K & K Audio - Lundahl Transformers, audio DIY kits and more

Five years have passed since releasing the original RAKK dac Kit. Our on-going research and development work has resulted in another substantial advance in the reproduction of digitally recorded music. It is our distinct pleasure to announce the availability of the latest incarnation of the RAKK dac, the Mk III.

It's commonplace for audio companies to hype their latest product as something better than the best thing since sliced bread. While others hype their successes, we actually get a lot of value from our failures. Rather than rush to market a design based on some newly announced chip, we build and thoroughly test a new design. Perhaps it is our high standards for sonic performance or maybe just bad luck, but most of what we evaluate falls short of our expectations - which is simply a substantial audible improvement over our present product line.

When a new design fails our rigorous testing, the DIY DAC builder benefits in two ways. First, you don't have to buy someone's marginal design that is based on the latest DAC chip with spectacular specifications but falls short in sonic performance. Second, you can rely on the fact that our product has been thoroughly evaluated and represents what we consider to be state of the art, providing the best possible audio in kit form that we know how to provide.

We have learned not to rely on the data sheet specifications, adherence to a particular converter technology or the famous-name audio guru's advice as to what will sound good. We do, however, spend a lot of time and effort keeping up with prospects. If we hear of a report that some chip or design is really good, it goes on our list of things to be evaluated. Sometimes we feel like prospectors sifting through a pile of sand and gravel hoping to find a bit of gold. Even though there may not be a nugget in that pile the sifting still results in learning - either what will work or alternately, what won't work. And occasionally, while looking for gold, we discover an unexpected gem stone.

We have been working diligently behind the scenes for nearly two years evaluating candidates to be included in the suite of RAKK dac Mark III products. We had no idea when this journey started how difficult it would be to follow our latest digital audio "act", the superb Mark II products, but we are now proud to announce that the RAKK dac Mark III products are indeed a substantial audible improvement over the Mark II suite. The improvements come in three areas: new circuit design, selection of higher performance parts and new features.

The improvement in sonic performance of the RAKK dac Mark III is split evenly between the on-board power supply regulators and the digital design. We found that high-quality low-noise regulators are absolutely required to provide the best sound. But not just any regulator will do - we tested and evaluated several cutting edge designs before selecting one design for the digital section and another design for the analog section. Not only was the circuit topology important, but the selection of resistors and capacitors was critical in the success of the designs.

Our selection of the DAC chip was a surprise - after evaluating several candidates, we settled on the TI PCM1794 that we had used in the original RAKK dac and Mark II as the clear winner. Some other chips were sonically impressive in one way or another but the PCM1794 produced the finest overall musical experience. We did find that TI's new SRC4392 edged out the SRC4192 that we had been using for asynchronous sample rate conversion. The SRC4392 has an additional feature that was ultimately valuable, as well. Because the SRC4392 has an integral input receiver, we threw it into the evaluation of input receivers. It turns out that the SRC4392 receiver is better than any of the separate input receiver chips that we tested, including the CS8416 that we used in the earlier RAKK dac designs.

The RAKK dac Mark III has a few new features. First, we have increased its capacity for digital inputs by providing a third SPDIF interface. And to better accommodate AV system integration we have developed an external TOSLINK adapter to use with any of the three SPDIF interfaces. Second, over the past 5 years we have had several requests for an I2S interface for the RAKK dac, therefore, we added an I2S input to the Mark III. We also developed a suite of attachments for the I2S interface. There is a USB-to-I2S adapter board and another adapter board designed specifically to allow connection to the new PS Audio PerfectWave Transport by means its proprietary I2S output. And third, for the advanced DIY practitioner, we developed a way to attach a remote I2S interface to the RAKK dac Mark III. All RAKK dac Mk III inputs are galvanically isolated in order to preclude any possibility of ground loops.

To complement the improvements in the RAKK dac we have a new RAKK dac Power Supply for the RAKK dac Mk III that uses the superb LT3080 regulator as well as a Jensen 4-pole filter capacitor. Of course, this new supply provides an audible improvement over the Mark II power supply.

