Bridging Vintage R2R Magic with Modern Streaming: A High-Speed USB Isolation Project

Here is the setup of the streamer

Even though I thoroughly enjoy my main audio system using physical CDs and vinyl, high-resolution digital music files are becoming hard to ignore. To play these modern formats without sacrificing sound quality, I needed a clean, modern digital front end.

For casual desk listening, my setup is simple: Foobar2000 on a Windows laptop, connected to a USB-to-Coaxial receiver, feeding a Denon Heos Link HS2 streamer acting as a DAC. However, I wanted something significantly better for my dedicated listening room.

My goal was to build a high-performance streamer with a clean digital output to feed my prized Denon DVD-5000. Under the hood of this classic unit lies a phenomenal vintage DAC section: four Burr-Brown PCM1702 R2R chips paired with Denon’s AL24 Processing and a Pacific Microsonics PMD100 HDCD filter. It represents the absolute pinnacle of classic multibit digital audio.

The Problem: Computer Noise

Computers and laptops are notoriously noisy due to switching power supplies and shared ground lines. To prevent this high-frequency electrical noise from polluting the sensitive clocking and conversion stages of the DAC, galvanic isolation on the USB line is essential.

While several manufacturers like Texas Instruments and Analog Devices produce USB isolator chips, I chose the Analog Devices ADuM4165. Unlike older isolators restricted to Full Speed (12 Mbps), the ADuM4165 supports USB 2.0 High-Speed (480 Mbps), which is crucial for high-sample-rate, high-resolution audio.

Key features of the ADuM4165:

• True USB 2.0 High-Speed Support: Automatic detection of Low (1.5 Mbps), Full (12 Mbps), and High-Speed (480 Mbps) connections.

• Bidirectional Isolation: Protects upstream or downstream ports.

• Signal Integrity: Retiming and redriving functions to minimize input jitter and keep the eye diagram clean.

• Clock Flexibility: Versatile clock input options for stable data transmission.

Finding the Hardware on AliExpress

Rather than designing a custom PCB from scratch, I found a fully assembled board on AliExpress that utilized the ADuM4165.

The board's architecture is impressively robust, featuring:

1. Mornsun VRB0505S-5WR3: A high-quality isolated DC-DC converter capable of delivering up to a 1A current load to the clean side.

2. CH334H USB 2.0 Controller: Acts as an onboard USB hub controller to manage downstream data.

3. HUSB239 USB Power Delivery (PD) Controller: Handles negotiation for USB-PD power sources.

To verify its performance, I measured the noise before and after the isolator. The noise fell by a factor of 10, dropping from approximately 20mVp-p on the input supply down to just 2.5mVp-p on the isolated output against a stable 5V rail.

The measurement on Pin 2/6 of the PI's GPIO

Measurement of the power on isolated USB Out with load, SNR = 66dB

The Build: Transport and Streamer Integration

For the USB-to-coaxial conversion, I chose a classic CM6631A receiver board. I housed both the ADuM4165 isolator board and the CM6631A board together inside a single plastic project box to keep the digital signal path as short as possible.

The tiny switch is to switch off the CM6631A board 

This interface box acts as the bridge for the heart of the streamer: a Raspberry Pi 4B running Volumio, which I salvaged from a retired mining rig.

Because the Pi was going into a shielded aluminum chassis, I made one essential hardware modification: soldering an IPEX socket directly onto the Pi's PCB to run an external high-gain Wi-Fi antenna.

Here is the setup of the streamer

Next Steps: Power Delivery

While the streamer is up and running beautifully, the final phase of this project will be building a dedicated low-noise power supply for the Raspberry Pi.

While the Pi itself generates plenty of internal switching noise—meaning a power supply upgrade isn't the absolute highest priority for jitter—a high-current, ultra-low-noise linear power supply (LPS) will ensure the Pi's voltage rail remains rock-solid under load, especially when powering clean USB peripherals.