With this latest DAC project of mine, I got frustrated in inventing new and impressive sounding names for my projects. Instead, I simply adopted the convention used by Mark Levinson: I just number the designs.
"No.3" contains all I have learned about digital audio DACs since I started designing out-board DACs. It contains judiciously picked set of known high-performance digital components, combined with selected no-compromise analog circuit blocks. Yet, the parts are relatively easily available at reasonable costs. With different grade components, the price of the system can be tailored to each one's budget. A low cost system would use AD1893 and SM5843A. A cost-no-object assemblage utilizes AD1890 and SM5842AP instead.
I have used every trick in my book to minimize the clock jitter and analog noise and distortion in "No.3". A special feature is the Digital Expansion Bus for adding new digital modules. Thus it forms a basic platform for upgradable and expandable digital audio system (see Digital Audio as a Hobby).
For DIY builders, I have designed a printed circuit board, which is manufactured by a professional board maker.
JT-DAC No.3 and Tubed DAC Output in piggy-pack installation.
JT-DAC No. 3 is my latest DAC project. It contains everything I have learned during the years from digital audio DACs. It includes a number of circuit items from the Audio Amateur magazine. I have combined all the best circuits to this unit:
I have used my own electronics design expertise in combining these superior pieces into a well tuned system. In my humble opinion, this board provides the best value in any outboard DAC board available, either of commercial or amateur origin.
A special feature is the Digital Expansion Bus for adding new digital modules. This bus utilizes the standard IIS bus for data transmission, plus necessary auxiliary signals and power supplies. Since its introduction in 1995, I have simplified the interface in "No. 3". It is now fully compatible with the interface used by Audio Alchemy in their expandable DAC products.
In my first DAC project (the System DAC), the jitter reduction was separated to an expansion bus module, since I was skeptical about the use of AD1890. The idea of recalculated sample values seemed rather questionable. I was anticipating that some other and better jitter reduction methods might emerge in the near future, and the expansion module would be easier to replace than the whole unit. Today, the AD1890 seems to be an established method, and no significantly better solutions have appeared. So it made sense to include it on the system board.
I finally managed to locate a local representative for NPC digital filters. Consequently, I have upgraded the oversampling filters to modern types. In fact, the board can be fitted with two alternative filters, SM5842A or SM5843A, which differ a great deal in price.
I have spent considerable time with my layout software to design a printed circuit board to complement the finely crafted electronics. I believe proper grounding and power supply distribution are an important aspect in digital audio boards to maintain low jitter and interference levels. Consequently, I have used ground and power supply planes extensively, still the PCB has only two layers to keep the manufacturing costs low. I believe this two layer board, with it's three separate copper planes for digital/analog grounds and digital power equals the more expensive four-layer board in performance.
The schematics and the parts list are available for viewing and downloading on this page. I have also prepared detailed assembly and testing instructions for DIY constructors. And if you encounter difficulties in building this DAC, I will try to help you as much as I can (to a reasonable degree) by email.
Comments, questions and feedback are highly appreciated:
This page was last modified on 27.02.2004.