Classé CP-800 D/A preamplifier
I was setting up for some musical demonstrations I was to present for a Music Matters evening at the ListenUp! store in Boulder, Colorado, in May 2011. For these events, an audio store invites manufacturers (and the occasional journalist) to demonstrate to local audiophiles the musical benefits of high-end audio playback. In Boulder, I was to share the store's big listening room with Dave Nauber, president of Classé Audio, who had set up a system with B&W Diamond 802 speakers, a Classé stereo amplifier, and a preproduction sample of Classé's new CP-800 preamplifier ($5000), all hooked up with AudioQuest cable. I unpacked my MacBook, with which I was going to play the high-resolution master files of some of my Stereophile recordings, and looked around for a DAC. There wasn't one.
I hadn't realized that the CP-800 is an example of a new breed of audio component: Not only is it a two-channel line preamplifier, it offers a complete set of digital inputs, including USB and an iPod connector. It can serve as a system's one-box heart, replacing the D/A processor and the cables connecting it to a conventional preamp. It even has a headphone output and a complete set of equalization features.
Plugging my laptop's USB output into the CP-800's rear-panel USB port, I was good to go chez ListenUp! I also vowed that the CP-800 would spend some quality time chez Atkinson.
What it does
The CP-800 shares the curved aluminum front panel Classé has used for its Delta-series products since the beginning of the century. This starts life as a flat extrusion with raised edges, and is gently bent into a U to form the front and side panels. Dominating the front panel is a large color LCD touchscreen that, in combination with the chunky metal remote, gives the user access to all functions via the usual hierarchical menu tree.
Flanking the touchscreen, within its bezel, are the Menu (left) and Mute (right) buttons, these duplicated on the remote. A black horizontal styling strip conceals: the Standby On/Off button to the left of the screen and, to the right, the infrared remote receiver window; a USB host connector for an iPod (Apple portable devices only; the CP-800's remote can control the iPod's transport functions); and a ¼" headphone jack. The large black knob for the shaft encoder that controls volume is at the right end of the black strip.
On the bottom of the rear panel are, from left to right: three pairs of unbalanced analog inputs on RCA jacks; two pairs of balanced analog inputs on XLR jacks; and two pairs of balanced outputs on XLRs, along with a single XLR output labeled Suball with their unbalanced counterparts on adjacent RCAs. The second pair of outputs can be assigned to double the main output pair, to permit biamping; alternatively, they can be used to provide stereo subwoofer outputs. The Sub output passes a mono low-frequency signal when enabled, but when the CP-800 is set up via the menu to manage bass, there is full control of crossover frequency and high-pass filter slope.
From left to right along the top of the rear panel are: the On/Off switch, and the AC input on an IEC jack; a USB port for connecting to a computer for audio streaming; a single AES/EBU digital input; three electrical S/PDIF digital inputs on RCAs (all digital inputs are galvanically isolated); four optical S/PDIF inputs on TosLinks; and various trigger and comms ports, including RS-232 and Ethernet. (On the review sample, the latter had still to be implemented.)
How it does it
Inside the CP-800, in front of the rear panel, a large, six-layer printed-circuit board runs the full width of the chassis. This carries the analog circuitry and the A/D, D/A, and DSP sections. Above this board and connected to it with two ribbon cables, a smaller, full-width, six-layer board carries the digital input circuitry. A small board behind the touchscreen, again connected to the main board with two ribbon cables, as well as to the screen with another ribbon, carries the CP-800's microcontroller.
Next to the controller board is the power supply. This is a switching type, but unlike conventional switch-mode power supplies, which have a bad rap in high-end audio circles for their propensity to introduce noise and enharmonic spuriae, the CP-800's supply uses Zero Volt Switching (ZVS), in which the primary switch operates when the incoming DC voltage is at a minimum, thus allowing the supply to have a low-noise RF footprint. In addition, the CP-800's supply is fully power-factor corrected, meaning that the incoming AC voltage and current are sinusoidal and in phase. (A white paper on this and the other technologies featured in the CP-800 can be downloaded here.)
The CP-800's rear-panel USB port operates in asynchronous mode, in which the flow of data is controlled by the DAC clock, not the computer. But the CP-800's operating mode, which Classé calls Optimal Asynchronous with Single Clock Substrate, differs from topologies used in competing products. Usually, the microcontroller in the asynchronous USB receiver chip controls the master clock. In the CP-800, a high-precision clock signal is buffered by a high-speed Field-Programmable Gate Array (FPGA) chip placed next to the DACs and master-clock oscillators. This is said to result in increased clock purity and more accurate D/A conversion. Additionally, when the CP-800 is processing data encoded at 44.1kHz and its multiples, the 48kHz master clock is turned off, and vice versa, to avoid cross contamination.
In Analog Bypass mode, analog signals are fed straight to the volume control and output circuits and the digital clocks are turned off. (The volume control is implemented with two two-channel Burr-Brown PGA2310 programmable-gain chips, one per channel used as a differential volume control.) However, the tone or equalization controls are implemented using two Analog Devices DSP chips. So if the user wants to use these controls, the analog input signals are converted to 24-bit digital data with a Cirrus Logic 5381 A/D chip. Digital data are turned back to analog using two Wolfson WM8741 DAC chips, each of these a high-performance, multi-bit, sigma-delta, two-channel type capable of operating with 32-bit data. Each DAC chip operates in differential mode, one per channel, and runs at a constant rate of 176.4 or 192kHz. The voltage-output DACs are followed by a fourth-order reconstruction filter with a 100kHz passband.
While the CP-800 offers myriad customizing options via its touchscreen menu, the default settings out of the box proved to be all that I needed. Pressing any part of the touchscreen's Home screen or the Source Select button on the remote allows you to choose a digital or analog input. The chosen source is then displayed on the bottom left of the Home screen. If digital, the Home screen displays the current sample rate in small print at the bottom. Large numerals in the top half of the screen indicate the current volume setting.