Featured image courtesy of Darko.Audio 
I’m going to go out on a bit of a limb here and recommend a piece of kit that I don’t actually own or use but that is a close cousin to kit in use here at Dismal Manor.
When Jason Stoddard, co-founder of Schiit Audio, has one of his periodic ask me anything interviews, a viewer invariably asks if Schiit will make an inexpensive streamer. Jason invariably answers, “no, that’s not our thing”. Jason has wisely chosen to avoid all the pain of what works with what in Linux Land.
But the question comes up sufficiently often that folks might like an answer. Back when I was looking for a streamer to replace AirPort Express and ChromeCast bits, I happened to discover John Darko’s Darko Audio YouTube channel. One evening, John reviewed the Allo Digital DigiOne Signature streamer and gave it marks that placed it firmly in high end sonic company with the Bluesound Node. After a bit, I ordered the boards and enclosure and put one together that I use in my lounge as my main stream receiver. I have a secondary HiFiBerry stream receiver that I use with the TV and video players.
In this article, I’ll tell you about a newer Allo Digital product that takes a bit different tack that will work nicely with today’s kit. That product is Allo Digital’s USBRIDGE Signature . We’ll also look at what the more commercial products offer that is not part of the DIY streamers.
Allo DigiOne Signature
John Darko’s YouTube video linked as  introduced me to the Allo Digital products and specifically to the Allo DigiOne Signature transport in the form of an on-camera review. Based on John’s review, I ordered the necessary boards and put one together that I continue to use.
John is a thoughtful reviewer and an understated presenter. John’s reviews are trustworthy and discloses the ethical considerations that guide them. When your preferences align with John’s, you can make a purchase decision based on them.
I’ll describe the DigiOne Signature to establish a reference for the USBRIDGE Signature and John’s website’s review of it. I have not heard the USBRIDGE Signature.
The DigiOne Signature consists of two bonnet boards for the Raspberry Pi. The mezzanine board functions as the “dirty side”. Its job is to short stop the Raspberry Pi’s noise that is present on the GPIO header. This board galvanically isolates the signals that pass between the Pi and the DigiOne streamer.
The top board is the actual streamer. This device receives the I2S audio signal from the dirty board, buffers and re-times it, and develops SPDIF and TOSlink signals from it.
This board delivers clean, well-timed SPDIF and TOSlink sample streams to coaxial cable and optical cable outputs. The optical interface uses the inexpensive TOSlink connectors made popular by Sony. The SPDIF interface is an RCA phono jack.
The Allo DigiOne signature has operated for several years without fuss. I have found the DigiOne Signature to be very musical paired with Gumby via SPDIF. Gumby has an unusual feature, a “buy better stuff light”. When this light is on, it indicates that input bit stream jitter exceeds a tolerance above which Gumby uses more aggressive buffering and retiming of the digital input stream. When this light is off, less extreme jitter buffering and re-clocking is in use. DigiOne passes Gumby’s acceptable jitter test. Jason reports that devices like Apple Airport Express tripped the jitter indicator.
Allo USBRIDGE Signature
The Allo Digital USBRIDGE Signature has a different physical architecture but a similar functional architecture to that of DigiOne Signature. Allo designed a mother board that accepts a Raspberry Pi Compute Module. The Compute Module is in the SODIMM form factor but actually contains a subset of the Raspberry Pi’s functionality intended for embedding in other devices. This motherboard holds the Ethernet, HDMI, and USB audio output.
The Raspberry Pi Compute Module 3+ provides the video, ARM Cortex-A53 (ARMv8) 64-bit SoC processor, and memory. The remainder of the system resides on the Allo Digital designed main board.
The main board has a clean zone with the USB Audio 2.0 interface and supporting buffers and timing. The main board processes the audio transport protocol and queues the PCM samples to the clean USB output section. It also derives an I2S bit stream that puts on the GPIO pins.
Those using USB DACS are ready to go by adding a good 5Volt 3Amp USB-C power supply. A Raspberry Pi 4 power supply is sufficient but the well heeled may prefer one of Allo’s low noise power supplies.
The whole point of the USBRIDGE design is to keep digital grunge at bay. The main board provides extensive filtering and isolation for each input and the bulk DC power. The board has 3 USB connectors in addition to the USB-C power connector. Two are for USB WiFI and USB Bluetooth interfaces. The third is the clean USB Audio 2.0 protocol DAC interface.
