Digital Audio Page

The sonic character of recordings made using digital audio technology has been controversial since before the CD was launched over twenty years ago. Many have felt that the best music recorded using the “Red Book” CD standard (16 bits/44.1KHz sampling rate) was markedly inferior to that commercially available on the long playing record. As a result, many music lovers have reluctantly embraced the CD medium as a means to add music to their collections. Excellent CD playback capability has been available for perhaps 5 or 6 years, but requires “big bucks”. Well, that reluctance and big financial outlay are a thing of the past with the RAKK dac.

The RAKK dac is the result of a cooperative effort of K&K Audio and Raleigh Audio to bring you the cutting edge, affordable digital music playback solution. We have employed the outstanding Burr Brown / Texas Instruments PCM1794 DAC, which is the latest generation current output 24 bit / 192KHz sampling D/A converter. Designed primarily for the high-end professional audio market, we found this converter to have the most lifelike sound. We coupled this DAC with the newly released 24 bit / 192KHz capable input receiver from Cirrus Logic, the CS8416. In addition to the word depth and sampling bandwidth considerations, the CS8416 has excellent jitter rejection specifications and a built-in digital input selection function.

In addition to the digital audio integrated circuit building blocks described above, a first-class regulated power supply sporting dual regulation for the digital as well as the analog voltages, and extensive local capacitive power supply bypassing, and a carefully conceived layout. The analog section of the DAC is supplied with a shunt regulator for the best possible sound. These design features have resulted in a product that conveys music both with technical excellence and emotional coherence.

Kit construction is a breeze -- most ot the work is already complete. The 2.35” x 4.9” (6.0cm x 12.5cm) RAKK dac circuit board has all of the surface mount components pre-soldered to the top of the 4 layer PC board. You need only solder the digital input transformer(s) and connector(s) to the underside of the board to complete the kit.

The RAKK dac Kit has provisions for two digital inputs. Designed into the PC board are pads for a Sound Connections (Vampire) right angle PC mount RCA or BNC (75 ohm) jack and pads for a Neutrik PC mount female XLR jack. Either or both of these can be used. However, if you prefer to use different input connectors, you are welcome to wire these to the RAKK dac board and receive credit against the unused connector in the kit. Please specify your connector(s) preference at the time of ordering. One Lundahl LL1572 digital input transformer is supplied with the RAKK dac Kit. An additional one may be purchased for a second digital input.

The RAKK dac was also designed to interface easily with Tent Labs’ XO-DAC 2nd phase locked loop (PLL) jitter reduction circuit. The XO-DAC adds a low jitter oscillator reference for a 2nd PLL to reduce clock jitter beyond that possible with the CS-8416. Physically, the RAKK dac mounts above the XO-DAC on the same stand-offs. Bare wires are dropped and soldered between the two PC boards to interconnect them after cutting a single trace on the RAKK dac board.

The Tent Labs XO-DAC module contains a secondary PLL, with a carefully designed low frequency low-pass filter. Within that loop a low noise opamp drives a Tent Labs VCXO with low intrinsic jitter (3ps RMS, 3 sigma). The resulting clock is one that nearly matches the VCXO jitter. The XO-DAC PC board mates with the RAKK dac in a straightforward way to offer the ultimate in low jitter performance. The stock XO-DAC is provided with a VCXO suitable only for CD replay (11.2896 MHz). Other frequencies are available by special order.

The RAKK dac requires two supporting functions: a power supply and an output stage. The power supply must provide 12vDC at about 100mA. Because the RAKK dac provides a low-level current output directly from the DAC chip, additional circuitry is required on an output stage board to provide current-to-voltage conversion, isolation, and gain. To help you complete your RAKK DAC based D/A converter, we have various supporting kits available.

The current output of the PCM1794 is high enough to enable use of a Lundahl LL1674 1:4 step-up amorphous core transformer for a passive output stage. A special connection of the primary is used to very effectively cancel the DC bias voltage present on the output terminals of the PCM1794. A high quality resistor in the secondary circuit of the transformerprovides for the current-to-voltage conversion. A pair of RCA jacks mounted on the PC board completes the kit. This output stage is actually small enough so that it is physically the same size as the RAKK dac board and can be stacked on top of it for a particularly compact installation.

For those of you who want to “roll your own” output stage or purchase the Passive Output Stage Kit, we have designed an excellent, compact standalone 12vDC supply. It features sonically advantageous Minimal Reactance with shunt regulation and a low stray field toroidal power transformer.

The differential class A triode parafeed output stage is a refinement of the circuit used in the excellent Raleigh Audio Mood Indigo Line Stage Preamplifier. This 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 6N1P dual triode. Current-to-voltage conversion is accomplished using high quality low value resistors.

The kit uses all top quality parts, among them: AuriCaps, Mills wirewound resistors, Kiwame carbon film resistors, and Caddock metal film resistors. See picture below.

A minimal reactance (MR) power supply has been designed specifically to power both the RAKK dac and the Active Output Stage. It features vacuum tube rectification, a vacuum tube shunt regulated B+ supply, AC filament supply, and a dedicated shunt regulated 12vDC supply for the RAKK dac board. This supply is physically joined to the Active Output Stage PC board and these are intended as an integrated unit, however, either can be obtained separately. The Lundahl LL1683 is the perfect power transformer for this combination.