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Die 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.
This is the ”reference” DAC in my audio system. It’s based on Twisted Pear Audio (http://www.twistedpearaudio.com/landing.aspx ) Buffalo III SE board and consist in my implementation for best use this module.
The heart of Buffalo III board is the ES 9018 ESS Sabre32 Reference DAC chip. Whith its amazing performance is probably the best DAC actually available. It can handle 32 bit PCM data via I2S input as well as DSD or SPDIF data with DNR of 135dB and THD+N of -120dB. To obtain a USB port and connect a personal computer as player for high-resolution files I use the Amanero Combo 384 module (http://amanero.com/). Amanero Combo 384 module support a sample rate of 44.1Khz 48Khz 88.2Khz 96Khz 176.4Khz 192Khz 352.8Khz 384Khz with output format I2S 32bit or DSD64, DSD128, on Windows DSD256,DSD512 too. For Mac OSX 10.6+ and Linux with UAC2 compliant kernel no drivers needed otherwise you can download KS/Wasapi/WDM/ASIO Drivers for MS OS XP to Win8 32-64bit. The Buffalo III SE board need a power supply of 5,25 V at 440 mA. I tested various type of power supply and in particular it’s “sound characteristics” and at last I adopted a mosfet shunt regulated type equipped with Schottky diode as rectifiers. The most important feature of this DAC is the adopted two audio transformers to match the output signals. The Buffalo III need an I/V stage buffer to match the differential current output from ES 9018 DAC. I tried various circuit and device ( tube, opamp, fet ) but the best performance I obtained with Triad HS-56V audio transformers, in 600:600 ohms configuration, connected directly in the +, – pin out on the Buffalo board. No DC current flow in primary side due the equal DC voltage on the +, – DAC sides. Usually a I/V low value resistor is put on transformer primary side but after various listening test I prefer to omit I/V resistor. About the sound, I love this DAC. Very dynamic, open, spacious, wide and deep sound stage, realistic voices even with DSD, PCM high resolution files and also 16/44 khz music from CD. All kind of music is played at best quality very, very close to the best vinyl set up.