Topping D900 1-Bit DAC – First Look

All Image Credit: Topping

Topping, a Chinese Hi-Fi manufacturer, has released its new flagship D900 DAC that is designed around a proprietary discrete 1-bit conversion technology rather than employing conventional, and readily available digital-to-analogue converters. Topping calls this 1-bit approach a Precision Stream Reconstruction Matrix (PSRM) and uses a technique known as time-domain density modulation.

For those of you not in the know, this technology implements 32 discrete elements, and is driven by an extremely high speed stream of 1-bit pulses, with the density of ones versus zeros representing the instantaneous waveform level. An analogue reconstruction filter then smooths these pulses into the final analog output signal.

Unlike conventional DAC chips that process the incoming bit words simultaneously like Burr-Brown, AKM and ESS, a 1-bit DAC works with single-bit values switching at extremely high speeds, the approach being applied to both PCM and DSD signals. Toppings proprietary PSRM design uses discrete logic components operating at extreme nanosecond switching speeds rather than established FPGA-based designs from companies like dCS and Chord Electronics.

Topping addresses jitter and timing errors in the digital signal that can cause audible distortion using a Complex Programmable Logic Device (CPLD) for clock optimisation by Altera. According to Topping, CPLDs offer more predictable timing than FPGA chips, making them particularly suited to time-critical digital audio applications. Complementing the digital signal chain the current-to-voltage conversion stage that converts the DAC’s output into usable line-level signals and employs low-distortion integrated op-amps combined with discrete components. This I/V stage was specifically developed to complement the PSRM architecture.

The DAC provides; Bluetooth 5.1 reception supporting; LDAC, aptX Adaptive, aptX HD, plus standard aptX, AAC and SBC codecs, all of which are lossy to varying degrees. The USB and I2S inputs handle: PCM up to 32-bit/768kHz plus native DSD512. The S/PDIF and AES/EBU connections supporting 24-bit/192kHz PCM and DSD64 in a DoP wrapper. Volume is adjusted by a discrete analogue preamplifier stage with relay-switched volume control using 18 relays (nine per channel) to provide 0.5dB adjustment steps. With Topping claiming a total harmonic distortion that sits below -140dB and a signal-to-noise ratio measuring 131dB, quite outstanding.

Control is available using the supplied remote and 2-inch colour display that runs Topping’s Aurora UI with nine selectable colour schemes. Touch-sensitive buttons flank a programmable rotary encoder that defaults to volume control but can be reassigned to other functions. Users can configure the screen to show incoming sample rate and bit depth, display a real-time frequency spectrum analyser or VU meter animations.

The D900 also includes 10-band parametric equaliser that is controllable using the free Topping Tune software for Windows and macOS that lets users create and save multiple EQ profiles. Once uploaded, profiles can be selected from the front panel without a computer connected. This EQ runs in the digital domain via an FPGA chip that is separate from the main PSRM conversion path.

Rear panel digital input connectivity includes; 1 x USB-C, 1 x USB-B, 2 x coaxial, 2 x TOSLINK, 1 x AES/EBU and 1 x HDMI that handles I2S signals in LVDS format. Analog outputs include two pairs of balanced XLR connectors one fixed level the other variable.

Finally, the D900’s chassis uses 3mm aluminium panels with an 8mm front plate with heatsink fins that run along both sides, while the feet incorporate vibration-absorbing pads.