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Sheherazade dsd-pcm Sabre32 DAC….

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.  

Falstaff: hybrid circlotron power amplifier….


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There has been considerable debate over the characteristics of tubes versus transistors…..The goal of this hybrid power amplifier is to take advantage of best performance of both technologies tubes and solid state device.‭ ‬Tube is highly linear without negative feedback,‭ ‬smooth clipping,‭ ‬wider dynamic range,‭ ‬characteristics highly independent of temperature,‭ ‬and so on‭…‬but its disadvantages such high-impedance devices that usually need a matching transformer for low impedance speakers.‭ ‬I prefer to use active devices in places where they operate the most linear.‭ ‬For voltage amplification,‭ ‬I rate tubes highest because they have a lot of headroom‭ (‬if the operating points and bias are correct‭) ‬and a long straight usable working range.‭ ‬In the power and/or current amplification stage,‭ ‬I rate discrete power transistors best.‭ ‬With them there is no need for an output transformer and it’s easier to achieve very low output impedance to match complex speaker loads.‭ ‬For a hybrid amplifier,‭ ‬MOSFETs combine well with tubes because they need no gate current.
‭ ‬To build this amp I was inspired to a classic Thirties-style, tube push-pull‭ ‬interstage transformer coupling amplifier due to his highly symmetric phase splitting that was invariant with signal level. ‭ ‬In the output stage I‭ ‬tested various topologies and at last I chose‭ ‬Mosfet Circlotron configuration with a single device per phase biased in class A and without feedback.‭ ‬It‭’‬s preserved the fully symmetrical architecture of the amplifier as a standard push-pull‭ ‬ amps but eschews the output transformer.‭ ‬The output mosfet,‭ ‬biased in class A,‭ ‬never switch off but have a constant well-defined drain-source current,‭ ‬there are no crossover or switching distortions.‭ ‬OK let‭’‬s look at first stage:‭ ‬this is a full push-pull tube stage‭ ‬. For the lowest distortion,‭ ‬the drivers need to have essentially horizontal load-lines,‭ ‬i.e.‭ ‬a very high impedance load relative to the plate impedance‭ ‬…‭ ‬preferably more than ten times higher.‭ ‬Transformer bandwidth is improved by having a low impedance on the primary.‭ ‬Tubes with low plate impedance that fit this application are the‭ ‬6h30,‭ ‬5687,‭ ‬7044,‭ ‬7119.‭ ‬After some listening test session‭ ‬I chose‭ ‬5687‭ ‬biased at‭ ‬12mA per triode,‭ ‬210‭ ‬V to anode.‭ The input transformer T1 is the excellent Lundahl LL1676 in 1:1 configuration. Multiturn trimmer on tube bias ( TB ) allows to perfect balance the stage. For GS gate stopper resistors use carbon composite type: any value from 220 and 1kohm is OK.
‬In the output stage‭ ‬I choose the Exicon‭ ‬ECX10N20R N channel lateral mosfet,‭ ‬high performance device designed specifically for linear audio amplifier. ‬It‭’‬s biased at‭ ‬1A of current and‭ ‬38‭ ‬V drain-source voltage.‭ ‬Two device per amplifier are needed and they must be well matched for Vgs.‭ ‬This reduce the output offset and distortion.
‭ ‬For the power supply‭ ‬in the stereo amplifier‭ ‬are needed four‭ ‬independent‭ ‬( floating‭) ‬units‭ ‬able to give‭ ‬38‭ ‬V.‭ ‬I use two toroidal power transformer‭ ( ‬one per channel‭) ‬with‭ ‬two‭ ‬independent‭ ‬secondary‭ ‬at‭ ‬30‭ ‬V,‭ ‬112,5‭ ‬VA,‭ ‬Schottky‭ ‬diode as rectifier‭ ‬and‭ ‬excellent‭ ‬Siemens Sikorel 3.300 uF/63V capacitors‭ ( ‬36‭ ‬total capacitors in the stereo amplifier‭ ‬).‭
In the PS tube stage I use an R-Core power transformer with‭ ‬260+260V at‭ ‬100mA‭ ‬ for‭ ‬5687‭ ‬anode voltage‭ ‬,‭ ‬6,3‭ ‬V at‭ ‬2‭ ‬A‭ ‬for‭ ‬5687‭ ‬filament‭ ‬,‭ ‬5‭ ‬V at‭ ‬3‭ ‬A‭ ‬for GZ34‭ ‬rectifier,‭ ‬6,3‭ ‬V at‭ ‬0,1‭ ‬A‭ ‬for bias power mosfet‭ ( MB ),‭ ‬12‭ ‬V‭ ‬at‭ ‬0,1‭ ‬A for‭ ‬negative‭ ‬bias‭ ‬of‭ ‬5687 ( TB ).‭ ‬5687‭ ‬anode power supply is a very fine choke input type with split L-C filter after first‭ ‬47uF capacitor for best L-R separation.‭ ‬All capacitor are MKP type bypassed with‭ ‬0,1‭ ‬uF fast type.
This amplifier run in class A. Put a DC voltmeter across 1 ohm/20W resistor and rotate the multiturn trimmer P1 for 1V reading ( 1A current ). Adjust P2 multiturn trimmer for minimum VDC output terminal blocks.