John’s magazine review does an excellent job describing the USBRIDGE Signature in a system. Based on my experience with the DigiOne Signature, I can recommend the USBRIDGE Signature.
So why only $250?
The Allo Digital and HiFiBerry products are inexpensive for several reasons.
- Allo sells directly and through DIY mail order shops.
- They forego expensive enclosures for simple stamped metal enclosures
- They forego licensed software like Roon Endpoint, Tidal, Qobuz, Spotify, AirPlay, etc.
- They take advantage of open hardware like the Raspberry Pi
- They take advantage of open source software like Raspbian OS for Raspberry Pi, the Roon endpoint, Shairplay, etc.
- Manufacturers need only write Raspbian compatible drivers for their hardware.
- These products make extensive use of surface mount components with automated assembly and test.
- You provide a DC power source. Design is simplified by using an external power supply and internal conversion and filtering as needed.
- They let you do the fiddly assembly and final test.
- They let you supply the DAC of your choice.
A manufacturer who is careful, as is Allo Digital, can achieve results on a par with the Bluesound Node product used as a streamer . The Bluesound is a well regarded product sonically, includes a competent DAC, and supports many licensed services. It is likely that your DAC will sound identically with the Bluesound Node 2 or the USBRIDGE Signature. Both support USB Audio 2.0 protocol.
What does the Bluesound Node Add?
The Blue Sound Node has all those things Jason and Mike are trying to stay away from.
- It has a UI app
- It has interfaces to many licensed streaming services
- It offers several licensed streaming protocol receivers like AirPlay, Qobuz, Tidal, Spotify, etc.
- The internal converter is MQA (master quality assured) capable.
- Some licensed products require compliance checking
- The case is prettier but plastic.
Bluesound has taken on these challenges. Mike, while at Angstrom, had an unpleasant experience licensing and using Dolby and THX home theater technology. The specialist companies always came last in product access and certification testing. These things all have development costs and per-unit costs.
The USBRIDGE Signature main board has a Raspberry Pi GPIO header allowing use of the USBRIDGE Signature with a second audio bonnet. Allo specifically intends this option to be used to stack a DigiOne Signature mezzanine board and bonnet board. A second case back panel plate is drilled to pass the bonnet board’s RCA and TOSLink connectors. In this configuration, you have a $500 streamer that supports USB, SPDIF, and TOSLink interfaces at high quality.
This option is useful when the DAC/amplifier combination will be located away from the streamer, for example on a chair side table or night stand. The SPDIF/TOSLink distance limitation is less restrictive than USB.
Darko Audio did not review the USBRIDGE Signature – DigiOne Signature bonnet combo.
The USBRIDGE Signature Manual states that the USBRIDGE is compatible with DietPi, Moode, RoPieee, and Volumio. Using DietPi, or RoPieee, the USBRIDGE Signature may be used as a Roon Endpoint. Both DietPi and RoPieee include the DigiOne Signature drivers and USBRIDGE USB hardware drivers. Linux provides the UAC2 drivers.
You need a good enough one. A regular Raspberry Pi 4 5 volt 3 amp supply is very good in this application. These provide more than enough power for the USBRIDGE and have low noise. There will be no audible noise in an idle system.
Allo’s Power Supplies
Allo makes two power supplies, the $60 Nirvana switch mode power supply and the $160 Shanti linear supply. Both are designed to be quiet and low impedance.
The catch is that the nice output circuitry in both of those supplies is redundant as USBRIDGE Signature and DigiOne Signature have substantial sophisticated internal power clean up. Over 30 low drop out regulators clean up the power to each section of circuitry on the USBRIDGE board and the DigiOne Signature board. My system is dead quiet using regular good quality USB 5V 3A Raspberry Pi supplies.
Any product needing more than 5 volts or positive and negative rails will include switch mode power conversion parts to make the required voltages from the bulk 5 volts DC. With careful design, hum and noise will be inaudible.
Where to Buy?
I bought my original DigiOne Signature directly from Allo. Today I would buy from PiShop.us here in country to avoid customs delay, foreign exchange, and Allo’s somewhat crufty website. PiShop.us, here in Delaware, has prepared its own product pages for its Allo Digital offerings that correct the omissions and ambiguities of the original Allo pages. Pi Shop clearly tells you what is included, what else you need to buy, etc and makes manuals readily available. You’ll save about a percent plus foreign exchange buying directly.