The Schiit Modi Multibit: A little wiggling ensures this DAC won’t quit

Sometimes, when an audio component dies, the root cause is something electrical. Other times, the issue instead ends up being fundamentally mechanical.

Delta sigma modulation-based digital-to-analog conversion (DAC) circuitry dominates the modern high-volume audio market by virtue of its ability (among other factors) to harness the high sample rate potential of modern fast-switching and otherwise enhanced semiconductor processes. Quoting from Wikipedia’s introduction to the as-usual informative entry on the topic (which, as you’ll soon see, also encompasses analog-to-digital converters, i.e., ADCs):

Delta-sigma modulation is an oversampling method for encoding signals into low bit depth digital signals at a very high sample-frequency as part of the process of delta-sigma analog-to-digital converters (ADCs) and digital-to-analog converters (DACs). Delta-sigma modulation achieves high quality by utilizing a negative feedback loop during quantization to the lower bit depth that continuously corrects quantization errors and moves quantization noise to higher frequencies well above the original signal’s bandwidth. Subsequent low-pass filtering for demodulation easily removes this high frequency noise and time averages to achieve high accuracy in amplitude, which can be ultimately encoded as pulse-code modulation (PCM).

 Both ADCs and DACs can employ delta-sigma modulation. A delta-sigma ADC encodes an analog signal using high-frequency delta-sigma modulation and then applies a digital filter to demodulate it to a high-bit digital output at a lower sampling-frequency. A delta-sigma DAC encodes a high-resolution digital input signal into a lower-resolution but higher sample-frequency signal that may then be mapped to voltages and smoothed with an analog filter for demodulation. In both cases, the temporary use of a low bit depth signal at a higher sampling frequency simplifies circuit design and takes advantage of the efficiency and high accuracy in time of digital electronics.

Primarily because of its cost efficiency and reduced circuit complexity, this technique has found increasing use in modern electronic components such as DACs, ADCs, frequency synthesizers, switched-mode power supplies and motor controllers. The coarsely-quantized output of a delta-sigma ADC is occasionally used directly in signal processing or as a representation for signal storage (e.g., Super Audio CD stores the raw output of a 1-bit delta-sigma modulator).

Oversampled interpolation vs quantization noise shaping

That said, plenty of audio purists object to the inherent interpolation involved in the delta-sigma oversampling process. Take, for example, this excerpt from the press release announcing Schiit’s $249 first-generation Modi Multibit DAC, today’s teardown patient, back in mid-2016:

Multibit DACs differ from the vast majority of DACs in that they use true 16-20 bit D/A converters [editor note: also known as resistor ladder, specifically R-2R, D/A converters] that can reproduce the exact level of every digital audio sample. Most DACs use inexpensive delta-sigma technology with a bit depth of only 1-5 bits to approximate the level of every digital audio sample, based on the values of the samples that precede and follow it.

Here’s more on the Modi Multibit 1 bill of materials, from the manufacturer:

Modi Multibit is built on Schiit’s proprietary multibit DAC architecture, featuring Schiit’s unique closed-form digital filter on an Analog Devices SHARC DSP processor. For D/A conversion, it uses a medical/military grade, true multibit converter specified down to 1/2LSB linearity, the Analog Devices AD5547CRUZ.

That said, however, plenty of other audio purists object to the seemingly inferior lab testing results for multibit DACs versus delta-sigma alternatives (including those from Schiit itself), particularly in the context of notably higher comparative prices of multibit offerings. Those same detractors, exemplifying one end of the “objectivist” vs “subjectivist” opinion spectrum, would likely find it appropriate that in the “Schiit stack” whose photo I first shared a few months ago (and which I’ll discuss in detail in another post to come shortly):

I coupled a first-generation Modi Multibit (bottom) with a Vali 2++ tube-based headphone amplifier (top), both Schiit devices delivering either “enhanced musicality” (if you’re a subjectivist) or “desultory distortion” (for objectivists). For what it’s worth, I don’t consistently align with either camp; I was just curious to audition the gear and compare the results against more traditional delta-sigma DAC and discrete- or op amp-based amplifier alternatives!

