When Joe Claussell DJs he'll often seem overcome by the music he's playing. He'll lean downwards before snapping violently upright. His elbows will start to flail. He'll shake his head and contort his features like a musician in the throes of a breathtaking solo. These movements are all connected to Claussell's manipulation of a rotary mixer. He'll make the mixer part of his performance, removing frequencies, injecting drama and accentuating musical riffs. He'll create a tremolo effect by rapidly twisting the EQs. At other moments he'll use the volume knobs to chop the music, producing new drum riffs.

Few DJs can use a rotary like Claussell does. In fact, rotary mixers are unhelpful for many DJ styles. If you're a DJ who likes to scratch or mix lots of records quickly, a rotary mixer is probably not for you. Yet rotaries have amassed a cult following. DJs like Motor City Drum Ensemble, Hunee, DJ Harvey and Sassy J all prefer them. World-class clubs such as Jaeger, Robert Johnson and fabric have them permanently installed in their booths.

The appeal of the rotary remains mysterious. Rotary mixers use knobs to control the sound rather than faders. Few have flashy gimmicks or effects. Many models are filled with expensive components, promise high-definition audio and come with a hefty price tag. And yet, rotaries do not necessarily equate to great sound. Using a knob to control sound is not intrinsically a good thing. But vintage models hold a legendary space in DJ culture. Nowadays, newer rotary mixers, such as the ARS MODEL 6700, are among the best-sounding on the market.

The rotary mixer's appeal is as much cultural as it is practical. Rotary mixers were there at the birth of DJ culture. The first commercially available club mixers, the Bozak CMA-10-2DL and UREI 1620, were rotary. Heavy, expensive and powerful, these handmade mixers were made with the best components to generate amazing sound. They were adopted at legendary clubs such as Studio 54 and Paradise Garage. Today, this gives rotary mixers an aura of authenticity that's alluring to dance music's traditionalists.

Top: Larry Levan, Paradise Garage, 1979. Bottom: Junior Vasquez, Tunnel Club, 1995

After disco's golden era, rotary mixers became less common in clubs. Cheaper and arguably more versatile fader mixers replaced them. The GLI PMX 7000, released in 1977, was the mixer that started this trend. Equipped with a cross fader that allowed DJs to make quick transitions between records with one hand, it was a tool that powered hip-hop DJs and transformed the art of DJing. Rotaries slowly became the preserve of audiophiles and music history nerds. Many diehard fans persevered with vintage models—the audio-geek equivalent of owning a vintage car, with similar challenges of upkeep.

But in recent years rotary mixers have made a resurgence. Leading the charge has been the French audio company E&S. In 2010 it released the DJR400, a rotary mixer small enough to carry in a DJ bag and plug into a club's soundsystem. The idea came from Cyril Etienne, more widely known as DJ Deep. "I fell in love with my UREI when I bought it in the early 2000s," said Etienne. "In 2003 I wanted to fix a minor issue with the UREI. I met Jérome Barbé, an electronic engineer who had a nice reputation in fixing vintage equipment. Barbé did not know about house or techno or DJing. Soon the idea of creating a prototype that would use the UREI as a model grew in our mind."

Barbé and Etienne refined their prototype to create the DJR400. The mixer offered great sound. Frank Wiedemann from Âme, Kerri Chandler and Danny Krivit gushed about the mixer's best-in-class audio. The mixer faithfully recreated that elusive "feel" of the much-loved rotary mixers from the '70s.

"When you're playing on a rotary mixer and you're turning two knobs there's definitely a different sense, within the flow, and the transition is very smooth," said Louie Vega, a DJ who has been using rotary mixers since 1985. DJ Deep added, "With UREI or E&S there is a smooth feeling when mixing tracks together, you don’t have to adjust EQs while mixing, there is a smooth compensation from one channel to the other. So that's a very almost a sensual way to feel transitions. You can accentuate parts of the tracks in a very personal way. So yes, it makes it a great instrument of expression."