Finally, we turned our attention once again to the Active Output Stage, looking for ways to further improve its performance. Both of us had some ideas that we accumulated over the course of the Mk II Output Kit production run that merited consideration for Active Output Stage improvement, so we wiped the slate clean for the Mark III Active Output. We evaluated several radically different designs as well as many variations for those designs and found that there were one or more nuggets of sonic excellence in all of the designs. We worked hard to isolate and understand the origins of the nuggets and then combine them all into a final, improved design. When the dust settled, the result looked similar to the differential class-A parafeed design used in the Mark II Active Output. However, the sound was much improved due to the many refinements to that basic design topology. For the sake of convenience, the circuitry of the Combo Input Kit is now integrated onto the Active Output Stage PC board, although, as if by magic, the footprint is smaller in area due to clever stacking of components on daughter boards.

The only component that did not change substantially is the RAKK dac Passive Output. The LL1674 is still the best transformer for the passive output application. Because the output of the RAKK dac Mark III is electrically identical to that of the Mark II, the same Passive Output configuration and component values are used.

As part of our ongoing commitment to the quality of our documentation and convenience for our customers, we now provide the assembly and installation manuals for our kits on-line. The manuals can be accessed at our online manuals archive.

Capable of converting 16-24 bit digital audio data with a sampling rate of up to 192KHz to an analog stereo signal using the Texas Instrument PCM1794 D/A converter.

All valid PCM digital audio data are upsampled to 192KHz by the Texas Instruments SRC4392 ASRC on the RAKK dac Mk III board for the minimum digital-to-analog aliasing artifact generation.

Three transformer-coupled S/PDIF inputs are present and selectable via wire jumper or off-board switch (not included). One LL1572 digital input transformer is supplied with the RAKK dac Mk III Kit. Additional LL1572s can be ordered at any time to enable additional inputs.

One I2S PCM audio data buss input is present and selectable via wire jumper or off-board switch (not included).

Analog output signal is two channel differential current, requiring current to voltage conversion in an analog output stage, such as the Passive Output Stage or the Active Output Stage Mk III.

Optional on- or off-board PCM digital audio data lock LED and output mute LED are supported.

RAKK dac Mk III PC board is professionally assembled and tested. Only digital input transformers and external connections need to be soldered to the PC board for installation.

PC board dimensions are 5.2" x 2.85" (14cm x 7.25cm). Screw hole spacing is the same as that used for the Mk I and Mk II for retrofit compatibility.

Power supply requirement is 12VDC at 250mA. Power supplied to the RAKK dac Mk III board should have less than 200mV of AC ripple and be within the nominal limits of 11.0VDC to 13.5VDC.

Note: For a complete digital-to-analog converter, you will need a RAKK dac Mk III Kit, a power supply (12VDC), and an analog output stage. Please see further information about power supply kits and analog output stages below that were designed to work with the RAKK dac Mk III.

The Diagram illustrates the interconnection of the RAKK dac Mk III with the output stages and power supplies.

RAKK dac Mk III Digital-to-Analog Converter Kit $459 ($399 for original purchasers of the RAKK dac Mk II Kit)

There is an old saying: "you are what you eat" and the same goes with audio electronics - the RAKK dac requires superb power in order for it to produce its best. Therefore we have expended a lot of time and effort in the power supply development and not skimped in this important area. As with the Mark II Power Supply kit, we offer two versions: a "bare bones" Basic version, providing what we consider to be the minimum acceptable power and a second Premium version that takes the RAKK dac sound over the top to its limit.

Both versions are "minimal reactance", meaning that minimal capacitive storage is used in the raw filter prior to the voltage regulator, resulting in less power supply noise in the regulated output. The Basic version provides adequate capacitance on its single 12V output. The Premium version provides additional capacitive storage, falling just short of excessive and has two separate outputs, one "superfiltered" by a Jensen 4 pole capacitor to supply the RAKK dac MK III itself for the best sonic performance and a second output for adapter/interface boards, such as the RAKK USB -> SPDIF, RAKK USB -> I2S boards or RAKK I2S Interface Adapter.

