Challenges in Digital ConversionT
he close-up above shows one of two tubes used in the buffer stage of the Audio Aero DAC (6Moons review
). Engineers have been incorporating vacuum tubes into DAC's since the DAC stage itself was separated from the transport/laser mechanism. The concept is to isolate the op-amp and other sensitive electronics from the motor noise and vibration of the transport. Originally Audiophiles discovered early CD-players were sonically inferior this prompted a company to modify a Phillips CD player. That company was the now defunct CAL (California Audio Labs) whom released their Tempest CD Player 22-years ago in 1986 (Stereophile review
). Today Audiophiles are still suffering from digital hangovers, which have brought new lifeblood into the industry. The Cayin CDT-17A CD player utilizes the highest quality Burr-Brown PCM 1792 DACs and vacuum tube buffer/output stages. The Cayin is what I would define as a work of Audiophile Art (seen below, PCB and circuit close-ups in TNs).
The vacuum tube has been used to ameliorate digital psycho-acoustic character defects
inherent in the conversion process, since Audiophiles first heard these phenomenon. Early Digital introduced new terms to the Audio Lexicon such as digital harshness
, digital fatigue
and listener fatigue
. Ironically, Audiophiles were the first to define these digital detriments for the same reasons they were plagued by them, high End hardware reveals all. While jitter is one cause among several, it’s the transfer process (D/A > A/D) itself at the crux of the issue. I am sure most of you are familiar with digital conversion, yet the following definition is one of the best I’ve read
because it explains the topic from a physics perspective:
In a digital recording system, sound is stored and manipulated as a stream of discrete numbers, each number representing the air pressure at a particular time. The numbers are generated by a microphone connected to a circuit called an ANALOG TO DIGITAL CONVERTER, or ADC. Each number is called a SAMPLE, and the number of samples taken per second is the SAMPLE RATE. Ultimately, the numbers will be converted back into sound by a DIGITAL TO ANALOG CONVERTER or DAC, connected to a loudspeaker.
Early on, a great deal that was recorded and copied to CD was un-listenable on Audiophile systems, to this I can attest. One reason I quickly denounced solid-state systems was due to choosing a digital (CD player) source over a turntable. For the same reason Audiophiles loved their Krell gear they now loathed them when it came to CD's. Even the most "tube sounding" solid-state gear could sound forward or bright, in no time at all. This originally led to a cottage industry where any decent CD payer was modified, usually replacing the power supply/transformer and/or incorporating tube buffer/output stages. An entire industry has now grown with a singular vision, to improve the quality of playback from the Compact Disc. One manufacturer of DACs known as Lossless Audio
simplifies the subject of Jitter in their posting; The main reason Digital sounds bad
. The subject of peak meters excerpt below is taken from a Trillium Lane Labs Paper
, Digital Distortion in CD’s and DVD’s, The Consequences of Traditional Digital Peak Meters
It is nearly certain that this constant barrage of distortion that we, the consumers, are hearing on compressed and mastered CDs contributes to the ‘digital harshness’ still reported by the more sensitive audiophiles in the music industry. According to industry insiders, not a single off-the-shelf digital to analog converter chip made today can accurately pass a signal wherein the samples are under full scale but the waveform that they represent exceeds full scale. Only a few high-end converters in the professional market can do this. This means that the preponderance of consumer (and professional audio) playback equipment is not designed to deal with these ‘hotter than full scale’ signal levels.
When we think of what we are up against, a picture begins to form as to why $600 vacuum-tube integrated amplifiers and USB tube-DACs are becoming all the rage. It's not just because of the movement towards integrating better sound with your PC, the tube reigns supreme as the circuit of choice when it comes to the op-amp and buffering of the signal. Of course that's not all there is to it, and in future articles this is what I hope to share.
Onto Listening / Testing Methods....