The DJR400 had one particularly novel feature: an inbuilt isolator. More often, isolators were found in much bigger mixers or had to be installed as a separate piece of equipment. "You are able to isolate the entire frequency with an isolator," said Louie Vega. "You could almost make it sound like you have an acapella live right when you're playing." But an isolator also allows a DJ to subtly manipulate the sound that's coming from a mixer's master channel. This, in the right hands, can help improve the sound on the dance floor. "Isolators are not only fun to use," said Mafalda, a London-based DJ and rotary fan. "They also add something to the music that EQs don't."

Isolators and rotary mixers are sometimes spoken about in quasi-religious terms, but their defining qualities are actually quite simple. For instance, an isolator's sound can be explained by its ability to affect a wider range of frequencies than most EQs. The bass EQ on an average DJ mixer could cut between 10 and 100hz, while an isolator might go as high as 300hz (some isolators allow you to set this point yourself). Proponents of isolators and many engineers say that cutting or boosting wider frequency bands sounds more natural to the human ear than the relatively narrow cut of a traditional EQ, resulting in a more organic sound.

Another sonic factor is the slope of the filter shaping the sound. Isolators have a gentle slope that lets more frequencies through, contributing to the smooth and open sound for which they're known. So while top-of-the-line components are an important part of the rotary sound signature, the inherent character of the isolator has a big impact, too.

However, isolators can also be used recklessly. "Isolators are an advanced tool to EQ more dramatically," Rich Cufley from Sound Services told us. "Give Floating Points the E&S and it's an instrument. But many other people fundamentally misunderstand EQing—the golden rule is to subtractively use EQs, only use it to remove stuff. But boosting it is totally undesirable." A more subjective point is that too much fiddling with EQs can disrupt the flow of a set. Even the pros, such as Claussell or Theo Parrish, occasionally misjudge their isolator usage.

The DJR400 won over fans, but it was a relatively niche product. "In 2003 in Europe, not many people wanted to hear about anything rotary in terms of mixers," said DJ Deep. "Except a few friends, mostly American, there was not much interest in Europe. I remember travelling to my gigs with the DJR100 prototype in a bag. Most of the time people were a bit sceptical about the whole thing. It took a very long time, about ten years, from 2003 till 2013, to take hold."

In 2015 there was a tipping point. A flurry of new rotary mixers entered the market. Many replicated the main selling points of the DJR400. Rane, MasterSounds, Condesa, Bozak and ARS all released tabletop mixers complete with inbuilt isolators. Bigger rotaries were released, too, like Isone's FP mixer and the ARS 6700. Deluxe custom models made from solid brass gained attention. There was even a budget model, the Omnitronic TRM202MK3, which made owning a rotary more financially feasible. This has helped make the rotary mixer a more visible part of DJ culture than ever before.

Some of a rotary mixer's most lauded features, such as producing great sound, are more nuanced than you might imagine. Any mixer won't produce great sound on its own. You need fantastic sounding records, well-configured turntables and a perfectly executed soundsystem. David Mancuso, a DJ that's probably done more than anyone else to further audio quality in DJ culture, didn't use any kind of mixer. As the late DJ said in an RBMA interview, "The less components you use, the better the transparency of the sounds. With each audio component, it may be very high grade, but eventually it will colorize what you're hearing.You use the minimum." If you want the best sound, you might need to ditch the mixer altogether.

A rotary mixer is not going to transform an average DJ into a great one. If you're looking for a tool to blend records together, there are more cost-effective options, ranging from fader mixers to digital controllers. But for retro-minded DJs they're hard to beat. Their converts remain evangelical. Why else would Louie Vega go to the effort of flying a sound engineer around the world with him so he can build a booth for a rotary isolator before each gig?

"After listening over and over to DJs like Louie Vega and Joe Claussell, their control was perfect and the mixes were extremely smooth," said DJ Deep. "I couldn't figure it out on my straight fader mixer, until I got my hands on a UREI. I fell in love with it. Rotaries do not make you a good or bad DJ. They are simply a dope tool."

Isonoe 420 DJ mixer, Giant Steps, London