“MUS-IC” to an Audiophile’s Ears? Lessons on Hi-Fi Electronics From a New DAC
MUS-IC, ROHM Semiconductor’s specialized family of audio electronics has a new member: a 32-bit DAC designed for high-res playback. What does this DAC teach us about the tenets of Hi-Fi electronics?
Audiophiles often have heated qualitative arguments about the nature of audio recording and playback. While the playback quality may be subjective to the listener, the electronics are more quantitative.
This article will explore the basics of high-fidelity (Hi-Fi) audio electronics, including the recently released BD34301EKV from ROHM Semiconductor.
The BD34301EKV quad-flat package. Image used courtesy of ROHM Semiconductor
The BD34301EKV is the newest Hi-Fi 32-bit DAC converter from the ROHM MUS-IC product family. This converter has been selected by the Luxman Corporation, a high-end audio equipment manufacturer because it is said to have a “sound that is natural, and easy to listen to.”
What Electronic Parameters Produce Hi-Fi Audio?
Texas Instruments identifies a few key parameters to produce Hi-Fi audio electronics. These parameters are spread between three major electronic blocks: the DAC, the I/V amplifier (also called trans-impedance amplifier), and the difference amplifier.
The most crucial parameter is the THD+N (total harmonic distortion and noise), which is less than -110 dB, superior signal-to-noise ratios (> 110 dB), and a dynamic range that allows for the full range of human hearing (120 dB).
Before looking into specific electronics, it is necessary to consider system noise in more depth.
An Overview of Digital Audio Hardware
The road to Hi-Fi is a seriously quiet one—as in 3.16 μV of noise on a 1 Vrms signal.
With noise thresholds that low, engineers must focus on ultra-low-noise power supplies with appropriately designed power delivery networks (PDN). These PDNs should account for the impedance characteristics of bypass capacitors and the thermal resistance of each IC in the signal chain.
TI even recommends using low-noise amplifiers instead of a less cost-effective load-dropout (LDO). According to TI, the OPA1688 is a useful op-amp to produce the power required for the audio signal chain.
An ultra-low noise power supply design for Hi-Fi audio. Image used courtesy of Texas Instruments
Next, we'll look at ROHM's new 32-bit digital-to-analog converter and an example circuit for an audio amplifier output stage.
ROHM’s 32-bit DAC
ROHM's converter is available in a 12.0 x 12.0 x 1.0 mm 64pin-QFP and claims to possess all the key characteristics of a Hi-Fi DAC, including less than -115 dB THD+N and a signal-to-noise ratio (SNR) and dynamic range of greater than 130 dB.
Block diagram for the BD34301EKV DAC and external amplifiers. Image used courtesy of ROHM Semiconductor
The standard application circuit includes the DAC unit as well as a two-channel two-stage external amplification process. As is typical with most high-end audio DACs, the BD34301EKV is a current-mode output device.
The DAC output must be converted into a voltage source by a trans-impedance amplifier fed to a difference amplifier to produce a single-end voltage appropriate for single-channel analog audio output.
A power amp often follows the conversion from digital-to-analog audio, and that's where things can get very interesting as the filtering can get pretty intense.
Notable Filter Design for Hi-Fi Audio Power Amplifier
The LM3886 circuit diagram. Image used courtesy of Texas Instruments
The LM3886 amplifier states that it will easily pass any signals up to approximately 1 MHz. However, this can be a problem if you only want to pass in the range of 20 Hz to 20,000 Hz.
The solution to the passband problem is filtering. The inputs are heavily filtered to narrow the passband and limit DC bias (a significant noise contributor). The output is filtered as well to compensate for the parasitic elements of the speaker wires.
Evaluating Suitability with Reference Designs
That's the basics of Hi-Fi electronics, though we could certainly expand on many other parameters, including common-mode rejection ratios, power supply rejection ratios, left and right channel isolation.
The new evaluation card, complete with the reference design. Image used courtesy of ROHM Semiconductor
ROHM's new 32-bit DAC offers high-resolution conversion of digitally mastered audio for high-end audio equipment.
Featured image used courtesy of ROHM Semiconductor
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