Both versions of the power supply are split into two modules - an Input Module and an Output Module, which are offered separately. The Input Module, which provides the power transformer and line filtering, is not needed if there is an LL1683 power transformer already in the enclosure to power the Active Output Stage. The Output Module provides the rectification, regulation and storage.

The Input Module accepts a Mains voltage of either 120V or 230V user configured and provides 18VAC output at 15VA.

The Output module accepts 16VAC to 18VAC (14VA minimum) and provides 12VDC with a maximum current delivery of 350mA, which is adequate for a RAKK dac Mark III plus an adapter/interface board.

PC board dimensions are 4.5" x 3.5" (11.5cm x 8.9cm). The height is 2.75" (7cm). Screw hole spacing is the same as that used for the Mk II for retrofit compatibility.

The RAKK USB -> SPDIF converter uses the TI PCM2707 DAC, providing adaptive isochronous audio stream playback at 32, 44.1 and 48 kHz sampling rates.

The RAKK USB interface with the RAKK dac Mk III and the RAKK dac Mk III Power Supply is designed for complete galvanic isolation to avoid the introducing digital noise from the computer into the audio system. Galvanic isolation is very important for providing the cleanest audio signal.

Power supply requirements are 12VDC at 75mA, which can be supplied by the RAKK dac Mk III Power Supply Kit.

PC board dimensions are 3.8" x 1.6" (9.7cm x 4.1cm). The board can be mounted using stand-offs through the corner PC board holes or by L-brackets through the holes adjacent to the USB connector to a back panel with the USB connector protruding through a hole in the back panel or connected to a panel mount USB jack (such as that made by Neutrik) using a short USB cable.

The Diagram illustrates the interconnection of the RAKK USB -> SPDIF and the RAKK dac Mk III.

RAKK USB USB-to-SPDIF Converter including LL1572 for use with other DACs $124

The RAKK USB -> I2S converter uses the TI PCM2707 DAC, providing adaptive isochronous audio stream playback at 32, 44.1 and 48 kHz sampling rates. (picture coming soon.)

Power supply requirements are 12VDC at 50mA, which can be supplied by the RAKK dac Mk III Power Supply Kit.

PC board dimensions are 4.9" x 2.35" (12.5cm x 6.0cm). The board can be mounted using additional stand-offs through the corner PC board holes directly above or below the RAKK dac Mk III and connected to a panel mount USB jack (such as that made by Neutrik) using a short USB cable or by L-brackets through the holes adjacent to the USB connector to a back panel with the USB connector protruding through a hole in the back panel.

The Diagram illustrates the interconnection of the RAKK USB -> I2S and the RAKK dac Mk III.

The I2S Interface Adapter provides the means for attaching an external I2S interface to the RAKK dac. The external interface is defined by that of the PS Audio PerfectWave Transport (PWT,) therefore this adapter provides the means by which a PWT may be attached to the RAKK dac Mark III resulting in a stunning digital audio experience.

The RAKK I2S Interface Adapter Kit consists of two PC boards: The first one (I2S Interface Adapter Receive board) provides an adaptation between the external HDMI connector (the HDMI technology is used for its wide availability and excellent data integrity) and the internal RAKK dac Mark III I2S interface; the other PC board (I2S Interface Adapter Send board) is designed to be used in an attaching device, such as a CD player or transport with an internal I2S audio data path, providing an adaptation between an HDMI connector and the attaching device's internal I2S data path.

The use of the I2S Interface Adapter Receive board to attach a PWT is straight-forward, however the use of the I2S Interface Adapter Send board with other transports is complex and only for the advanced hobbyist. Because we have no knowledge of the internal structure of the myriad of devices that a hobbyist may want to attach to the I2S Interface Adapter, we strongly recommend that the I2S Interface Adapter Send board be used only by someone who has the necessary knowledge and skills to undertake the project without guidance.

The I2S Interface Adapter boards use the ubiquitous HDMI cable (not included) for interconnection.

Power supply requirements are 12VDC at 75mA for the DAC DOCK receive board, which can be supplied by the RAKK dac Mk III Power Supply Kit. The requirements for the DAC DOCK send board are 5VDC at 20mA, which should be derived from the host CD player or transport.

