The ACCUSTIC ARTS^® TUBE-DAC II 

A) 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.

B) 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:
First of all an extraordinary precise digital receiver is being used for reception of the incoming digital signal (for example from a CD transport), processing and routing of same to an elaborately designed 32 bit Microcomputer.

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 multiplying processors are scanning the incoming signal and finally calculate two digital 32 bit signals, one for the right and one for the left channel. All digital operations are therefore taking place with the accuracy of true 32 bit instead of the otherwise standard 16 to 24 bit.

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 digital input signal with a maximum word length of 24 bit which has been received and processed by the digital receiver is now first of all undergoing an 8-times oversampling.
Now follows the key step, as this signal is now scanned parallel thus exactly simultaneously by the means of the integrated 26 multiplying processors. >From these 26 single values the processors then compute an absolutely precise digital signal with the resolution of true 32 bit by the usage of the so called mean value method.

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.

(For comprehension: When using only one data line there is always a minimal time difference between right and left channel which can lead to an unnatural acoustic pattern.)

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.

C) 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:
- very high impedance
- very high bandwidth
- very low distortion factors and a “good-natured” distortion spectrum
- “analog” and very precise sound performance

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. E.g. even after an exchange of tubes it is “plug and play”. The typical disadvantages of tubes are eliminated with this concept.

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. 

TUBE-DAC II highlights

Audiophile reference D/A-converter with a so called "tube hybrid" concept

Special and unique 2 x 32 Bit technology with simultaneous digital signal processing, separately for left and right channel

Extremely elaborate digital signal processing supported by an unique 32 bit Microcomputer

The 32 bit Microcomputer contains e.g.:
- High precision crystal oscillator
- 32 bit digital filter (including "de-emphasis" technology)
- 26 parallel working multiplier processors

Advantages of this "tube hybrid" technology:
- very high impedance
- very high bandwidth
- very low distortion factors and a "good-natured" distortion spectrum
- "analog" and very precise sound performance

Easy exchange of tubes without any adjustments ("plug and play")

Professional Class A output stage using technology derived from studio engineering

All used components are of outstanding quality (e.g. 10 pcs Burr Brown^® OPA 627) and additionally selected

2 premium quality rotary switches with gold-plated contacts

2 magnetically shielded, encapsulated toroidal core transformers ("Made in Germany") of premium quality for high output reserves

Very high power supply capacity (approx. 84,000 µF totally)

Extremely stable and resonance optimized housing, primarily made of massive aluminium plates; turning knobs made of massive and chromed brass

ACCUSTIC ARTS^® TUBE-DAC II is "Handmade in Germany"