DALI Callisto 6 C wireless loudspeaker

PS Audio’s Paul McGowan has been sending out a daily newsletter by email since 2011. In his May 29, 2019 epistle he asked, “What would our world of high-end audio look like if there were only active wireless loudspeakers? If even the half-a-million-dollar mega-beasts were internally amplified and connected via wireless and controlled from an iPad? No more boxes. No more wires and cables. Only speakers.

“Would we have come full circle, back to the days when music reproduction systems were self-contained?” he concluded.

As I read Paul’s newsletter, I had just started to write this review of a wireless speaker from Danish manufacturer DALI (Danish Audiophile Loudspeaker Industries), the Callisto 6 C, a pair of which has been living in my listening room for the past two months. The Callisto 6 C costs $5750/pair including a DALI Sound Hub fitted with a BluOS NPM-1 module (see later) and is indeed a self-contained music reproduction system.

Going without wires
Stereophile has reviewed other wireless speakers in recent issues. Jim Austin reviewed Apple’s HomePod in August and September 2018, and Herb Reichert and I reported on KEF’s LSX in May and June 2019. What these loudspeakers have in common, other than an absence of connected cables—they do have wires for power—is that they use class-D amplifiers and powerful digital signal processing (DSP) to implement the crossover and optimize the loudspeaker’s performance in the time and frequency domains.


Both the KEF and Apple also use DSP to adjust the speaker’s behavior to allow for its position in the listener’s room. The Callisto 6 C doesn’t include the latter capability, but what it does have when used with DALI’s Sound Hub is the ability to accept PCM audio data sampled up to 192kHz. (Internally, however, the Callisto system is limited to 96kHz.) The HomePod is restricted to sample rates up to 48kHz, and while the KEF can decode 176.4kHz and 192kHz PCM data, it can only do so with a wired Ethernet link.


The Sound Hub is a relatively small black box with a circular display centered within a front-panel volume-control ring, just below three control buttons. A second, square display on its top panel is active when a pair of Callisto speakers is connected (see later). According to DALI, the Hub connects to the speakers with either a 2.4GHz or 5.8GHz Wi-Fi link, whichever offers the best quality, using “a proprietary 30-bit protocol.” This link transfers uncompressed, 24-bit I2S-formatted audio data sampled at 96kHz, “utilizing the remaining bits to control volume, speaker ID, and other control data.” This link is bidirectional—swipe your finger left or right along the top panel behind the gently curved front baffle of one of the speakers and you can adjust the levels of both, with the volume indicated with white-LED bargraphs on their baffles beneath the woofers and in numbers on the Sound Hub’s front-panel display. Each speaker also has an analog input on a back-panel RCA jack; when the speakers are hard-wired, the analog signal is converted to 24/96 digital and the volume control is disabled.


On its back panel, the Hub has analog inputs (one pair on RCA jacks, another on a 3.5mm stereo jack) and three S/PDIF digital inputs(one coaxial and two optical TosLink). Wirelessly, the hub accepts Bluetooth input, including aptX-HD. A Type A USB port provides 5V power for a Chromecast Audio dongle or to charge phones, and there is a service port. Also on the rear panel is a pair of preamplifier outputs and a mono subwoofer output, plus two ports for plug-in modules. One of those ports can be occupied by the BluOS NPM-1 module, which is included with Sound Hubs sold in North America; the module adds Ethernet and Wi-Fi network connections to allow control of the Callisto/Sound Hub system and the transmission of data from a NAS, with MQA decoding, using the Bluesound BluOS Controller or Roon apps. The Sound Hub is supplied with a remote control that connects via Bluetooth.


The Callisto 6 C
I am 600 words into this review, and I haven’t yet discussed the loudspeakers. The Callisto 6 C is a tall, slim tower, just over 39″ tall. An optional pair of spiked outrigger stands provides stability on carpeted floors.

The last DALI loudspeaker I reviewed was the Rubicon 8, in March 2015; the Callisto 6 C features a line-up of drive-units similar to the Rubicon’s, but with two woofers rather than three, these operating below 2.6kHz. These woofers feature a unique cone material—paper pulp impregnated with a matrix of wood fibers, the combination said to give a better controlled transition from pistonic motion to break-up—and the cone is terminated in a soft, low-loss rubber surround.


Like the Rubicon, these woofers use what DALI calls their Soft Magnetic Compound (SMC) in the magnet pole pieces. Conventional magnetic materials suffer from hysteresis: It takes longer for them to be demagnetized than to be magnetized, which results in distortion, especially at high frequencies. If the pole pieces could be made from a material that is highly susceptible to magnetization but is also an electrical insulator, the hysteresis and the resultant distortion, especially third-order, would be very much reduced. SMC is formed from small particles of iron that are each coated with an insulating material to create a substance that is still highly ferromagnetic but has a very low electrical conductivity—about 1/10,000 that of iron. This dramatically reduces hysteresis. The Callisto woofer’s SMC pole piece is surrounded with a copper ring to minimize the modulation of the magnetic field by the current fed to the voice-coil.

The woofers are loaded with two flared ports on the rear of the enclosure. DALI CEO Lars Worre holds strong opinions on the behavior of reflex systems, feeling that the ports should be used more to reduce distortion rather than to achieve the ultimate low-frequency extension. Worre told me that the textbook reflex tuning, where a Q of 0.7 gives maximally flat extension and a response that is 6dB down at the port tuning frequency, compromises the woofer’s behavior in the time domain and results in too much bass in real rooms. (“Room gain,” the reinforcement of a speaker’s bass due to reflections from the boundaries, typically tilts up the low-frequency output by approximately 4dB/octave below a corner frequency that is inversely related to the size of the room.) DALI speakers use what Worre refers to as “Fast Tuning,” where a lower-Q woofer alignment gives sufficient in-room bass extension coupled with much lower group delay at low frequencies and reduced ringing at the port tuning frequency.

Also like the Rubicon, high frequencies are handled by a pair of units: a conventional soft-dome tweeter, this a little larger than usual (1.15″), and a ribbon supertweeter 0.67″ wide and 1.77″ tall. There is no crossover between the two high-frequency drivers: The ribbon tweeter rolls in above 8kHz and widens the horizontal radiation pattern above 10kHz, where the dome tweeter is starting its mechanical rolloff. This is intended to give a greater range than usual of seating positions at which listeners can hear a full high-frequency balance.

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US distributor: NAD Electronics International

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