I2S Interface Adapter Receive board dimensions are 4.9" x 2.35" (12.5cm x 6.0cm). This board can be mounted using additional stand-offs through the corner PC board holes directly above or below the RAKK dac Mk III and connected to a panel mount HDMI jack (such as that made by Neutrik) using a short HDMI cable. The I2S Interface Adapter Send board (dimensions 1.0" x 0.75" (2.5cm x 1.9cm)) can be attached to the chassis back panel by the integral HDMI connector bracket (and included Send PC board brackets) with the HDMI connector protruding through a hole in the back panel.

The Diagram illustrates the interconnection of the RAKK I2S Interface Adapter and the RAKK dac Mk III.

The RAKK TOSLINK -> SPDIF converts the optical digital signal originating from CD players, HDTVs, AV receivers, etc., to the electrical SPDIF format, making it compatible with the SPDIF inputs on the RAKK dac Mk III. (Picture coming soon)

The RAKK TOSLINK interface converts the optical digital audio signal to a high quality buffered SPDIF audio data stream and operates from a 12VDC supply (10mA), such as the RAKK dac Mk III Power Supply Kit.

PC board dimensions are 1.5" x 1.3" (3.8cm x 3.3 cm). The board can be mounted on the back panel of an enclosure using the mounting hole on the TOSLINK receiver module itself. The special self-tapping screw for this purpose is supplied.

The Diagram illustrates the interconnection of the RAKK TOSLINK -> SPDIF and the RAKK dac Mk III.

The PCM1794 output current is converted to an AC voltage by the action of the excellent Lundahl LL1674 amorphous core transformers and high quality tantalum I/V resistors.

PC board dimensions are 4.9" x 2.35" (12.5cm x 6.0cm). The height is 1" (2.5cm). The board can be mounted using additional stand-offs through the corner PC board holes directly above or below the RAKK dac Mk III.

Output level at 0dBfs: 2.0vRMS
Output impedance: 3000 ohms
Frequency response: 10Hz to 20KHz, -0.7dB, -0.4dB

The differential class-A triode parafeed output stage is a further refinement of the circuit, now in its fourth generation, first used in the excellent Raleigh Audio Line Stage detailed in the November 2004 issue of AudioXpressThis output stage uses a transformer-coupled output in a bridged parallel feed configuration that was used by Western Electric over 60 years ago. However, it employs a cascode connected depletion mode MOSFET constant current source (CCS) that is more effective than anything available to Western Electric engineers. The output transformers are the superb amorphous core Lundahl LL1674 units. The amplifying device is the Russian 6N6P dual triode. Dual high-voltage Minimal Reactance Power Supplies are integrated onto the Active Output Stage board, so only a power transformer (LL1683 or equivalent) is required to provide a complete vacuum tube output stage solution.

For DAC applications, current-to-voltage conversion is accomplished using high quality low value tantalum resistors. In addition to being a superb RAKK dac Mark III output stage, this kit can accommodate line-level inputs. The line-level input option is provided simply by adding LL1676 input transformers (not included) to locations on the Active Output circuit board.

An optional volume control (not included) has also been accommodated (see Stepped Shunt Attenuator).

The kit uses all top quality parts, among them: Lundahl transformers, Cardas film capacitors and tantalum resistors.

The Active Output Stage kit is offered in two levels. The levels differ in the amount of parts provided versus the parts that you must procure on your own. The Level 1 kit provides the minimum parts, the Level 2 all of the parts needed.

Included:

Circuit boards
Assembly manual (on-line)
Schematic diagrams
Parts list

Your involvement:

Order the parts from five suppliers
Cost of additional parts needed - approximately $540 (as specified)
Assemble the kit

Active Output Stage Level 2 Kit for Mk II owners without Mk II trade-in: $849

Included:

All parts, including circuit boards and two LL1674 transformers
Assembly manual (on-line)
Schematic diagram

Your involvement: Assemble the kit.

Fully assembled and tested Active Output Stage for Mk II owners with Mk II trade-in: $849

Fully assembled and tested Active Output Stage for Mk II owners without Mk II trade-in: $999