When Tim de Paravicini set out to design the DACute, I’m fairly certain that he sought to maximize its analog footprint. After all, analog is his métier. In fact, he freely admits to trying to equate digital performance to good analog practice, the payoff being that smooth and soothing sonic sensation analog tape and vinyl provide so well. Take, for example, the DACute’s DAC chip, a Wolfson WM8741 multi-bit delta-sigma DAC. It’s a high-performance stereo DAC designed for audio applications. It supports PCM data-input word lengths from 16-to 32-bits and sampling rates up to 192kHz. The folks at Wolfson included a smorgasbord of features such as fine resolution of volume and soft-mute control, digital de-emphasis, and a range of advanced digital filter responses. The digital filters include several selectable roll-off and performance characteristics. Tim’s approach was to minimize use of the internal digital filters. Because they cannot be totally bypassed, he set them for the highest frequency point and then implemented analog LC elliptic filters for 3dB down at a frequency of 40kHz.
This is the sort of analog filter Tim has always used on analog tape recorders for bias and other ultrasonic-noise filtering. An LC elliptic filter is rather economic in terms of parts count for a given slope, but does produce a nonlinear phase response over its passband. In Tim’s view, the filters “are quite good as far as phase response is concerned over the audio band to about half an octave away from 3dB down, so up to 20kHz is more than good enough for me.”
According to Tim, all these DACs by Burr-Brown, TI, Wolfson, or whomever produce significant high-frequency (HF) noise. Tim believes that it’s important to filter out HF hash lest these artifacts upset some amplifiers and tweeters. However, he still requires that response at 20kHz be within 0.1dB of the midband and frowns on the practice of rolling off the top end for so-called sweetness.
The DACute uses a Cirrus SPDIF receiver and accepts up to 24/192 digital data from USB, coaxial SPDIF, and TosLink SPDIF inputs. After passing through the analog filters, the signal is fed to a line preamp stage that is configured like a single-ended amplifier. A 6922/ECC88 twin triode is used per channel. The two sections are cascaded with the second stage being transformer coupled. The output transformer incorporates two secondary windings, one of which provides unbalanced and the other balanced output. A tertiary winding provides some feedback. Maximum output is said to be 5V into 500 ohms with reasonable distortion figures.
Tim says that he’s not interested in vanishingly low distortion levels at max output since “we hear best the stuff that goes on at lower levels, just where many digital systems fall down.”