Part 5: The Last Piece of Pi – The Allo Boss I2S DAC

Raspberry Pi with Allo Boss I2S DAC v1.2 

(The Last Piece of Pi, I promise)

By Steve Graham

In the previous Raspberry Pi installment Raspberry Pi Postscript: S/PDIF Vs. USB Sound Comparison, I pitted a Pi with S/PDIF-out HAT board against another Pi employing USB out.  The short version of the tale was that I came away underwhelmed by the USB connection.  My plan had been to employ an underutilized, USB-connected, Meridian Explorer 2 DAC in my workshop music system.  With that scenario falling flat, a Plan B was needed. 

Plan B: Equip a Raspberry Pi with a D to A converter HAT.  For about $87 CDN ($65 US) I bought an Allo Boss DAC I2S v1.2.  It’s not exactly an entry level-priced DAC HAT, but it’s not the most expensive either.  I also splurged a bit on an acrylic case, $19 CDN ($15 US), and Allo’s $15 CDN ($10 US) audio-specific power supply.  The supply is touted as, “made to specifications guaranteeing the lowest ripple and noise possible.” 

The Boss has a couple of tricks up its sleeve that sets it apart from less sophisticated DAC HATs.  Firstly, it allows for individual powering of the Pi and HAT boards.  This can lower electrical noise to the HAT, thus improving audio performance.  Many HATs receive (noisy) power from the Raspberry Pi board.  I used a standard Pi wall wart to power the Pi board and the above mentioned Allo audio-specific supply – a noticeably larger wart – to power the Boss DAC.  Both are switch mode power supplies.  

The second trick is the way clocking is implemented.  Typically, the Pi board sends data containing music and clock signals to a HAT board.  The Boss DAC, true to its name, dominates the Pi board.  The Boss’s clock signals, based on individual 44.1 and 48 kHz sampling rate oscillators (and their respective multiples), control the timing of data transfer from Pi to DAC.  This domineering behaviour reportedly lowers jitter in the data stream for better sound.  A bit of subservience in the interests of better music reproduction?  I’m up for that.  The Boss DAC has other notable features as well.  See Allo.com for a full listing. 

 

Pi Programming:

Nearly identical to the Pi using the S/PDIF HAT.  The only change was to select the Allo Boss DAC HAT on the SqueezeLite page of the setup procedure.  See page 6 of this PDF doc. 

On first power up, the network scanner utility on my PC couldn’t find the Pi.  After swapping a few Ethernet cables, with no result, I rewrote the PiCorePlayer onto a fresh micro SD card.  I think the first card was the one I used to program the Pi in the first three parts of this series.  The setup then went easily with the fresh card.  I probably over-wrote the original card about twenty times in the process of debugging and verifying the programming instructions I described in Part 2.  At ten bucks or less, it may be worthwhile having a spare card or two as backup.   

The analogue RCA outs from the Boss board were connected to my line stage.  I let the Pi/Boss combo run for more than a week streaming various web radio stations for break-in purposes.    

 

System:

Same as part three of the Pi series.  Scroll down to System Context in this link. I’ve received a few questions recently about the actual gear I was comparing.  My apologies if I wasn’t clear.  This time around I was comparing two discreet digital front-ends: 

The Challenger:

A $60, 1TB USB hard drive containing music files feeding a Raspberry Pi 4B with an Allo Boss DAC v1.2 HAT connected to my line stage with RCA-type single ended cables. 

The Incumbent: 

The many, many times more expensive NAD 50.2 player feeding a PS Audio DirectStream DAC (NAD/DS) connected to my line stage using XLR-type balanced cables.  The actual cable in both sets of interconnects was the same but the connectors were obviously different.

 

I used PiCorePlayer as I’ve had good luck with it and if truth be told, I’m a bit lazy and didn’t want to learn the ins and outs of another operating system.  Once programmed, I used the iPeng app on an iPad to control music play.  It’s possible to control volume from the iPeng app but this could induce digital truncation.  To avoid this potential sonic compromise, I set the app volume slider to max and controlled volume at the line stage. 

 

Music Selections and SQ Evaluation:

Handel, Messiah, Christopher Hogwood et al, (16/44.1)

An absolutely stunning performance and recording by some of the best musicians in the classical music world.  I haven’t played this recording in ages.  In fact, I don’t think it’s been played through my current digital source components or even my Spendor D9 speakers.  Played with the NAD/DirectStream combo it’s even more stunning than I remember.  The virtuosity of the voices and the orchestral accompaniment is in a word, riveting.  The NAD/DS (and the rest of the system of course) just nail the performance.  The speed with which the music can start, stop, change tempo and change volume is startling. 

