Faders

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The crossfader is the component of the DJ mixer that is most important to turntablists.

Intro

Early DJ crossfaders were a simple carbon-track design, and contained two pairs of Logarithmic curves. This made the crossfader essentially equivalent to two back-to-back input faders. These were adequate for looping and beat-juggling, but made performing even basic scratches very difficult. In addition, the faders simply weren't designed to withstand the number of cycles a typical turntablist would put them through in a day, let alone a few years. As a result, many/most early scratch DJs used the phono/line selector switch to cut instead.

By the early 90s, performance mixers (such as the mx-2200) had faders with sharper curves, and used a carbon/plastic hybrid design to increase the durability. Still later mixers, such as the Vestax PMC Pro series, switched the passive fader design for VCA circuits. These allowed advanced features like control voltage filtering (increasing fader life even further) and curve adjustment.

The current generation of performance mixers have a wide variety of novel non-contact methods of sensing fader position.

Fader requirements

Turntablists have the following requirements in a crossfader :

  • Low friction - the wiper should move with as little friction as possible.
  • High durability - the fader should be able to withstand significant lateral and forward-backward force at all wiper positions.
  • Long operational lifecycle - the faders lifecycle should be specified in the millions of operations, ideally tens of millions.
  • Small dead zones - faders often have dead zones or "lag" at each end of the wiper travel, where the movement of the wiper has no effect. Large dead zones affect the ability of the mixer circuit to reduce the cut-in distance.

Other factors to consider include :

  • Wiper weight - The inertia of a fader depends on its mass. For example, the Focus Fader is extremely light and thus has very little inertia, whereas the large metal stem base used in Vestax PCV faders gives them a much larger inertia.
  • Rebound damping - different faders have different means of damping the wiper when it hits the interior wall of the enclosure. This affects both the amount of mechanical noise generated by the fader, as well as the efficacy of some scratch techniques that might involve bouncing the fader against the damping material. The ALPS faders used in early Vestax PMC Pro mixers have no damping whatsoever, whereas Pioneer's Magvel fader used in its most recent performance mixers have a full range of interchangeable rubber stoppers with different rebound characteristics.

Design approaches

Mechanical characteristics

Generally speaking there are two types of fader mechanical designs :

Spring suspension

Wipers in spring suspension faders are suspended between two metal springs - an upper spring that pushes upwards against the top of the metal enclosure, and the lower metal brush that actually makes electrical contact with the fader's carbon tracks.

These faders are cheap as they contain very few parts, and can be made very small. However, they are not very durable due to there being no strong wiper support structure and generally have a short lifespan due to the brush contacts being an integral part of the support mechanism. In addition, they are usually lubricated with a fairly viscous lubricant so are not ideal for turntablists.

Most DJ faders are of this type, including (until recently) those found in premium mixers such as the Pioneer DJM series and inexpensive passive designs such as the Raiden and Frisk faders. They are not however generally found in performance mixers for the above reason.

Railed

The wiper slides along one or more polished steel rails, which allows for greatly improved durability and lifespan as the mechanical support is separated from the electrical mechanism. This also allows for position sensing methods other than the traditional metal brush/carbon track combination.

Most of the faders found in performance mixers are of this type, but can be separated into two categories :

  • Short bodied - these have the rails placed horizontally side-by-side. This allows the fader enclosure to be of a similar size to spring suspension designs, but some designs do not have great angular stability in the wiper as the rails are not very far apart. Performance-oriented faders in this format include :
  • Long bodied - these have the rails places vertically, one above the other. This design keeps the wiper very stable, but require a lot more space inside the mixer to accommodate the taller fader enclosure. Performance-oriented faders in this format include :

The Stanton CFF1 (aka Focus Fader) and Ecler Eternal are both combinations of long-bodied and short-bodied designs, in that the rails are side-by-side like a short bodied fader, but the enclosures themselves are long-bodied to fit in the sensors.

Electronic characteristics

Fader electronics can be again divided into two groups, carbon-track-based and contactless.

Carbon track

Most faders are based on the principle of metal brushes, usually brass or stainless-steel, contacting a PCB covered in a carbon track. The carbon track increases in resistance along its length, and this change in resistance can be used to control an audio circuit.

Early mixer designs actually made the fader a part of the audio signal path, allowing the change in resistance of the wiper to adjust the relative volume level of the output. Later mixers used the fader to generate a control voltage for a Voltage Controlled Amplifier, allowing techniques such as filtering and curve adjustment. A more in-depth discussion of the crossfader support circuitry can be found in the crossfader circuit page.

Disadvantages to the carbon-track design include :

  • Increased friction - since the metal brush has to be in contact with the carbon track, naturally this means there will be some friction. This can be moderated by reducing the force of the brushes and lubricating the carbon track.
  • Limited lifespan - every time the wiper is moved, a few atoms of the carbon track are scraped off, and as such the lifespan of the fader is limited. Exactly how limited this is depends on the formulation of the carbon track and the maintenance level of the fader.
  • Susceptibility to contamination - The brushes and tracks can gather dust and dirt, which reduces the quality of the contact and can introduce unwanted noise.

Contactless

As an alternative there are a number of contactless faders in the market, each based on one of these technologies :

  • Photointerruptor - Invented by DJ Focus, these were the first contactless performance faders to be created, and relied on the principle of the wiper interrupting a beam of infrared light at each end of the wiper travel. Faders based on this technology include :
    • Stanton CFF1, based on Focus' earlier homebrew prototype.
    • Stanton CFF2, which expanded the principle to include photointerruptors along the entire length of the wiper travel for full analogue position sensing.
  • Photo Reflector - designs based on reflecting light off the wiper. Designs of this sort include:
    • American Audio QD5 MK2 optical fader
  • Magnetic hall effect which use Hall effect sensors to detect the position of a Neodymium magnet attached to the wiper. Faders based on this technology include :
    • Rane Magnetic Fader, which was the first magnetic fader to be released, and first seen on the Rane TTM-56 mixer.
    • Ecler Eternal, available as an option for many Ecler HAK/NUO mixers
    • Vestax CF-X2, as seen in the Vestax PMC-05ProIV mixer and available as an option for the VCI-380 controller.
  • Magnetic Reed switch, which actuates when a magnet attached to the wiper is close enough. This technique was recently impletemented by Wax Inspektor in his Waxcutter, but other people had experimented with them previously. It has the distinction of being the only truly passive contactless design, but it has never been commercialized.
  • Capacitive sensor technology, which uses the wiper to modify the effective relative permitivity of the dielectric of multiple linearly spaced capacitor plates innit. So far the only faders based on this technology have come from Audioinnovate, i.e. :
  • Inductive sensor technology - lord knows how this works. I'm still investigating.
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