DAC, USB/SPDIF-converter og Headamp i eet!
Foranlediget af et bidrag fra LarsHPs til NOS DAC tråden gik jeg for en måneds tid siden på opdagelse, og uvis af hvilken årsag endte jeg på denne side, AllDacInfo. Her var der meget spændende at kigge på, og siden der blæser en vind fra øst i disse dage, så var min opmærksom nok specielt rettet mod mere obskure dac’er fra den kant.
Egentlig var jeg på udkig efter en NOS dac, men denne lille, hjemmebyggede alt-i-een dac til en plovmand fangede alligevel min interesse. Måske har jeg opholdt mig for længe på hoved-fi (@Claus
), måske er det blot oceanet af billige kinesiske rør-forstærkere, jeg har været ved at drukne i de sidste par måneder, uanset årsag så har jeg tydeligvis udviklet en smag for low-tech-low-cost produkter 
Så, Varna II blev bestilt, og viste sig selvfølgelig at være udsolgt i dagene inden. Men, i dag, nøjagtig en måned senere, er min egen lille Varna så produceret, testet, og klar til den lange rejse mod nord. Før eller siden ankommer den nok fra ingeniør Ivelin Yovchevs lille værksted (køkkenbordet?) i Bulgarien, men indtil da kan man forlyste sig med beskrivelsen herunder, eller selv gå på opdagelse på nettet.
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SPECIFICATIONS:
Word Lenght -16bit
Sampling Rate -32kHz/44.1kHz/48kHz
Processor Type -BurrBrown, Texas Instruments, PCM2704CDB
Output Signal Level - 0,65V (RMS), Aprox. 1Vp-p
THD+N with load 10K - 0,006%
THD+N with load 32Ohm - 0,025%
Output power at 32Ohm load - 12mW
SNR - 98dB
Input Interface - USB 1.1
Output Interfaces - line out type RCA; 3,5mm headphone jack; Digital Coaxial S/PDIF (5 times buffered, galvanic insulated). All outputs can operate simultaneously.
Power options - Selectable with switch power supplying:
Position 1 - By USB, when the power selecting switch is in position "USB", no need any external PSU.
Position 2 - With external PSU (5Vdc/100mA)-optional
Power Consumption - Aprox. 40mA (5Vdc) at full load (32 Ohm headphones at maximum level and working S/PDIF out)
Dimensions: H-49mm; W-96mm; L (with plugs)-78mm
Weight: 165g
Designed and handcrafted in Bulgaria by eng.Ivelin Yovchev.
DESCRIPTION:
It's great pleasure for me to present to you my new model USB DAC. Basically, it is an evolution of the highly successful "VARNA", which brought us popular not only at home but also abroad. It is logical the successor to be called "Varna II". I kept all good solutions from the platform "VARNA", but also I developed the model in several directions:
The main improvement is in using of a two-layer PCB, which allows several excellent optimizations of the topography and especially of the grounding plane. As a result, increases dramatically the conductive section of the ground. This significantly improves the performance of both the IC itself and it's serving periphery (mainly at the power filtration). Apart from this, the new ground plane is much more effective EMI screen for all incoming interference.
The second direction of improvements has focused on improving the filtration of the power supply. The major percentage of our customers uses the USB DAC supplied by USB. Because of this I decided to revise the power filters and their topology on the board . Among other things, I used double larger inductances in the filter units and dramatically increased the capacitance of all local filtration. When I say drastic , the exact number is 10 times. The model " VARNA " works with 2,2 uF local filtration capacitors, soldered directly to the pins of the IC. In the new platform the capacitors are already 22uF. The larger capacitance requires more space . Therefore the topology was overhauled and optimized. When did this "dirty " work, I provided a place for larger electrolytic capacitors, too.
The third direction was related to the addition of S/PDIF coaxial output . Many people say that the digital output of PCM2704 is not good enought . I disagree with this statement. If the scheme of the digital output is not well designed , then of course we will not have good results. I accepted this task seriously. I studied foreign experience and I was able to compile a personal view. The direct using of the PCM2704's digital output not causes anything good . Output is weak and unable to provide the necessary power to achieve high results. Therefore it is necessary to use buffer . In this manner the digital output of the IC is unloaded and starts to generate pulses with an ideal shape. And to insulate all interferences comming from the external S/PDIF line, it is imperative that it be galvanically separated after the buffer (even the ground !) . As a buffer I used a logical IC that provides five-times buffered digital output, which gives huge strenght of the signal . The galvanic separation is made by using of a high quality transformer "MURATA". After the listening tests , I can say that PCM2704 got a very high quality S/PDIF coaxial output, that sounds equal or may be better than my asynchronous transport M2TECH "HIFACE".
What to expect from the sound of the new " Varna II" ? Expect well-known warm and pleasant sound of the old "VARNA" , supplemented by a few strokes microdetail, caused from the newly introduced enhancements. The sound is clean, tight , yet natural and therefore less intrusive . No tension or aggression, so characteristic of most sound cards on the market. I've tried to get a sound that does not burden ears and mind and can be listened to for a long time without causing any negative reactions in the listener. This is actually a natural sound, all synthetic and sterile irritating and often cause discomfort and even headaches . Even after hours spent with headphones on my head , " VARNA II" does not cause fatigue in me . This test is sufficient for me that I've achieved my goals.
http://forum.audioplatinum.com/viewtopi ... 909#p19909