The dynamic range of this recording demonstrates the compression used on many contemporary recordings.  The detail is stunning but not fatiguing or harsh.  The beauty of tenor soloists can almost make you weep, and the soprano soloists will take your breath away.  But these attributes and many more audiophile touchstones don’t detract from the musical whole.  The sense of the recording venue is so real it’s spooky. I was so swept away by the entire performance that I had to go back and replay various parts and think less dispassionately to capture written comments.   

How does the Pi/Boss DAC combo do?  Surprisingly well.  The treble is a little laid-back compared to the NAD/DS.  There isn’t quite the detail of the expensive rig, but I’ll take less detail any day rather than hard, nasty treble.  The sense of the recording venue isn’t as solid, but still decently portrayed.  One thing that I did find very good with both sources was the portrayal of individual voices in the various choruses.  The NAD/DS was better, but the Pi/Boss DAC was still quite decent.  The Pi/Boss’s placement and solidity of instruments and voices in the recorded space was better than I expected, not in the same league as the NAD/DS, but respectable all the same.  Dynamics, both micro and macro, are decently portrayed by the Pi/Boss.  Where the NAD/DS just sails through fortissimo levels, the PI/Boss gets a little congested and strained, but it does so without calling a great deal of attention to itself.        

 

Liala Biali, Revival, from Out of Dust. (16/44.1) 

A Jazz/Soul protest song?  Sure, why not?  Ms. Biali is a bit difficult to categorize.  Her style is informed by Jazz but not limited to it.  I think the production has a slight opacity, with Ms. Biali’s voice just a little buried in the mix.  There is, I think, a smidge too much compression too, but not hair-pullingly awful like it is on Bonnie Raitt’s Dig In Deep.  Ms. Biali’s voice is noticeably clearer and stands out a bit better through the NAD/DS gear.  In some ways, the Pi/Boss make this recording more listenable with its soft-ish high frequencies.  The Pi/Boss’s bass seems to have a slight mid-bass bump, with a bit of low bass roll-off like some small speakers exhibit.  It’s not overwhelming, but noticeable compared to the NAD/DS’s flat and subterranean bass response.  Dense mixes like this song are better unravelled by the NAD/PS DAC.  YouTube link to Revival.  There are many other videos of Ms. Biali singing and interviews as well on YouTube.

The Monolith is another great song on the album, and really highlights Ms. Biali’s vocal chops.  I first heard this song on Jazz FM91 and immediately thought of Joni Mitchell in her youth.  As it turns out Ms. Biali has covered many of Joni’s songs, and they are accessible on YouTube.  The fact that Laila was a pianist first and a singer second makes her lovely voice seem all the more incredible.  Especially so to someone like me who can’t carry a tune in a bucket.    

     

Miles Davis, Kind of Blue. (24/192)

This high-resolution download always sounds electrifying on my usual system running the NAD 50.2/PS Audio DirectStream combo.  When playing 24/192 files in part 3 of this series (Pi with S/PDIF-out HAT into the DirectStream DAC) I noticed more sound quality degradation compared to the degree of performance reduction with 16/44.1 files.  The 24/192 files, as dispensed by the S/PDIF HAT, were still better than the 16/44 files, but a little too diminished from what I knew was possible.  

The Boss DAC didn’t leave me feeling let down with its 24/192 performance.  I’ve yakked enough about this recording in previous reviews so I won’t bore you kind WoS followers any more.  I will give you a neat summary instead.

 

The Boss

Not the DAC, my long-suffering editor.  (I call him Boss and he calls me Chief.  I think that’s because I’m Noam’s chief tormentor.)  Anyway, the Boss likes numbers.  So, let me try to sum this up as succinctly as possible.

 

The Boss DAC

Let’s assign the NAD 50.2/DirectStream DAC an arbitrary score of 100%.  When playing 16/44 files I’d rate the Pi/Boss DAC combo as capturing 75% of the big rig’s “goodness”. 

Playing 24/192 files, I’d again rate the Pi/Boss DAC combo as capturing 75% of the big rig’s “goodness”.  The performance of the 24/192 files was higher than 16/44 files but relatively speaking, the Pi/Boss performed as well as it did with 16/44 files.

