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Technical Secrets of the Crossfader
By Johnathon Novik

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.

  • ALPS is a brand name for a Japanese manufacturer of quality faders. ALPS faders can be used in direct attenuation or VCA style circuits.

  • Some manufacturers do not advertise the fact they use VCA circuitry. 
  • Better mixers will use VCA circuitry for the channel faders in addition to the crossfader.
  • Most 19" mixers come with a fixed fading profile that is suited towards the wedding or club DJ. However, the manufacturer may sell a replacement crossfader with a different profiles.
  • There is no correlation between fading profiles and the type of crossfader circuitry being used (VCA or direct attenuation).
  • Many narrow profile mixers (e.g. 10" models) used by scratch DJs offer adjustable fading profiles. Some provide three preset profiles while others are continuously adjustable.
  • Faders come in different lengths to suite individual DJ styles. Longer crossfaders offer more precise control whereas shorter crossfaders offer a faster response.
  • A fader that is "hot swappable" can be replaced while music is still playing. While the fader is removed the channels assigned to the fader will go to minimum attenuation (maximum output).
  • Every crossfader will have a different tactile feel that cannot be adjusted. Some require a light touch, some heavier. There is no substitute for hands on product testing to see if one meets your liking.

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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