The Truth About Guitar Connections

Some guitarists swear they can hear differences between guitar
cables. Although people may ridicule that idea — wire is wire, right? — different
cables can affect your sound. And there’s a technically valid reason:
cables that sound a certain way with one amp may sound different with a different
amp or audio interface or when using different pickups. In
this excerpt from Sweetwater
Publishing‘s best-selling book, How
to Record and Mix Great Guitar Tracks, we’ll
explore why these differences occur and how to compensate for them.

Even before your guitar’s signal reaches its first effect, amp
input, or audio-interface input, much of the sound is already locked in due to
three factors (apart from strings, pickup type, and pickup adjustment).

Pickup output impedanceCable capacitanceAmplifier input impedance

You don’t need to know the theory behind this, just the practical
results. In fact, if you don’t want to know the “why,” you can just skip ahead
to the section at the end called “Guitar Connection Tips.” For those who want
to know more, we’ll start with cable capacitance, which is nothing more than a
second “tone control” applied across your pickup.

A standard tone control connects a capacitor from your “hot” signal
line to ground (fig. 1). With this tone-control circuit, the capacitor passes
higher frequencies more readily than lower frequencies. Shunting the higher
frequencies to ground reduces the treble. However, because the capacitor
doesn’t pass lower frequencies, they continue along to the output. (For the
technically minded, a capacitor has two conductors separated by an insulator — which
also describes shielded cable.)

Figure 1: How a guitar’s tone control and cable capacitance combine to reduce treble.

Any cable has some capacitance, although not nearly as much as a
tone control. The longer the cable, the greater the capacitance and the greater
the potential to dull the sound. Whether this has a noticeable effect depends
on the other two factors: amp or interface input impedance and guitar output
impedance.

Guitar
Amp Input Impedance

When sending a signal to an amplifier, some of the signal gets
lost along the way — sort of like having a leak in a pipe that’s transferring
water from one place to another. Whether this impacts the sound depends on the
amp’s input impedance. With passive guitar pickups, lower amp input
impedances load down the pickups, which produces a “duller” sound. Tube
circuits generally have a high input impedance, which may be one reason why
some guitarists prefer tube amps.

Although a high amp input impedance preserves the pickup level and
provides smooth tone-control action, it also accentuates the effects of cable
capacitance. A cable that reduces highs when used with a high-input-impedance
amp may have little, if any, audible effect with a low-impedance input.
However, the downside of a very high-impedance input is being more susceptible
to picking up noise, RF, and other electrical interference. The goal is to find
the sweet spot between too low and too high an input impedance.

Audio
Interface Input Impedance

Instruments
with passive pickups aren’t compatible with an audio interface’s XLR mic inputs
or with 1/4-inch line-level inputs designed
for electronic gear with high output levels. So, most
audio interfaces include at least one high-impedance (abbreviated Hi-Z), 1/4-inch
instrument input that’s optimized for electric guitar and bass (fig. 2). Use
this input to retain level and high frequencies from your guitar and to send
the cleanest sound to your DAW.

Figure 2: All Universal Audio Apollo interfaces with mic preamps have Unison inputs. When enabled, they re-create the electrical characteristics associated with amp inputs and mic preamps — like input impedance and circuit behavior. The interfaces shown above each have two high-impedance inputs.

If an interface lacks a high-impedance input, then precede it with
an effect or a direct box designed for guitar. Your guitar feeds the effect’s
high-impedance input, while the effect’s low-impedance output feeds the audio
interface’s input.

An
audio interface’s instrument input is designed for the highest fidelity, not
necessarily to be electrically identical to a typical amp input. Some guitar
players feel an interface’s high-impedance guitar input lacks the subtle
interaction between passive pickups and a physical guitar amp. Furthermore,
some guitar players don’t always want a clean
or bright sound.

To
better emulate an amp input, some audio interfaces (Avid Eleven Rack, Universal
Audio interfaces with their Unison technology) include amp-like input circuitry.
Radial Engineering makes direct boxes with a “drag” control that emulates cable
loading and lower impedances to insert between the guitar and the interface
input. MOTU’s ZBox (fig. 3) is
a passive device that inserts between your guitar and interface. Its circuitry
is more like a guitar amp’s input.

Figure 3: MOTU’s ZBox gives more of an amp “feel” when inserted between a guitar and a high-impedance audio interface input.

Patching a high-quality, guitar-specific cable between your guitar
and audio interface will provide the sound most players want. However, if
you’re accustomed to a cable dulling your sound, then use a long cable between
your guitar and the interface input. Also, a coiled guitar cord (fig. 4)
typically has more capacitance than a conventional
guitar
cable.

Figure 4: Unlike some coiled cords, this one from Vox is well made. It comes in five different colors to match studio decors or live-performance setup themes.

Pickup
Output Impedance

Our final interactive component is the guitar’s output impedance.
This is like inserting a resistor in series with the guitar that lowers volume
somewhat. Like amp input impedance, pickup output impedance interacts with your
cable to alter the sound. Here’s the takeaway:

Almost all stock pickups have a relatively high output
impedance, which is more susceptible to dulling from cables.Using stock pickups with a high-impedance input
minimizes high-frequency and level losses.Active pickups with low output impedances are
relatively immune to an amp’s input impedance and to any dulling effect from
long cables.

Guitar
Connection Tips

To achieve a guitar setup that’s relatively immune to amplifier
loading and cable capacitance, you have three main options:

Replace your stock pickups with active types.Add an impedance converter (“buffer board”)
immediately after the guitar output.Follow the guitar output with any effect (compressor,
distortion, etc.) that has a high input impedance and a low output impedance.

If you’re committed to connecting a stock guitar with passive
pickups into a high-impedance amp or interface input, then follow these cable
tips:

Keep the guitar cord as short as practical. Longer
cables have more cable capacitance.If you make your own cables, choose cables with the
lowest “pF per foot” specification (picofarads are a unit of capacitance),
consistent with cable strength.If artistically possible, keep your guitar’s volume
control turned up all the way.

If your guitar doesn’t sound quite right, don’t
immediately reach for the amp or amp sim control. A lot happens to your guitar
signal, even before it hits an amp’s or audio interface’s input jack. And, if a
guitarist swears that one cord sounds different from another, that could be
true — and now you know why.

(Excerpted from How to Mix and Record Great Guitar
Tracks, “Chapter 03 – Setting Levels, Recording, and Miking.” To purchase the
entire book, please visit Sweetwater
Publishing.)
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