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Most DJs have
probably experienced some sort of problem with a crossfader. Maybe
the lever broke off, maybe it produced static or popping sounds or
maybe it just had some residual stickiness from that Coke spilled on
it last year. The problem was annoying at the time but you probably
didn't think too hard about it. The replacement crossfader cost less
than $75 and it is just about the easiest fix possible on a piece of
DJ equipment. It's not until the third or fourth time a problem
occurs that you realize you have spent more on replacement
crossfaders than you did for your mixer. Then you start to appreciate
a quality crossfader.
So how does one
find a quality crossfader? Consumer Reports isn't going to review DJ
mixers (instead of food mixers) any time soon so you'll need to learn
how to recognize quality on your own. This article attempts to
demystify crossfaders so you can appreciate the difference between a
good one and a great one.
The Crossfader's Function
A crossfader is
designed to predictably control the outputs of two separate mixer
channels based on the relative position of the fader's knob between
its endpoints. It's a simple sounding task but there are many
different ways the job can be done, electrically and mechanically.
Crossfader Construction
Electrically
speaking, most crossfaders are just variable resistors. They consist
of a resistive material placed between two contacts. A third contact
called a wiper slides between the end contacts as you move the
crossfader knob. The quality of the resistive material and wiper
contacts impacts the sound quality (popping and cracking noises) and
the longevity of the mixer. The bearings used and type of
construction will impact the smoothness of the crossfader and its
susceptibility to mechanical stresses and dirt.
Crossfader Circuit Types
Most mixer
manufacturers buy their crossfaders from the same handful of
vendors. This does not create equality though because circuit
implementations can be very different. Most crossfader circuits are
implemented in one of two basic schemes. The first and most common
implementation is based on direct attenuation of the audio signal by
the crossfader. The second and more preferred implementation in
quality mixers uses a voltage-controlled amplifier (VCA) to perform
the actual attenuation of the audio signal.
The block diagram
below outlines the basic configuration of a direct attenuation
crossfader. The audio signal travels right through the crossfader's
wiper. As the wiper moves, it can skip, scratch or wear away the
resistive coating. This produces audible popping and cracking noises.
Dirt build up will make these problems worse. Another problem with
this approach is that the actual crossfader consists of two variable
resistors tied together (for the left and right channels). These
resistors may not be perfectly matched and may wear differently with
age. This produces slight differences in the left/right channel
fading profiles. Improved mechanical construction techniques and new
materials have reduced these problems tremendously in recent years
and thus some high-end mixers still use direct attenuation circuits.
In general though, direct attenuation is more commonly found in older
mixers and budget mixers.
The crossfader in
a VCA implementation is mechanically similar to the one used in
direct attenuation design. The operation is significantly different
though. A DC current (double lines) is passed through the crossfader
instead of an audio signal. The crossfader position determines the
control voltages sent to separate Voltage Controlled Amplifiers for
each channel. This arrangement has several key benefits. First,
electrical noise caused by the wiper action does not create pops and
crackles in the audio section. Second, only one resistive track is
needed on the crossfader thereby eliminating left/right differences.
Third, better left/right channel separation can be maintained because
there are no audio signals present at the crossfader.
Optical Crossfaders
Optical
crossfaders are a relatively new addition to the high-end mixer
market. These are mostly implemented in VCA style circuit. An optical
fader typically consists of an LED transmitter aimed at a
photo-detector with a shutter that slides between the two. As the
shutter blocks the light path the output of photo-detector's is
changed. This control voltage is then sent to the VCAs as in the
previous block diagram. The benefit of this design is the absence of
physical contact between the shutter and the optics. This eliminates
friction, wear and tear, and travel noise. These faders often have
precision ball-bearings that offer extremely long life. There are
cheap optical faders though. These use a photo-resistor in the
detection circuit instead of a transistor or diode. The resistor
designs respond slower to changes and they're fading profiles are not
as repeatable with time and temperature.
Fading profile
The fading profile
characterizes how quickly one channel is faded out and the other is
faded in. Different profiles are needed to accommodate different
mixing styles. To cut and scratch a DJ needs a sharp profile for
quick transitions. On the other hand a club DJ that beat-mixes needs
a more gradual "constant power" profile. This profile
attenuates both channels slightly at the midpoint to insure that the
combined output of the two matched beats doesn't double the output
volume. Finally, wedding and radio DJs that don't beat mix would use
a profile where each channel reaches full volume at the midpoint and
then linearly decreases as the slider is moved to its endpoints.
Since the two songs aren't beat matched and one recording is fading
in while the other is fading out, the net result is a constant output level.
Shopping considerations
Appreciating the
differences between crossfaders only makes shopping for mixers more
difficult. Specific crossfader details like those discussed in this
article aren't found easily on web sites or in catalogs. You'll need
to do some research and contact a few manufacturers to insure you are
getting what you want. Here are a few other helpful shopping hints.
Choosing the right mixer
You will probably
never choose a mixer by crossfader alone. Inputs, outputs, features,
size and price will still dominate your shopping search. Hopefully
this article has convinced you that the design of the crossfader
should also be an important part of your consideration too. And while
crossfaders are becoming more versatile, there is no design that does
everything best. You'll need to examine your needs and
determine what tradeoffs you can make.
Bio: (I don't know
if you use these) Jonathan Novick began djing in the late seventies
at a small FM radio station and continues to DJ weddings and
corporate parties on a part time basis. He holds a BSEE degree from
Rensselaer Polytechnic Institute and currently works for a Fortune
500 technology company in a technical management role.
Credits: The
author wishes to acknowledge Chris Roman of Numark Industries and
Rick Jeffs of Rane Corp for their valuable assistance in providing
data for this article.
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