A sideways wiggle did the trick

That said, I almost didn’t succeed in getting the Modi Multibit into the setup at all. My wife had bought it for me off eBay as a Valentine’s Day gift in (claimed gently) used condition back in late January; it took me a few more months to get around to pressing it into service. Cosmetically, it indeed looked nearly brand new. But when I flipped the backside power switch…nothing. This in spite of the fact that the AC transformer feeding the device still seemed to be functioning fine:

The Modi Multibit was beyond the return-for-refund point, and although the seller told me it had been working fine when originally shipped to us, I resigned myself to the seemingly likely reality that it’d end up being nothing more than a teardown candidate. But after subsequently disassembling it, I found nothing scorched or otherwise obviously fried inside. So, on a hunch and after snapping a bunch of dissection photos and then putting it back together and reaching out to the manufacturer to see if it was still out-of-warranty repairable (it was), I plugged it back into the AC transformer and wiggled the power switch back and forth sideways. Bingo; it fired right up! I’m now leaving the power switch in the permanently “on” position and managing AC control to it and other devices in the setup via a separately switchable multi-plug mini-strip:

What follows are the photos I’d snapped when I originally took the Modi Multibit apart, starting with some outside-chassis shots and as-usual accompanied by a 0.75″ (19.1 mm) diameter U.S. penny for size comparison purposes. Front first; that round button, when pressed, transitions between the USB, optical S/PDIF, and RCA (“coaxial”) S/PDIF inputs you’ll see shortly, selectively illuminating the associated front panel LED-of-three to the right of the button at the same time:

Left side:

Back, left-to-right are the:

  • Unbalanced right and left channel analog audio outputs
  • RCA (“coaxial”) digital S/PDIF input
  • Optical digital S/PDIF input
  • USB digital input
  • The aforementioned flaky power switch, and
  • 16V AC input

Transformers vs voltage converters

Before continuing, a few words about the “wall wart”. It’s not, if you haven’t yet caught this nuance, a typical full AC-to-DC converter. Instead, it steps down the premises AC voltage to either 16V (which is actually, you may have noticed from the earlier multimeter photo, more like 20V unloaded) for the Modi Multibit or 24V for the Vali 2++, with the remainder of the power supply circuitry located within the audio component itself:

Fortunately, since the 16V and 24V transformer output plugs are dissimilar, there’s no chance you’ll inadvertently blow up the Modi Multibit by plugging the Vali 2++ wall wart into it!

Onward, right side:

Bottom:

And last but not least, the top, including the distinctive “Multibit” logo, perhaps obviously not also found on delta-sigma-implementing Schiit DACs:

Let’s start here, with those four screw heads you see, one in each corner:

With them removed, the aluminum top piece pops right off:

Next up, the two screw heads on the back panel:

And finally, the three at the steel bottom plate:

At this point, the PCB slides out, although you need to be a bit careful in doing so to prevent the steel frame’s top bracket from colliding with tall components along the PCB’s left edge:

Firmware fixes

Here’s a close-up of the PCB topside’s left half:

AC-to-DC conversion circuitry dominates the far left portion of the PCB. The large IC at the center is C-Media Electronics’ CM6631A (PDF) USB 2.0 high-speed true HD audio processor. Below it is the associated firmware chip, with an “MD218” sticker on top. The original firmware, absent the sticker, had a minor (and effectively inaudible, but you know how picky audiophiles can be) MSB zero-crossing glitch artifact that Schiit subsequently fixed, also sending replacement firmware chips to existing device owners (or alternatively updating them in-house for non-DIYers).

And here’s the PCB topside’s right half:

Now for the other side:

In the bottom left quadrant are two On Semiconductor MC74HC595A (PDF) 8-bit serial-input/serial or parallel-output shift registers with latched 3-state outputs. Above them is the aforementioned “resistor ladder DAC”, Analog Devices’ AD5547. Above it and to either side are a pair of Analog Devices AD8512A (PDF) dual precision JFET amplifiers. And above them is STMicroelectronics’ TL082ID dual op amp.

Shift your eyes to the right, and you’ll not be able to miss the largest IC on this side of the PCB. It’s the Analog Devices ADSP-21478 SHARC DSP, also called out previously in Schiit’s press release. Above it is an AKM Semiconductor AK4113 six-channel 24-bit stereo digital audio receiver chip for the Modi Multibit’s two S/PDIF inputs. And on the other side…

is a SST (now Microchip Technology) 39LF010 1Mbit parallel flash memory, presumably housing the SHARC DSP firmware.

Wrapping up, here are some horizontal perspectives of the front, back, and left-and-right sides:

And that’s “all” I’ve got for you today! In the future, I plan to compare the first-generation Modi Multibit against its second-generation successor, which switches to a Schiit-developed USB interface, branded as Unison and based on a Microchip Technologies controller, and also includes a NOS (non-oversampling) mode option, along with stacking it up against several of Schiit’s delta-sigma DAC counterparts. Until then, let me know your thoughts in the comments!

—Brian Dipert is the Principal at Sierra Media and a former technical editor at EDN Magazine, where he still regularly contributes as a freelancer.

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