The philosophy of the TUBE-DAC II The ACCUSTIC ARTS® TUBE-DAC II is an outstanding, highly precise digital to analog converter layouted in a so called “Tube-Hybrid” concept. This concept combines the advantages of the solid state technology with the advantages of the tube principle, however without the specific disadvantages of tubes. To use the advantages of this concept to full capacity, a very elaborate, digital signal processing was integrated, which only enables the extraordinary precision and musicality of this Tube-Hybrid concept. The TUBE-DAC II is the ACCUSTIC ARTS® reference in the high end D/A converter segment and intended for the music lover who appreciates the precision of digital sources, yet doesn’t want to abandon the outstanding sound of top analog units. The TUBE-DAC II is therefore the perfect solution for “audiophile gourmets”, who do not so much care about superficial grandstanding but are looking for a maximum of precision, authenticity and musicality. Preferably the TUBE-DAC II should be operated with a high quality CD transport, for example the ACCUSTIC ARTS® DRIVE I-MK2.
The digital signal processing of the TUBE-DAC II 1. A brief overview During the development of the digital signal processing great importance was attached to highest precision and attention to detail, which is why an improved method compared to the currently common “upsampling principle” has been implemented. You will learn more about it in the next chapter. Digital signal processing: This excellent Microcomputer is the heart of the digital section
in the TUBE-DAC II and, apart from a high precision quartz
crystal-oscillator, also contains a 32 bit digital filter and 26(!)
parallel operating processors (so called multiplying processors). The following conversion of these two 32 bit signals into analog signals is taking place in two separate D/A conversion chips, and therefore also completely discrete by channel and of highest precision. The resulting two analog signals are finally processed by an elaborate filter circuit in accordance with the Tube-Hybrid concept and leave the TUBE-DAC II either via balanced XLR-outputs or unbalanced RCA outs.
2. The digital signal processing in detail A common upsampling D/A converter scans the digital input signal, for example from a CD with a frequency of 44.1 kHz, with a higher frequency (e.g. 192 kHz sampling rate). Indeed the individual time segments between the scans are getting very short by this means and the accuracy is increased, however as a matter of principle all upsampling methods have the disadvantage that with higher frequencies the noise is also increasing significantly. This unwanted noise has to be removed afterwards by the usage of filters, which again can lead to sonic interference of the music signal. The 32 bit Microcomputer of the TUBE-DAC II is also recomputing the digital input signal, however with superior and more elaborate technology in comparison with the common upsampling principle. This works as follows: The accuracy achieved by this method is significantly superior to the upsampling principle. In addition to that nearly no high frequency noise is generated, which is why there is no need to use high frequency filters, which would interfere and change the music signal in the high frequency range and have a bad influence on the sound. But that’s not all! The integrated 32 bit digital
filter of the Microcomputer operates absolutely
isochronal for both the right and the left channel. This
is possible because 2(!) data lines are used for the new
computed 32 bit signal and not just one data line as usual. The following conversion from the digital into an analog signal is taking place completely separated for the left and the right channel and as mentioned previously 100% isochronal. To fully utilize these advantages, two D/A converter chips are used, one for the right and one for the left channel.
The analog signal processing: The Tube-Hybrid concept Apart from the very elaborate technology used for the conversion of the digital into an analog signal, the analog signal processing is based on a Tube-Hybrid concept. In this concept the tube is used only where it makes sense physically and soundwise, which is the second stage of our analog filter stage, a so called “GIC filter”. The advantages of a tube in this filter circuitry: We use only tested and additionally hand selected tubes. The tubes are run more than 100 hours in a special lab test prior to delivery. After completion of this lab test the unit is tested again by our Audio Precision® test system. During development of this concept it was additionally paid attention
to the fact that the working points of the tubes are set automatically
and without readjustments. The following impedance conversion stage operates with the world-wide renowned Burr Brown® amplification chip OPA627®. Totally 10 pieces of this exclusive and extraordinary amplification chips are being used in the TUBE-DAC II. The TUBE-DAC II provides balanced XLR-outputs as well as unbalanced RCA outs. The balancing is accomplished by means of OPA627® chips.
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