FETAudio FDA-2A Manual - A. Introduction:
FDA-2A Manual.doc
Rev. 1.0
Page 2
31-Aug-2012
A. Introduction:
FDA-2A AD1955+WM8805 DAC use the same “super sound discrete IV converter” of FDA-1B.
The main difference is the change of the main DAC chip from Texas Instruments PCM1794A
to Analogy Devices AD1955 in Single Ended output mode. The second minor change is the
digital receiver chip from WM8804 to WM8805. Due to the use of AD1955 DAC chip in FDA-
2A, it sounds smoother, more laid back, and less dynamics relative to FDA-1B PCM1794A
DAC. In fact many people like this type of sound and regard it as more musical. This
reminds me the sound of PCM63 in D1V33 DAC. Overall, FDA-2A DAC sounds more like the
mixture of D1V33 and FDA-1B with the 24bit high resolution.
So who is the winner? To me it is a matter of taste for the choice of DAC chip to match with
the rest of Hi-Fi system. Personally I think FDA-1B PCM1794A DAC is a better match to valve
amplifiers and FDA-2A AD1955 DAC suit more on solid states amplifiers.
In FDA-2A, the AD1955 DAC chip will work in its reset default mode, i.e. PCM mode. Thus
FDA-2A natively is able to work up to 24bit/96k with “NOS Daughter Board” due to the
limitation of AD1955 default PCM mode. To let it play 24bit/192k input signal, an “ASRC
Daughter Board” is required (same board used in the FDA-1B) to down sample 192k digital
signal back to 96k for the AD1955 chip to decode.
On the WM8805 receiver side, it is preset to hardware mode but input logics allow it to
have 4 digital inputs (2 SPDIF, 1 AES and 1 Toslink) and one I2S input with a new front panel
control board. The FDA-1B front control board is also usable but it only can control 3x
digital input plus 1x I2S or 4x digital input without I2S input. Moreover one of the two RCA
socket (top one) on digital input section can be set to SPDIF out of WM8805 by on board
jumpers. It functions as a buffer and jitter filter/cleaner of the selected digital input signal
(not for I2S input).
Since this is a DIY DAC, it will work as a standalone DAC. But there are “control bus” sockets
delicate to control both the AD1955 and WM8805 chips on board in software mode. This is
made for DIYer to improve the DAC further for functionality and user interface. If the DIYer
knows SPI & MCU software program, the DAC can be transformed into full software mode
to extend the full functions of the DAC without sacrificing performance of sound. Some
possible additional functions are 24bit/192k native support, DSD direct input support, , 4
more digital inputs (total 8), testing of mono mode, external digital filter mode via new
module at U18 daughter board, adding display to show sample rate and working status of
the DAC, slaving the internal DAC system clock to external low jitter Super Clock reference,
adding a USB2.0 to I2S module via I2S input at J5 etc…
Thus this DAC design is really flexible for user to experiment with different modes of the
AD1955 and WM8805 chips! The price is just a faction of the demo board of Wolfson or
Analogy Devices and it has two chips on one single board. With the super sound discrete
IV converter, it is able to explore the best sound from these chips available for PCM and
DSD format.
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