Updated: 24-Jan-01
The heart of the whole Converter: The DAC stage it self

It is recommended to download first the PDF Document DDDAC1.pdf with the schematics.
For pictures of all the parts please see the Technical Backgrounds Section and Picture Galery

picbks11.jpg (8501 bytes)
The Analog Output from the DAC is filtered and amplified by the little Sowter transformers


Finally this is where the newly sampled and re calculated data is being transformed in an analog signal.  Currently, a lot of discussion is going on, on what type of DAC would be the best. I am giving my more detailed opinion on the Technical Background pages DAC's. But you will have noticed, that I selected the PCM63PK for this Job. Simply because in my (and others !!) opinion, this is the best and most suitable DAC Chip for use in a configuration like this; I/V conversion with a resistor and amplifying and filtering with a step up transformer. No other DAC will perform in this configuration like the PCM63. Simple reason is that the PCM63 is a resistor ladder converter with "class-A" output. So straight current coming out of the chip, WITHOUT any intervention of OPAMPS, or what so ever. This just screams for short signal path treatment with Tubes. To have a very clean and stable current source output, it is evident that the power supply for the DAC is clean and stable and separated for the digital and analog side. With the extensive set up of the power supply this requirement is more than full filled.

The schematic itself is actually very simple. All capacitors are NOT critical and values were taken with parts on hand !! Finally  the 3 main signals go into the chip and the analog current is converted in a voltage by the (how simple...) 100 Ohm resistor. Of course you can use OPAMPS for this purpose, but only if you like a flat un-spatial soundstage, loose pin point focus and lot of details. If you, like I, do not want this, than spend 10$ct on the resistor. The smart people will say, why not a 1k Resistor, you will have 10 x the output voltage and you won't need a transformer or Tube stage. Well, there are 2 good reasons not increase the resistor!!

  1. The output impedance will go up, together with cable capacitance and Miller C in the amp, you will loose big time bandwidth, easily under 20kHz !!!
  2. Although the output is a current source, the output voltage is NOT limited, actually it is capped at .7 Volt (!!!) this is caused by protection diodes in the output circuitry. An OPAMP has no problem with this, because of the virtual ground of the input, keeping the output voltage on 0Volt......

If you are interested in the more technical background of why the Resistor doing the I/V conversion should not exceed 100 Ohm, please read this technical bulletin with a digital scope and spectrum analyzer

Finally, the signal goes through the Sowter transformers for gain and filtering. This results in extreme linear phase behavior compared to standard filters, which I believe is fundamental for obtaining a detailed and open spatial soundstage.


IMPORTANT: The information provided on this page is intended as guide for DIY activities and therefore free to copy and or publish. If any one wishes to use any of the information from my WEB site, please make sure to refer and footnote to my URL Link as source! Doede Douma