Sonic Frontiers SFCD-1 CD player
As digital processors and transports have improved, however, it has become increasingly apparent that the jitter-inducing S/PDIF or AES/EBU interface connecting them, where the word clock is embedded within the data—is becoming the limiting factor. Some manufacturers—Linn, Spectral, Sumo, Krell—have invented custom systems that use a separate clock cable to lock the transport to the master clock in the DAC. One—Audio Alchemy—is promoting the use of the Philips i2s interface, where the word-clock signal is carried on a separate line (footnote 1).
A third strategy is simply to design a one-box CD player that doesn't need an interface. Other than Krell, however, with its excellent-sounding KPS-20i, and Naim with its CDS (footnote 2), high-end designers have avoided the single-box solution. Isolating the various analog, digital, and mechanical subsystems from each other is a daunting challenge in a single chassis. A CD transport mechanism, with its motors, servo systems, and EFM decoding and error-correction electronics, throws off electronic noise and RFI that can affect the crucial analog output stages. Moreover, these subsystems can interact with each other through the power supply, and pollute the power supply rails with noise.
The trick is to create a single-box CD player that doesn't suffer from these problems, makes sure that the clocking circuit takes full advantage of the lack of a jittery interface, uses state-of-the-art digital filters and DACs, and features a well-designed analog output stage.
Enter the SFCD-1
That's exactly what Sonic Frontiers has attempted with their new SFCD-1 CD player. The SFCD-1 is essentially the company's SFT-1 CD transport and SFD-1 Mk.II digital processor in the same chassis. But rather than scale down the power supply to fit in one box, Sonic Frontiers has increased the power supply size and improved the isolation for the SFCD-1. The SFCD-1 is also the first CD player to use the UltraAnalog DAC module; it is a collaborative design effort between Sonic Frontiers and UltraAnalog.
From the front panel, the SFCD-1 CD player appears identical to the SFT-1 transport with one important exception: the presence of the HDCD LED that lets you know when the player is decoding an HDCD source. And instead of digital output jacks on the rear panel, the SFCD-1 has single-ended and balanced analog outputs. No digital output is provided; Sonic Frontiers believes that no improvement can be made in the SFCD-1's sound quality by adding an external digital processor. The SFCD-1 also has no digital inputs for decoding other sources (unlike the Krell KPS-20i, which can also function as a digital processor or transport). Adding digital inputs and outputs would, of course, have increased the SFCD-1's price.
Although the SFCD-1 bears many similarities to Sonic Frontiers' SFT-1 transport and SFD-1 Mk.II digital processor—the power supply, DAC, digital filter, and analog output stage are variations on the basic topology found in the separates—some important differences distinguish the CD player. I mentioned earlier the problem of lack of electrical isolation between the noisy transport mechanism and the sensitive audio circuits in a one-box CD player. The analog circuits, digital circuits, and mechanical systems "talk to each other" through the common power supply provides a conduit for noise to get from one subsystem into another. Obviously, a separate transport and processor, with separate power transformers and even separate AC cords, won't suffer from this. Putting a transport and processor in the same chassis to create a CD player invites problems with circuit interaction through the power supply. Sonic Frontiers has attempted to minimize or eliminate this problem by giving the SFCD-1 a well-isolated supply.
More than half the SFCD-1's circuit-board real estate is dedicated to the power supply. Overall, the SFCD-1 uses more than 31,000µF of filter capacitance. Three power transformers supply 12 regulation stages, two of which are discrete. One transformer powers the front-panel display, microcontroller, display driver chips, and other such ancillary functions. Another transformer feeds the audio-circuit supply rails through four secondary windings. These audio-circuit supplies include two separate sets of ±12V rails and the ±115V B+ and B- supplies to the tubes. One ±12V regulated output powers the analog section of the UltraAnalog DAC, the other feeds the DC servo circuit around the output tube. The B+ and B- supplies are regulated by discrete regulation circuits rather than IC regulators. This second transformer is a custom toroidal that replaces the laminate transformer used for the same purpose in the SFD-1.
The third transformer also has dual secondary windings, one of which feeds a +5V regulation stage for all the SFCD-1's logic circuits, with the other secondary powering the transport mechanism, including the spindle motor, drawer, and transport servos.
Sonic Frontiers also junked the transport servo and decoder board that comes with the Philips CDM 12.4 transport mechanism and designed their own board using the same servo and decoding chips. They discovered that the stock board contaminated the rest of the power supply, adding jitter to the clock signals.
Digital filtering is provided by the Pacific Microsonics PMD100 HDCD decoder/filter IC. The 6dB of attenuation required by Pacific Microsonics on non-HDCD-encoded discs is performed in the analog domain, an approach that gives better sound than does the use of PMD100's digital-domain attenuation.
Of course, the SFCD-1 needs no input receiver, no input switching circuits, no phased-locked loop circuit, and no input termination networks. In fact, getting rid of the jitter-prone S/PDIF interface and its attendant circuitry is the SFCD-1's raison d'être. Rather than "recovering" a jittered clock from the transmitted signal, the SFCD-1 uses an ultra-low-jitter crystal oscillator (located 1" away from the DAC), which serves as the master clock for the entire system. The crystal oscillator's clock is input to a flip-flop along with the 8fs deglitch signal from the PMD100 chip (the signal that controls when the DAC converts its digital samples to analog values). The crystal oscillator thus reclocks the 8Fs clock needed by the UltraAnalog DAC.
The DAC is a two-channel UltraAnalog D20400A, custom-made for Sonic Frontiers. The 20-bit D20400A is a combination of monolithic and discrete components in an encapsulated module. Each DAC is tested and calibrated by hand-soldering tiny metal-film trim resistors on the "rungs" of the DAC's 20-bit "ladder" to achieve good low-level linearity.
The SFCD-1 is balanced in the analog domain, not fully balanced by using two DACs per channel. The DAC is followed by a passive RLC filter and a tubed output stage using a Sovtek 6922 dual-triode for each channel. Each half of the 6922 amplifies one phase of the balanced output signal. The left- and right-channel single-ended outputs that appear on the RCA jacks are therefore driven by only half of the 6922.
Sonic Frontiers' SFD-1 and SFD-2 processors use large-value coupling capacitors at the output to block DC. Not only are these capacitors physically large, and expensive (they are premium MIT MultiCaps), they also increase the output impedance at low frequencies (8k ohms at 20Hz in the SFD-2, 1350 ohms at 20Hz in the SFD-2 Mk.II). This may be the reason the SFD-2 and SFD-2 Mk.II sound somewhat woolly in the midbass, and tend to lack deep bass extension.
For the SFCD-1, Sonic Frontiers replaced the coupling capacitor with a feedback-based DC servo system. The servo prevents DC from appearing at the output, with no need for the coupling capacitor. In addition, a separate, ultra-fast-acting clamping circuit disconnects the output from the rear-panel jacks if any DC is detected. This circuit protects against the catastrophic possibility of more than 100V DC appearing on the analog output jacks if a tube fails. According to Sonic Frontiers, this circuit has never failed in any simulated failure mode.
Footnote 1: Some readers have asked why no one has thought of using the Sony SDIF system, where the word-clock signal is carried on a separate conductor. I have no idea why, other than in some professional installations, such as Sony Classical's studios in New York, the SDIF interface has been ignored.—John Atkinson
Footnote 2: Naim's CDS CD player puts the transport and D/A section in one chassis, with the massive power supply in a single chassis.—Robert Harley