious property
that two random, uncorrelated
noise sources add up as a vector
sum. To add two noise sources,
each source is like the side of a
right triangle and the sum is like
the hypotenuse. So, instead of
1+1=2, we get something like
1+1=1.4 and 1+1/2=1.1. If we
look at this in dB, we can take
away two useful benchmarks:
Adding a pedal that has as
much noise as everything before it
will result in increasing the total
noise by 3 dB.
Adding a pedal with half as
much noise as everything before
it will increase the total noise by
only 1 dB.
“Where’s the fuzz?” When it
comes to noise in guitar rigs, the
most critical component is anything
that adds gain. Adding gain
is another way of saying, “Make
it louder.” Overdrive, distortion,
and fuzz pedals, as well as your
amp, are among the devices that
do this. As far as noise is concerned,
they are all pretty much
the same: They take the guitar
signal and make it louder up
to the point at which they clip
off the tops and bottoms of the
signal to create distortion. We
hope that the noise signal is small
enough that it won’t clip, so even
though the guitar signal might
not seem to get louder because it
is clipped, the noise will definitely
get louder. A typical overdrive
can add 40 dB of gain, and a fuzz
might offer 80 dB or more.
Using what we learned a
moment ago, we can make the
following conclusion: If a pedal
adds a significant amount of gain,
the noise generated by everything
before that pedal is critical and
the noise generated by everything
after it ... well, not so much.
“What order do I put my
pedals in?” To minimize noise,
the answer is easy: Put an overdrive
or distortion pedal first
and then the rest in any order
you want. What about amp distortion?
This is where you can
use the effects loop. The preamp
section of your amplifier
plays the role of the overdrive/
distortion pedal, so to minimize
noise connect the other effects
between the send and return.
“How do I troubleshoot
noise issues?” A common complaint
is, “My distortion pedal
is noisy and every time I turn
it on, the noise gets louder.” A
distortion pedal amplifies both
the noise coming in
and its own
internal noise. To figure out
who is being naughty and who
is being nice, plug your guitar
directly into the distortion pedal
and then into your amp. Turn
the guitar’s Volume knob all
the way down. Note the noise.
Next, mute the strings and turn
the guitar’s Volume knob up. If
the noise gets louder, then the
pedal is only amplifying what is
coming out of the guitar and the
dominant source is the guitar. If
the noise stayed the same, then
the pedal is the dominant source.
“What about frequency
response?” So far, we’ve
ignored how a pedal’s frequency
response affects the noise. Once
our signal and noise have been
joined for life, we can’t reduce
one with out reducing the other.
One thing we
can do is take
advantage of the fact that the
most objectionable noise is at
the very high end of the audio
band. If we roll off some high
frequencies—that is, turn the
treble down—the reduction in
noise may be worth the reduction
in signal. This is a critical
part of the design of a distortion
pedal. If we cut too much, the
sound is muddy. And if we cut
too little, it is thin and “hissy.”
“Will a noise gate help?”
When we’re playing nice and
loud, noise is not much of a
problem. Our signal-to-noise
ratio (SNR) is high. We have
lots of signal. The problem is
the worst when we’re not playing—
then we have no signal
and all noise. What if we were
to hire an assistant to turn
down the gain whenever we
stop playing? The noise gate is
a more cost-effective method to
achieve that goal. It senses when
you’re not playing and turns
down the volume for you.
Digital processors that model
amps or distortion pedals usually
build in a noise gate that can
make a noisy system appear to be
quiet. The fact that many younger
players’ first amps are modelers
with built-in gates sometimes
results in a poor understanding
of how gain impacts the noise
in a rig. This can be frustrating
when guitarists start putting
together a pedalboard.
I can’t say we have even
scratched the surface of noise in
audio systems, but hopefully this
will get you thinking and experimenting
with developing an
intuitive understanding of where
the noise coming out of your
speakers originated and how you
can go about keeping it under
control. Stay tuned for more
ideas in the coming months.
Gregg Stock
is
Senior Analog Guru and
Engineer at Strymon.