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A high-value, high-per­for­mance DAC de­signed for PCM sig­nals up to 24 bits and 192kHz that’s up­grade­able?

Re­mem­ber when the first two CD play­ers ap­peared on the mar­ket? The Sony of­fer­ing was a 16-bit unit, al­beit with a DAC that switched be­tween each chan­nel, mis­align­ing them by a small amount. (*See foot­note.) Philips, though, felt that 16-bit DACs weren’t suf­fi­ciently ac­cu­rate at that point, so it launched with a 14-bit DAC. The in­her­ent 12dB higher noise floor re­sult­ing from that 2-bit short­age still put it be­low the noise floor of just about any phono pre-am­pli­fier avail­able at the time.

The prob­lem was lin­ear­ity. Was it pos­si­ble to cre­ate a cir­cuit that could ac­cu­rately re­con­struct ana­logue sig­nals from dig­i­tal num­bers which spanned four or­ders of mag­ni­tude? That is­sue soon be­came moot by means of a nifty bit of side-step­ping. As sil­i­con got bet­ter (hey, com­pare the ca­pa­bil­i­ties of a mod­ern com­puter with an early 1980s model: it’s at least three or­ders of mag­ni­tude faster and more pow­er­ful). In­stead, over­sam­pling and delta-sigma con­ver­sion be­came the norm. In ef­fect these turn PCM to some­thing like Di­rect Stream Dig­i­tal, al­low­ing for a very sim­ple fi­nal DAC stage. (Yes, there are ad­di­tional com­pli­ca­tions.) This also dealt with the com­plex­i­ties of cre­at­ing an ex­tremely sharp anti-alias­ing fil­ter, and the draw­backs of such fil­ters.

I bring all this up be­cause the Schiit Bifrost DAC re­viewed here—a high-value, high-per­for­mance DAC de­signed for PCM sig­nals up to 24 bits and 192kHz sam­pling—is the Multibit ver­sion.

the eQuIp­Ment

The stan­dard Schiit Bifrost DAC ($649) uses the Asahi Ka­sei AK4490, a 256× over­sam­pling delta-sigma DAC chip. The re­view Schiit Bifrost Multibit model is $979 and uses a very dif­fer­ent DAC tech­nique. Since the Bifrost is up­grade­able, you can con­vert the for­mer to the lat­ter, but the board will cost you $549. This is not a user up­grade. Your Bifrost will need to go back to the distrib­u­tor for in­stal­la­tion, and the nec­es­sary firmware up­grade for the moth­er­board.

The only vis­ual dif­fer­ences be­tween the Bifrost and the Bifrost Multibit are stick­ers, one on the box and one on the back panel of the unit.

I will re­turn to the Multibit thing shortly, but first a lit­tle about the unit. It’s a mid-sized (229×58×172mm), three in­put model that weighs 2.3 ki­los. There’s a USB Type-B socket which you use with a com­puter (or, if you’re that way in­clined, a smart phone) to turn dig­i­tal au­dio into ana­logue, plus an op­ti­cal in­put and a coax­ial dig­i­tal au­dio in­put. All three sup­port the res­o­lu­tion men­tioned—192kHz, 24 bits. There is no sup­port for Di­rect Stream Dig­i­tal (nei­ther, ap­par­ently, is there in the ba­sic Bifrost, even though its DAC chip sup­ports up to quad speed DSD). Out­put is via stan­dard RCA sock­ets. The top, bot­tom and front of the case are formed from one folded sheet of 3mm alu­minium.

It pro­vides high per­for­mance and ex­cel­lent value for money for those who don’t need DSD ca­pa­bil­i­ties...

Three small in­di­ca­tor lights on the front in­di­cate which in­put has been se­lected, while a but­ton cy­cles through the in­puts. There are no in­di­ca­tors for show­ing sam­pling fre­quency, so un­less you’re us­ing au­dio soft­ware that makes a pos­i­tive state­ment about what is be­ing de­liv­ered to the DAC, you’ll be pro­ceed­ing some­what on trust that you have your com­puter con­fig­ured cor­rectly and it is not re­sam­pling the au­dio. A small hard-wired switch on the back turns the power on and off. The FAQ in the slim man­ual says that you can leave the Bifrost switched on ‘all the time’.

That man­ual (and the Schiit web­site) are rather cute. On the back cover next to the com­pany name is a tag line: ‘It Hap­pens’. It notes of the seem­ing ven­ti­la­tion holes on the top that they ‘Don’t Go Through’, say­ing: ‘Hey, you try get­ting FCC-friendly ra­di­ated noise on a DAC that has vents in the cover. Go ahead, try. But hey, they look cool, right?’ I con­fess to hav­ing a soft spot for a com­pany that doesn’t take it­self too se­ri­ously.

What is se­ri­ous is the tech­nol­ogy in­side. In­stead of an off-the-shelf au­dio DAC chip, Schiit has gone for an in­dus­trial chip, not de­signed at all for au­dio ap­pli­ca­tions, and has built its own cir­cuitry around it. Schiit is very up front about this. It de­scribes the sys­tem as us­ing ‘Schiit’s pro­pri­etary closed­form, time- and fre­quency-do­main op­ti­mised DSP-based dig­i­tal fil­ter… cou­pled to a pre­ci­sion Ana­log De­vices AD5547CRUZ dig­i­tal-to-ana­log con­verter—a D/A never be­fore used in any other au­dio prod­uct.’

Is this some form of heresy? Shouldn’t a high-qual­ity au­dio de­vice use com­po­nents de­signed for au­dio prod­ucts? The data sheet for the AD5547 (Rev.D) sug­gests as ap­pli­ca­tions test equip­ment, in­stru­men­ta­tion, dig­i­tally con­trolled cal­i­bra­tion and dig­i­tal wave­form gen­er­a­tion.

