2019 AGS Magazine_V5

Most solid-state amplifiers with either an acoustic-focused
design or multi-purpose, multi-channel
design are going to have a special mic input and
phantom power. Otherwise, it’s best to use a special
mic preamp or, in cases where a combination
of a pickup and a microphone are being used, a
blending device will condition the signal. Many
pickup/mic systems have this blend control built
into the guitar, or are sold with the device.
Once the signal is generated, it is now off to the
amplifier. There are three basic components to
an amplifier: the preamp, the power amp and the
speaker.
The preamp is where most of the character of
an instrument amplifier is developed. The primary
job of the preamp is to take the signal sent from
the instrument and bring it up to the appropriate
level for the power amp to do its job. It is in this
stage where the tone controls are used along with
the preamp gain to condition the instrument’s signal.
It is also at this stage where a lot of unwelcome
sounds and distortions can be introduced.
Often acoustic instruments, because they use
piezo-electric pickups, require or work better with
an additional preamp because they don’t output
enough signal that a traditional guitar amp can accommodate.
Modern amplifiers and piezo electric
pickups are high enough quality that an additional
outboard preamp isn’t really necessary, but there
are a lot of very high-quality devices designed to
improve, enhance or just clean up the signal on its
way to the amplifier.
Most preamps have some type of tone controls.
Musicians are universally familiar with a basic tone
control, where you turn up or down the bass and/
or treble of an instrument. These passive shunting
circuits are standard in almost any kind of audio
amplification because they are the most intuitive
way to change a signal to suit personal preference.
Beyond that, there are also active controls and
equalization to further fine tune the signal. One
reason it’s important to have an amplifier designed
specifically with an acoustic guitar in mind, is that
general market amplifiers are designed with a
“scoop” built into their preamp, meaning the midrange
frequencies are artificially turned down because
when playing rock on an electric guitar those
frequencies can become “muddy” and unpleasant.
With acoustic guitars, or archtops and jazz music,
you need those frequencies because you want a
much more piano-like response.
The preamp is really where there can be a real
“art” to audio design. Different components can
make a difference in whether something is dark
or bright, even if they have the same specification.
Just like choosing a type of wood, choosing
a brand and type of capacitor can change the
sound; for example going from a less expensive
type of capacitor to a polypropelene capacitor will
add brilliance to a sound, but the finer elements
can even change greatly depending on the manufacturer
and the individual components tolerances.
Even something as simple as a different batch
can make a difference in the sound and although
subtle, not necessarily insignificant. This is typically
where math and engineering take a backseat to using
one’s ears to make design decisions.
The next stage in amplification is the power amp.
Power amplifier designs are a little more straight
forward and have traditionally been broken into
classes (A, A/B, etc…) but in the simplest terms,
these days there are three basic types of power
amplifiers on the market: Tube amps, traditional
transistor solid-state amps, and class D (switching)
amplifiers.
Tube amplifiers are the oldest of the technologies,
and their design is an art unto itself -- every
little thing matters including where wires are
placed when connecting components together.
Solid state power amplifier architecture uses a
transistor as opposed to a vacuum tube, reducing
weight, size, cost and maintenance, but requiring
more complex circuits. Most recently, class D amplifiers
have become more available and are the
predominant amplifier on the market for acoustic
instruments as they put out the most efficient power.
In the beginning, class D amplifiers had both
quality and fidelity issues as well as noise problems
when applied to instrument amplifiers (as opposed
to consumer audio applications), but those problems
are largely a thing of the past.
When designing an amplifier for acoustic instruments,
or any instrument where you do not want
the signal to contain any distortion, you want as
much power as is practical. The more power you
have, the more volume and dynamic control you
can get from the instrument without mud or distortion;
however that brings with it size and weight.
Class D amps are the most economical way to do
this, but can be missing the analog warmth you get
from a tube amp design. As with the preamp, this
is where designs need the attention of the designer’s
ears as much as anything.
If weight is not so much of a concern, then a
well-designed tube amp can deliver a few things
that aren’t possible with even the most sophisticated
digital circuitry. One of the unique characteristics
of vacuum tube technology is that tubes can
produce harmonics naturally. Why might this be
important, and doesn’t this constitute some type
of distortion of the input signal? A purist would say
yes – it’s harmonic distortion, but when you take
into consideration the loss of the “total” sound and
nuances that a single-point pickup system doesn’t
deliver to the amp, then it’s a reasonable trade-off.
When playing your favorite acoustic guitar,
you’re hearing all kinds of sounds coming not just
from the sound hole, but from the sides, the back,
the neck and the strings themselves. A lot of the
complexity of that sound is lost when you use a
single-point, or even a dual, pickup system that
samples the sound from a very narrow space on
the guitar.
Vacuum tubes help by reintroducing some of the
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