Most of the DAC's featured here utilise S/PDIF of some sort as a data transport, but increasingly there are alternatives being offered in the form of USB, Firewire and other computer related serial interfaces. All the USB based DAC's featured here are a standard USB Audio Class A or B devices, those that require a unit specific driver are considered to be computer soundcards. A standard USB audio device should work out of the box with Windows 8, 7, Vista, XP, 2K, ME and 98SE (Win98, Win95a/b/c and NT4 can be made to work), Linux 2.4.22 and higher, most recent BSD variants, Mac OS9 and up (late v8 can be made to work as well) and so on, Class A devices should also work on OS/2 v3 and up or any version of eComStation.
Alouette Audio Makes the MP-DAC, features a 24bit Delta Sigma converters and filtering section made out of discrete transistors.
AQVOX Makes the USB 2 D/A, it is unusually flexible for a consumer device offering coax and optical S/PDIF, balanced AES/EBU and USB inputs and balanced and unbalanced outputs, epecially notableat the price which is only about 650€, it also offers upsampling to 24/192, jitter correction and low latency drivers for MS Windows.
Burmester Audiosysteme Manufactures 2 high end converters , the reference design is the 970 SRC which is an upsampling DAC with 5 digital inputs including Toslink, Balanced, AT&T and RCA, it has a wordclock output ton enable you to slave CD transports to it and unusually enough a couple of digital outputs. Another unusual feature on this DAC is the inclusion of a multiple variants of filters which you can switch between and set up differently and memorise the set-up of, allowing you to change the characteristics of the DAC to suit different CD's or transports/devices player, in addition it has fixed and variable balanced and unbalanced analogue outputs. The somewhat cheaper 980 SRC retains almost all of the electronic features of the 970 except for the AT&T digital input, were savings are made is in the power conditioners, stabilisation and acoustic suspension/isolation, the 980 is almost 20 KGs lighter than the 970 and is built more like conventional high end hi-fi product, stunningly good nevertheless and gives it's big brother a run for it's money.
Candeias Engineering Makes the high end Modelo DAC, although intended to mate with the company's Modelo CD transports it was a firm favourite with the German audio press and much more so than the transport itself so you will often find this mated to other items, it features 24bit converters and a Wordclock input.
CEC Makes DX 71 MkII and the DX 51 Mk III both are modular and thus upgradable converters that feature an upsampling converter, CEC's Superlink and balanced inputs, the main difference lies in the improved electrical and acoustical isolation of the DX 71 model over the 51 and the 71 also has digital outputs in addition to the analogue ones.
Dodson Audio Makes the DA-218 that is a high end upsampling unit capable of operating at bitrates of up to 24 and at sampling frequencies as high as 768kHz, unusually enough the analogue section of the DAC have been cryogenically treated.
JMF Electronique This French company manufactures 2 models of DAC's, the high end Stereo D/A is a traditional 2 channel converter but the company does not provide lots of information on that model in English. The DXC6.10 however is something else altogether, it is a reference class modular unit that comes standard with 2 channels but can be expanded to up to 6 channels or more with analogue expander cards. All inputs are S/PDIF and AES/EBU and all outputs are balanced, in addition to being able to upsample up to 768kHz it offers a host of advanced digital processing options, more info can be found here.
Metronome Technologie Makes the C2 Signature DAC that uses the same 24 bits @ 192kHz bitstream converters as their high end players but features a valve output stage and an independent power supply. I/O includes balanced and unbalanced analogue outputs in addition to AES/EBU S/PDIF and Toslink inputs.
Reimyo Makes the Reimyo DAP-777, it's based on the electronics used in their acclaimed CDP-777 CD player and features balanced and BNC digital inputs in addition to the usual, one small but thought full feature is that it actually supports input at 48 and 32kHz, a great feature for DAT owners and something that many other high end converter manufacturers forget to do (they restrict the input to 44,1) .
Sonosax Manufactures the SX-DA2, this is a miniature rugged unit actually intended for portable applications in the professional sector but a has a much higher sound quality than the usual suspects in that field, expensive though, more info here.
Stello Has the DA220 a high end upsampling converter that supports 192kHz, they also have a somewhat more unusual device in the form of a the DP200 which is a combination of a DAC and an amplifier.
California Audio Labs The last couple of converters that the company made were the Sigma II and Alpha, both were 96kHz capable DAC's that featured tube filtering, they made the more conventional Gamma solid state model.
Candeias Engineering Owners of older versions of the Modelo converters should note that they can upgrade it to a more current specification fairly easily, more info here.
CEC Current owners of the DX 71 and the DX 51 MkI or Mk II can upgrade their models to the current 71 MkII pr 51 MkIII specification by getting hold of a new converter board from their local distributor, the new board features new upsampling converters with 24bit 356KHz capability and whatnot.
Rega Discontinued the manufacture of the Io DAC in 2002, it was designed to mate with the Jupiter transport from the company and was in fact seldod sold on it's own. The Io uses Burr-Brown converters and has no less than 5 power supplies built into the unit or one for each major component.
Also knows as black boxes, all CD players have an analogue filter to rid the signal from unwanted artefacts, many CD players especially the early ones has rather weak filters so there was a market for high quality external filters that were placed in the signal chain between the CD player and the amplifier, a number of those boxes also had some sort of "exciter" circuit, although these units are getting rarer these days there are some companies still making them. There are also buffer amplifiers sold as black boxes, these can obviously be used with any hi-fi products but ar at the best with a fairly coarse signal like the one put out by a CD player.