My original Predactor preamp used EF810 tubes to drive the output of a separate Sabre 9018 dac.
Version2 came about as a result of prototyping the little JLSounds I2SoverUSB/AK4396 dac with a tube output stage.
It sounded so good that when a client ordered a Cabernet Reference and was looking for a dac I suggested we integrate a tube output stage into the Reference to work with the dac.
The little JL Sounds usb dac is an interesting unit. From the website:
I2SoverUSB is especially designed for asynchronous audio transfer. It can transmits PCM and DSD (DoP) audio data. This board provides bit-perfect playback at sampling rates from 44.1 kHz up to 384 kHz with up to 32 bit resolution.
I2SoverUSB can work with most DIY DAC boards with I2S, LJ, RJ and S/PDIF inputs. Also I2SoverUSBsupports directly PCM1704, PCM63, PCM61, PCM58, PCM56, AD1862, AD1865, AD1861, AD1860, AD1851 and other compatible with listed DACs. Thus these DACs can work in NOS mode or PC oversampling.
I2SoverUSB uses reclock to reduce jitter significantly. I2S, S/PDIF outputs, oscillators and reclock are galvanically isolated from XMOS processor and USB ground. The galvanic isolation eliminates common noise originated by the computer. There is an galvanically isolated external master clock input. The board is equipped with NDK NZ2520SD low jitter oscillators 45.158MHz and 49.152MHz. LP5900 ultra-low noise linear voltage regulators are used.
The dac chip used is the AK4396 , which is sometimes called the "miracle" dac , supposedly for its sound qualities. It does sound somewhat similar to a R2R chip like the 1541 or PCM1704. It has a nice natural tone and is quite musical.
Unfortunately the stock unit uses op-amp outputs and in my opinion degrades the sound, making it sound closed in and a bit grey sounding.
I'm just not a fan of op-amps, they use considerable amounts of negative feedback, and usually exhibit these sound characters when used after dac chips, although there are exceptions.
Trans-impedance zero feedback op-amps or tubes are much better sounding and in this case the AK4396 sounds great with tubes.
Removing the opamp connections and re-wiring a direct connection to the dac chip is a tricky operation, definitely a magnifying glass required!
The AK4396 chip has balanced outputs so in the Cabernet Reference I built a differential tube circuit using both triode sections of the Russian 6N6 or 6H30 tube. The circuit converts the balanced input to single ended and passes the signal on to the line stage of the Cabernet Reference. It is transformer output coupled for drive and required gain.
I built the dac into a simple black box with power supplies ( 2 separate supplies, one for I2S-USB and one for dac.
Its connected to the preamp with XLR cables.
There are many dac chips available and many opinions about which one is "best".
However, what is much more important is the total result of input, dac chip, and I/V conversion/output. If everything is right a synergy will result that will give a far superior sound than a less well implemented solution.
The AK4396 with the 6N6 diff tube sounds very balanced, it has that great big 3D presentation that I like so much and its very easy to listen to.
With JRiver used as source this makes a complete computer usb system, relatively simple but top notch sound.
This application demonstrates how versatile the Cabernet preamplifiers are. The 6N6/6H30 tube can be configured as a dac output stage, a bass driver for subs or active speakers, or even as a completely independent line stage using IDHT tubes to compliment the DHT line stage.
