My most recent "Project 4" - Digital tube amp or PowerDAC.

The unit has 2xRCA, 1xBNC and 1xXLR Digital Audio inputs with maximum data rate 192kHz 24bit.
Also has one I2S input via LVDS interface (connected to the source by CAT6 STP) with maximum data rate 384kHz 24bit.
It will drive 4 or 8 Ohm speakers or headphones with impedance as low as 16 Ohm.

This project was designed and built with Hi-End philosophy in mind and features:
- no op-amps (even built-in to DAC chip);
- no feedback of any kind;
- no digital filter or oversampling;
- no analog filter after DAC chip (and it sounds good, especially with hi-res recordings!);
- no capacitors in sound path.
To achieve that I have used an industrial DAC chip with "open" R-2R ladder connected to a reference battery source from one end and to the triode grid from the other end, getting the shortest signal path possible.
As industrial DACs usually have SPI input only, there is a converter from I2S to SPI format implemented into Xilinx CPLD. Block diagram is provided for those who are interested in technicalities.
At this moment I am using MAX542 16-bit multibit DAC chips but it is upgradeable to 18-bit or 20-bit multibit DACs. It will be my next project . :)
To minimize noise power supply was built in a separate case.
Commercial off the shelf enclosures were used so don't be confused with "Vacuum Tube PREAMPLIFIER" and "TUNER CD DVD AUX" labels on the front panel - I am still working on the solution how to re-label panel in a nice, good looking manner. :)

Comments

• Raghu
• 5 June 2014 at 08:23AM
• Hi Dr.
  
  This project is very interesting and looking very nice. Any chance to share schematics to my email
  (nraghunadh@gmail.com). I am very much impressed this and i will try to build one for me.
  
  Thanks,
  Raghu

• Dr. Bar-Mental'
• 10 June 2014 at 06:07AM
• Hi Radhu,
  
  Thanks for your interest.
  
  First of all, this approach is quite expensive and time consuming. (Honestly, I wouldn't start it, if I knew that it would turn to be that complex).
  
  You have to be prepared to spend around $2000 for parts and match to 1-2% two pairs of tubes at 3-5 points (at least).
  
  The idea of this post was to show an alternative approach to high fidelity DAC design: omitting some parts (like filters) and still have a great result.
  
  So, apart from digital board, no documentation (adjustment instructions or complete set of schematics) was made.