usb audio interface circuit based dac

This is a high-quality preamplifier circuit with a built-in USB DAC designed for the Leachamp power amplifier. The schematic is derived from the PCM2902 datasheet. The circuit includes a DAC and ADC, SPDIF output and input, and an HID section featuring three buttons for MUTE, VOL+, and VOL-. To achieve high-quality playback, an external low-drop voltage stabilizer is required for the DAC section. The DAC used is the LP2951CM, which is readily available at local stores. The output voltage is set to approximately 3.7V using two resistors. The circuit board is designed with optimal ground placement, ensuring the separation of analog and digital grounds, which are connected at a single point at the USB connector. The PCM2902 datasheet recommends connecting a Low Pass Filter (LPF) to the DAC output to filter high frequencies above the audioband produced by oversampling conversion. The integrated circuit includes a digital LPF that filters frequencies above 100kHz. The application notes on the manufacturer's pages suggest using a 1st-Order LPF (simple RC) or a 2nd-Order filter with operational amplifiers, which also function as a preamplifier. A simple RC LPF with recommended values of R = 1kΩ and C = 4.7nF was implemented. It is advisable to use a roll-type capacitor instead of a ceramic capacitor. No noticeable difference in sound quality was perceived between connections with and without the filter; however, it is better to include it in consideration of other components in the audio chain. To achieve a higher cut-off frequency, the capacitor value can be changed to 3.3nF.

The described preamplifier circuit integrates a USB Digital-to-Analog Converter (DAC) with essential features for high-fidelity audio playback. The PCM2902 IC serves as the core of the circuit, providing a seamless interface between digital audio sources and analog output. The inclusion of both DAC and ADC capabilities allows for versatile audio handling, accommodating both playback and recording applications.

The power supply design utilizes the LP2951CM voltage regulator, which is known for its low dropout voltage, ensuring that the DAC receives a stable and clean power supply. This is critical for maintaining audio quality, as fluctuations in power can introduce noise and distortion. The output voltage of approximately 3.7V is achieved through a resistor divider, optimizing performance for the DAC's operational requirements.

The PCB layout is meticulously designed to minimize interference between analog and digital signals. Proper grounding techniques are employed to prevent ground loops and ensure a clean signal path. The single-point ground connection at the USB connector is a common practice to maintain signal integrity.

The recommendation for a Low Pass Filter (LPF) is crucial in audio applications to eliminate high-frequency noise that can occur during the oversampling process. The digital LPF within the PCM2902 is effective up to 100kHz, but additional filtering is advisable to further refine the output signal. The choice of a simple RC filter with a 1kΩ resistor and a 4.7nF capacitor provides a straightforward solution to attenuate unwanted frequencies, while the suggestion to use a roll-type capacitor ensures better performance compared to ceramic types.

Overall, this preamplifier circuit is designed with a focus on high-quality audio output, utilizing proven components and techniques to deliver a reliable and effective solution for audio amplification needs. The flexibility of the circuit allows for adjustments in filtering characteristics, accommodating various audio setups and preferences.This is high quality Preamplifier circuit with built-in USB DAC for my power amplifier Leachamp. Schematics is from datasheet of PCM2902. Circuit includes DAC and ADC, SPDIF output and input and HID part with 3 buttons for MUTE, VOL + and VOL-. For high quality playback is needed to use an external low-drop voltage stabilizer for the DAC part. DAC is used LP2951CM which was easily available at local stores. Output voltage is set to about 3. 7V with two resistors. Circuit board is designed regarding to good ground placement and separating of analog and digital ground. These ground are connected in one point at a USB connector. In datasheet of PCM2902 is recommended to connect Low Pass Filter to output of DAC for filtering high frequencies above audioband which are produced by oversampling conversion.

Integrated circuit includes digital LPF which filters frequency above 100kHz. In application notes for filter on the manufacturer pages are recommended 1st-Order LPF (simple RC) or 2nd-Order with operation amplifiers which works like preamplifier too. I used simple RC LPF with recommended values R 1k and C 4n7. It`s better to use roll-type capacitor instead ceramic. I didn`t hear difference in a sound between connection with filter or without it, but with respect to other components in a audio chain it is better to use it.

For a higher cut-off frequency we can change value of capacitor to 3n3. 🔗 External reference