It seems to me Schiit isn’t treat­ing dig­i­tal au­dio sig­nals as be­ing im­bued with some mag­i­cal mu­si­cal essence that will be lost with­out un­cer­tain, golden-eared hand-tun­ing. No, it’s treat­ing them as things to be con­verted as ac­cu­rately as pos­si­ble into the orig­i­nal ana­logue sig­nals from which they were formed. It is bring­ing en­gi­neer­ing to bear on that task.

The AD5547 is an R-2R lad­der-based DAC chip. That is, good old fash­ioned tech­nol­ogy, but ca­pa­ble of ex­treme pre­ci­sion thanks to the three-and-a-half decades of de­vel­op­ment since those early CD play­ers. Since the chip is a 16-bit model, and the DAC is rated at sup­port­ing 24-bit sig­nals, I’m guess­ing that some over­sam­pling still comes into play. The chip ac­tu­ally has a band­width of 6.8MHz. But that’s push­ing the bounds of my com­pre­hen­sion. What magic this ‘ pro­pri­etary closed-form, time- and fre­quency-do­main op­ti­mised DSPbased dig­i­tal fil­ter’ pulls off, we’ll see in the test re­sults.

Schiit rates the fre­quency re­sponse of the Bifrost Multibit at 20Hz to 20kHz ±0.1dB, and 2Hz to 150kHz ±0.5dB. The out­put is rated at 2 volts RMS. THD is rated at less than 0.005% across the full au­dio band­width, while IMD is rated at less than 0.008% and the signalto-noise ra­tio is rated at greater than 109dB (re­ferred to 2 volts RMS).

Fi­nally, I will note that Schiit has made its DACs mo­du­lar and up­grade­able. So should there be sig­nif­i­cant im­prove­ments avail­able in the fu­ture in either the dig­i­tal or ana­logue sec­tions, the in­ter­nal com­po­nents can be changed. This, again, would be a job for the distrib­u­tor. Schiit would like its cus­tomers to note how­ever, that al­though the Bifrost has a 5-year war­ranty, this cov­ers only labour and parts in the un­likely event that a re­pair is re­quired: it does not en­ti­tle you to up­grades.

In use AnD lIs­ten­InG ses­sIons

As seems to be now stan­dard, no driv­ers were in­cluded in the box. The man­ual—pre­sum­ably not too re­cent a pub­li­ca­tion—says to go to a spe­cific link on the man­u­fac­turer’s web­site to in­stall them.

But you may not need driv­ers any­way. Macs have sup­ported USB Au­dio Class 2 for ages, and so (be­lat­edly) has Win­dows, since early 2017. The driver page says: ‘Most newer Win­dows ver­sions al­ready have driv­ers for our stuff.’ I found that Win­dows sup­port un­sat­is­fac­tory when first in­tro­duced, but whether it’s this par­tic­u­lar DAC, or the sev­eral Win­dows 10 up­grades since then, there were no prob­lems this time. I plugged the Schitt into my desk­top com­puter and in­stantly the usual USB ‘de­vice con­nected’ sound emerged from the (al­ready pow­ered up) speak­ers. I checked the Win­dows Man­age Au­dio De­vices di­a­log box, and there it was, set as the de­fault de­vice.

So I fired up Foo­bar 2000, went to its out­put set­tings and found the Schiit listed there with three op­tions. ‘DS: Speak­ers (Schiit USB Au­dio Gen 2)’, ‘WASAPI (event): Speak­ers (Schiit USB Au­dio Gen 2)’ and ‘WASAPI (push): Speak­ers (Schiit USB Au­dio Gen 2)’. The first of those sim­ply runs the sound through the stan­dard Win­dows au­dio pro­cesses, and will re­sam­ple it if nec­es­sary to match the out­put sam­ple rate you’ve set in the Win­dows di­a­log. For hi-fi lis­ten­ing, this is best avoided, but is use­ful to have if your ma­chine is a gen­eral-pur­pose com­puter and you need to hear no­ti­fi­ca­tions and such.

But for the best per­for­mance, you’ll want one of the WASAPI set­tings. WASAPI stands for Win­dows Au­dio Ses­sion Ap­pli­ca­tion Pro­gram­ming In­ter­face. The stan­dard Win­dows au­dio han­dling has to mix your mu­sic with beeps and buzzes from your sys­tem, au­dio from YouTube videos and all that stuff.

What is se­ri­ous is the tech­nol­ogy in­side. In­stead of an off-theshelf au­dio DAC chip, Schiit has gone for an in­dus­trial chip not de­signed at all for au­dio ap­pli­ca­tions

Graph 4. Noise at 96kHz/24-bit re 2V. USB in­put–Win­dows (white trace) vs. Op­ti­cal In­put—MAC (green trace) vs. Op­ti­cal In­put–MAC but with USB in­put dis­con­nected (blue trace).

Graph 1. Fre­quency re­sponse (44.1kHz/16-bit). Left chan­nel (white trace) vs. right chan­nel (green trace).

Graph 2. Fre­quency re­sponse (96kHz/24-bit). Left chan­nel (white trace) vs. right chan­nel (green trace)

Graph 3. Fre­quency re­sponse (192kHz/24-bit). Left chan­nel (white trace) vs. right chan­nel (green trace).

Schiit has made its DACs mo­du­lar and up­grade­able. So should there be sig­nif­i­cant im­prove­ments avail­able in the fu­ture, the BiFrost can be up­graded

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