Halide Design – DAC HD
The DAC HD is the upgraded version of the original devilsound DAC. The HD keeps the same general form factor, joining a USB cable at one end, a short length of RCA cable at the other end for the analog audio signal, and building the electronics in an enclosure in the middle. Althought the RCA cables and the general idea are similar to the original devilsound DAC, the electronics inside have been completely revised, and the USB cable has been upgraded as well.
Digital
The digital section of the DAC, which converts the USB signal into the I2S signal used by the D/A conversion chip, is very similar to that used in the
USB audio took a huge leap forward with the development of asynchronous audio devices. The digital section has already been previously on the Bridge design page.
D/A conveter
The actual digital to analog conversion in the HD is handled by the Wolfson WM8716. The WM8716 combines a number of desirable features in a D-to-A chip. The ’16 can be programmed to use a “slow roll” digital interpolation filter. In listening tests, this results in a much more natural sound than the standard “fast roll-off” interpolation filter more commonly used in digital audio, even when the fast roll-off filters were apodizing or minimum phase. Other options frequently used in high-end audio are the NOS filter response (zeroth order holdover, frequently with additional analog filtering), which was used on the original devilsound DAC; soft knee minimum phase filters, pioneered by Ayre; or of course, the standard “brick wall” sinc filter, which is the traditional “standard” filter.
There are other notable advantages with the Wolfson chip. The analog power supply for the chip can go be run at 4.5V, meaning it can be powered from a power rail derived from a filtered / down-regulated USB line. The chip requires no negative power supply, which additionally simplifies design. The DAC HD uses a charge-pump to generate a clean -4.5 V rail, for the op-amp output. However, one of the design goals was to minimize any switch-noise caused by the DC-DC conversion, by minimizing the amount of current drawn by the negative rail. The fewer things powered by the negative rail – in this case, only one op-amp per channel – the less the current draw, and the lower the switch noise. Minimizing the switching noise allows for a cleaner analog output, and cleaner power to the clocks, which helps minimize jitter.
The WM8716 is run in software mode, and is programmed with a PIC microcontroller. The microcontroller puts the DAC chip into 24-bit I2S mode, selects the slow roll-off filter, and then goes into sleep mode.
Output stage
Perhaps one of the more useful features of the chip is that it provides a voltage output, rather than a current output. This allows the chip to directly drive an analog filter stage, to roll off the high frequency switching noise, without the need for an additional I/V stage. Since the DAC outputs at 128 * FS = (5.6 MHz for CD resolution), with an internal low-pass filter at 195 kHz, a fairly high frequency low pass filter suffices to completely and accurately reconstruct the original analog signal.
The DAC HD uses as single op-amp configured as a MFB (multiple feedback) two-pole Butterworth filter. The DC offset is removed in the same stage by connecting Vmid of the DAC (the voltage middle point) to the non-inverting input of the op-amp. A final small “snubber” resistor on the output insures that the op-amp can drive the line without any issues from cable capacitance.
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This allows for an extremely clean output stage, employing only one op-amp, and two capacitors (one for each pole), and a handful of resistors per channel. Note that the output filter is tuned using Butterworth coefficients, which allows for a flat passband and a steep cutoff. Commonly, Bessel coefficients are used in digital filters, which are in effect only slightly different. In comparison, the Butterworth filter will add a few microseconds of group delay to the higher frequencies, but results in a flatter passband and greater attenuation of the switch noise.
The analog filter employs silver-mica capacitors for the highest performance. Silver-mica is commonly used in high-end audio, especially for low-level phono amplifiers, and has two main advantages. First, silver-mica is extremely stable, linear, and keep the same capacitance values over a wide range of voltages. Second, silver-mica has no problem with high-frequencies, retaining its capacitance into the GHz range. Keep in mind that the output from a D/A converter will contain switch-noise at 12.3 MHz (for 96 kHz audio), and higher harmonic components. Thus filter must be able to operate properly at audio as well as radio frequencies, for best results.
Power supply
Most of the components in the circuit can run of 4.5 V or less, so this can be derived from the 5V USB power line. The USB power is filtered through separate LC filters for the Analog DAC supply, the positive Analog output supplies, the digital supply (3.3V), and the clock supply (3V), to minimize any power cross-talk between the various circuit components.
For the output circuitry, a negative supply in needed. A -5 V signal is generated with a MAX889S, and further CLC filtered and down-regulated to -4.5V. In total, the DAC HD contains 8 independent voltage regulators, for optimal performance.
Cabling
Since the rest of the DAC was being redesigned, we took another look at the USB cable. When the original devilsound DAC was being created, USB audiophile cables essentially did not exist, but currently, there are a number of companies trying to improve on the standard USB cable design.
We decided to use the Wireworld Starlight USB cable, since that was one of the few cables that we felt offered a compelling design to improve upon the standard USB cable. The Starlight runs the power separately from the data lines, and features full coaxial shielding around the power lines to minimize cross talk. In addition, the flat geometry of the datalines allows for a clean, controlled 90-ohm impedance, allowing the cable to run a full 7 meters without repeater electronics.
Note that the starlight uses silver-clad copper for the data and power lines. Silver plating is typical for high-speed data transmission lines, since silver has the greatest conductance of any metal. For high speed signals, plating (rather than solid core) is typically used, since the the signal is constrained to the outside of the cable, due to the skin effect.
The Halide DAC HD retains the same solid silver analog RCA cables, with the Eichmann silver bullet plugs.
Construction
As with the devilsound DAC, the electronics on the HD are contained in a custom CNC machined enclosure. Internally, the USB cable is connected to the board through mechanically and electrically reliable mini-B connector. The RCA cables are soldered directly to the board, to minimize the number of contacts. The enclosure is black annodized, and laser engraved