SOtM dx-USB HD USB to S/PDIF Converter Review

August 10, 2012

I have reviewed two asynchronous USB to S/PDIF converters – the Halide Design Bridge ($395) and the Audiophileo 2 ($579). Both worked as advertised. While the Halide Design Bridge is limited to sampling rates up to 96/24, the Audiophileo 2 adds compatibility with 176.4/24 and 192/24 files. With it, you also have the expense of a USB cable (and possibly a digital cable), whereas the Halide Design Bridge has captive USB cable with the converter circuitry built into an Eichman Bullet RCA or a BNC. Both derive their power from your computer’s USB port.

The SOtM dx-USB HD ($450) has the same capabilities as the Audiophileo 2, with the additional option of using a separate battery or linear power supply source. The idea is to provide power to the USB to S/PDIF converter from a clean power source, rather than what is likely the very noisy switch-mode power supply in your computer. It makes sense from both a technical and common sense viewpoint. SOtM offers a matching SOtM mBPS-d2s 9 volt power supply unit ($400), which combines rechargeable batteries and an intelligent recharger. In addition, the SOtM dx-USB HD provides AES/EBU and Toslink digital outputs on top of coaxial S/PDIF, so it is considerably more versatile than the Audiophileo 2.

The unit is housed in a small, sturdy, and attractive aluminum enclosure. The front panel of the SOtM dx-USB HD is straight-forward. There are indicator lights that tell you whether you are using power from the USB cable or an external power source, as well as indicator lights which tell you the incoming (and outgoing) sampling rate. The rear panel has the external power source input jack, and well as the AES/EBU, Toslink and coaxial S/PDIF digital output connections. The SOtM mBPS-d2s 9 volt power supply unit comes in a matching enclosure with a DC input for the wall-wart power supply used to recharge the batteries, and a DC output for connection to the SOtM dx-USB HD on the rear panel. The front panel has an on-off switch and indicator lights which tell you the status of the batteries and whether the unit is charging. Both units are well made, using quality parts.

My current setup is a Mac Mini with 16GB of RAM using iTunes and Pure Music playback software. I used the coaxial S/PDIF output of the SOtM dx-USB HD into one of the digital inputs of my Wadia 121 Decoding Computer aka DAC. I tried the SOtM dx-USB HD powered by the USB port on my Mac Mini and by the SOtM mBPS-d2s. I also made up a little connector so I could power the SOtM dx-USB HD with a 9 volt battery. The SOtM dx-USB HD draws 500ma, which a typical 9 volt alkaline battery can provide, but, to be safe, I used an UltraLife 9 volt lithium battery, which can provide up to 1200ma and lasts longer. Unfortunately, the SOtM dx-USB HD would lose lock, so using a 9 volt battery is not an option. To be honest, during my listening sessions, I could not tell a difference regardless of whether it was being powered by USB or by the SOtM mBPS-d2s external power supply. Nevertheless, I recognize the technical superiority of using a separate battery power supply, so perhaps with the right source material the differences might be audible.

There is also a variant of the SOtM dx-USB HD with an upgraded SOtM sCLK-2224 clock module ($950). Adding it is said to reduce jitter. The SOtM dx-USB HD with the clock upgrade, however, cannot be powered by USB. The SOtM dx-USB HD with the clock upgrade would not even light up with only a USB connection. You must use the SOtM mBPS-d2s power supply, which increases the total cost to $1350. I was able to audition both the standard SOtM dx-USB HD and the SOtM dx-USB HD with upgraded clock simply by switching the input on my Wadia 121. If differences exist they were subtle. Again, perhaps the differences would be more readily apparent with different source material. I think my system is resolving enough so that significant improvements would have been audible. Nevertheless, you can never have too little jitter.

I compared the SOtM dx-USB HD to the Halide Design Bridge at 44.1k, 88.2 and 96 sample rates and could detect no differences, so for an additional $55 ($395 vs. $450), plus the cost of a USB and digital cable, you pick up the ability to play back 176.4 and 192 sample rate files, making the SOtM dx-USB HD a good buy, plus you have the option to upgrade to an external power supply. I no longer have the Audiophileo 2 on hand so I could not do a comparison, but since I could hear no difference between the Audiophilleo 2 and the Halide Design Bridge, the fact that the SOtM dx-USB HD comes in at $145 less, also points to it favorably.


I could find nothing to fault with the SOtM dx-USB HD. Like the Audiophilia 2 and the Wadia 121, it played all my 44.1, 88.2, 96, 176.4 and 192 sample rate files without a hitch, switching among them automatically. If you already have a DAC and are interested in playing back high resolution files, then the SOtM dx-USB hD will get the job done at a very attractive price. The option to isolate it from the switch-mode power supply of your computer with an external rechargeable battery pack is a big plus. I like options. I like the SOtM dx-USB HD in all of its iterations. I have no hesitation in recommending it.

As a final note, SOtM stands for “Soul of the Music”. They get it.

- Frank Berryman

Associated Equipment

Analog Source: VPI Scout; Dynavector 20X2; Musical Surroundings Phonomena II
Digital Source: Meridian G08; Mac Mini; Wadia 121; Halide Design Bridge and DAC HD; Amarra and Pure Music
Preamplifier: Meridian G02; Meridian G68ADV
Power Amplifier: Meridian 557
Loudspeakers: Magnepan 1.7; Meridian DSP5500
Analog Cables: Kimber Select KS1016 and KS1116
Digital Cables: Kimber D60; Meridian Digital Link; Wireworld Starlight USB
Speaker Cables: Kimber Select KS6063 and KS9033
Power Cables: Kimber PK10G and PK14G
Headphones: Etymotic ER-4S
Headphones: Etymotic ER-4S
Accessories: GIK acoustic treatments; dedicated 20 amp circuit; Audience aR2p power conditioner