 

Rewind

Casting my mind back to the first parts of this Pi series, I’m inclined to assign some numbers to the digital-out HAT’s performance.  Before I do, let’s remember that the S/PDIF-out HAT costs about 2/3 the amount of the Boss DAC HAT.  

Using the same convention as above, assigning 100% to the NAD/PS DAC, I’d rate the Pi + S/PDIF HAT + PS DAC at 70% of the “goodness” compared to the NAD/PS combo when playing 16/44 files.  Observant readers will notice that I’m rating the Boss DAC slightly higher than using the digital-out HAT into my $$$ PS Audio DAC!      

When playing 24/192 files, I’d rate the Pi + S/PDIF HAT + PS DAC at 65% of the “goodness” compared to the NAD/PS gear.  Note again this is relative performance.  The 24/192 files still sounded better than the 16/44 files.

Perhaps a more accomplished S/PDIF-out HAT than the one used in parts one through three of this series would take the above “goodness” ratings higher.   

 

Let’s Take Stock

Pi 4B 2GB $48 (CDN) $34 (US)

Pi standard power supply $10 $8

Micro-SD card $9 $7

Allo Boss DAC v1.2 $87 $65

Allo audio power supply $15 $10

Acrylic case $19 $15

Seagate 1TB Hard drive $60 $50

 

Total $248 (CDN) $189 (US)

 

The Wrap

The level of performance exhibited by the Pi + Allo Boss DAC would be stunning at $1,000.  The fact that it can be had for $248 Canadian ($189 US), including hard drive, is quite simply astonishing.  There has been no better time than right now to get into file-based audio playback.

An Absolutely Amazing 5+ Star Value

So that’s me, done.  No more pie or Pi for that matter.

Except for the following:

 

Year-end Wishes (no rant this time)

2020 was one heck of a year.  Please be kind to yourself, your family and strangers too.  May all of our vaccine dreams come true in early 2021.

 

Additional Reading for the Digitally Obsessed

I2S (a very basic explanation)

IIS, usually abbreviated to I2S (I squared S), is short for Inter IC (integrated circuit) Sound.  This is essentially the process used in a CD player to get the data from the disc to the digital to analogue converter chip(s).  According to what I’ve read and seen online, this is the preferred method of data transfer.  When I2S is converted to other formats (S/PDIF, Toslink, USB etc.) is when things start to get messed up.  The trouble with S/PDIF, Toslink, USB transmission formats is that data and clock signals are transmitted together and can interfere with each other degrading sound quality.  There are professional digital transmission formats that transfer data and clocks separately but these require multiple cable links.  (dCS, the UK builder of extreme high end digital gear, uses multiple links on some of their eye-wateringly expensive gear.) 

I squared S avoids the compromises of the “flawed” transmission formats.  So, why don’t we have I2S connections between our digital sources and our DACs, he asked naïvely?  The short answer, as far as I’m able to discover, is that there are no standard I2S protocols.  Some builders of high-end gear use I2S between their digital sources and DACs but they have sorted out the send and receive protocols to accomplish this.  (No doubt, for reasons of commerce, they aren’t keen to share this with all and sundry.)  

Back to our Allo Boss DAC I2S.  Data is – I’m led to believe – transferred from the Pi board to the Boss using the I2S format.  Would you like to take the I2S signal and connect it to your external DAC?  From what I’ve read it’s possible if you are prepared to take a very deep dive into coding to sort out the various protocols.  (I suspect that during PiCorePlayer setup when a HAT is selected on the Squeezelite Settings page what really happens is the selection of the I2S protocol(s) necessary for a particular HAT and the Pi to play nicely together.)

Why not just use USB and avoid the S/PDIF compromises you ask?  USB is a whole other can of worms and one that is exacerbated buy the electrically noisy USB buss on Raspberry Pi boards.  

If a DAC HAT is used then it’s I2S right into the DAC.  If it’s a S/PDIF-out HAT, then I2S gets transmuted into a S/PDIF signal with the potential compromises that entails.  Of course, most of this is just conjecture and semi-educated guesswork on my part.  A well implemented S/PDIF setup could perform better than a poorly implemented I2S DAC HAT.  Like so much in audio, it’s all down to the details.     

In the end, it’s what sounds best to your ears that really matters.