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Die zauberflote‭!!‬…

Zauberfline2uuuuuuuuDie zauberflote‭!!‬…it‭’‬s‭ ‬a magic flute now I thought when I listened for first time this DAC.
‭ ‬Yes,‭ ‬the original Musiland monitor‭ ‬02Us was a honest good cheap USB DAC but nothing more. ‭ ‬All low cost DAC or CD player use a modest analog op amp based output stage.‭ ‬But a quick look in the schematic and specs DAC I saw a very good digital part‭ ‬:‭ ‬high speed USB interface,‭ ‬SNR and dynamic range of‭ ‬113dB,‭ ‬good PCM1793‭ ‬DAC,‭ ‬excellent‭ ‬Windows driver and so on.‭ ‬Well,‭ ‬this is a good,‭ ‬true‭ ‬24‭ ‬bit/192kHz digital device.‭ ‬So it‭’‬s worthwhile a serious heavy upgrade:‭ ‬a tube push-pull output stage.‭  ‬In this role a good candidate is the Russian tube‭ ‬6H30:‭ ‬very linear,‭ ‬low plate resistance,‭ ‬adequate gain‭…‬a good choice for transformer load stage. ‭ ‬The tube new circuit must replace the op-amp output stage on the Musiland board taking the input signals as in the schema below.‭ ‬CCS‭ ( ‬constant current source‭) ‬on the cathodes‭ ‬6H30‭ ‬is based on DN2540‭ ‬N channel depletion mode vertical mosfet.‭ ‬With the source resistor in the schema the current is set to‭ ‬40mA‭ ( ‬20mA per tube‭)‬.  
‭ ‬GS is a carbon composite gate stopper resistor:‭ ‬any value from‭ ‬220‭ ‬and‭ ‬1k ohms 0,5W is OK, for the other resistors use 1% precision metal or carbon type 0,5W.‭ ‬50‭ ‬ohms multiturn trimmer allows to balance current in primay push-pull transformer‭ ( ‬e.g.‭ ‬set the equal‭ ‬6h30‭ ‬anode voltage‭)‬.‭ A poor DC current balance might saturate the core and reduce the inductance of the windings ( worse low frequency response ). ‬Power supply‭ ‬it‭’‬s a very fine tube rectifier,‭ ‬choke input circuit ( choke-fed supply are much quieter due to the choke slowing down the rate-of-charge of the main 47uF cap ) and‭ ‬need a transformer with‭ ‬3‭ ‬windings:‭ ‬260+260V at‭ ‬100mA for HT,‭ ‬5‭ ‬V at‭ ‬2A‭ (‬min‭) ‬for‭ ‬5Y3GT filament,‭ ‬6,3‭ ‬V at‭ ‬2A (min‭) ‬for‭ ‬6H30‭ ‬filament.   ‭ ‬Note in Power supply schema,‭ ‬after first‭ ‬47uF cap,‭ ‬the anode voltage are splitted for best R-L separation performance.‭ ‬All capacitors used are MKP type‭ ‬bypassed with‭ ‬0,1uF polystyrene or silver mica.‭ ‬Don‭’‬t forget to ground one leg of‭ ‬6H30‭ ‬filament to reduce hum.  
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