As in Hans Christian Andersen’s fairy tale The ugly duckling, can a small, insignificant cheap DAC become a well-sounding high-end DAC? In most cases the difference between an economical DAC and a high-performance DAC is found in the analog output stage. Take for example the Douk Audio DSD 1796 (price below 100,00 euros): it is equipped with an excellent XMOS interface and an equally good DSD1796 DAC characterized by 24 bit resolution, 192 kHz sampling frequency, 4 mA current differential output.
The idea is to replace the analog output stage based on modest SMD op amp chips with a high performance I / V conversion transformer ( Sowter 8347F ) followed by a valve push-pull output design.
The following scheme is taken from DSD1796 data sheet but is almost identical to Douk Audio board.
Remove the 3 SMD opamp U16, U17, U18 and solder 4 22 ohms 1% precision resistors as in following foto.
Remove also the 3 RCA sockets and the headphones output. Cut out the DC 5V input power supply connector and the toggle switch. Solder a little jump wire ( as in the photo) for permanent 5 V internal power supply obtained using a free 6,3 V tap in the power transformer and a 7805 regulator. Use a bridge of schottky diodes in this power supply and a 150 uF Sanyo Os-Con capacitor soldered on the Douk Audio board ( 5V DC in ). In the analog output stage I tried several tubes ( 6H30, E182CC,….), the best result in terms of balanced sound, wide and depth of the acoustic scene I got with russian 6N6P.