RD30 – Quad multibit d/a converter
We all know how good a PCM1704 can be. Currently priced at 75$ per mono chip, and it seems to be the only 24bit R-2-R “dinosaur” available. I wanted to give to this chip that well-deserved digital front end, with a reclocker that offers superb jitter rejection(asynchronous FIFO style), a powerful digital filter, I2S, S/PDIF, AES and USB inputs (all accepting HD audio), convenient user interface with alphanumeric display and remote. I wanted the digital filter to be switched off when needed – for pure NOS operation. I wanted true balanced operation and this requires four PCM1704 on board. Could not stop it – I wanted the current outputs summed 2 by 2 for single ended output if needed – with the benefits of double current swing and double sample rate through analog linear interpolation.
The current outputs from PCM1704 are directly available, so any compatible I/V stage can be used, tube or solid-state.
The result is the RD30 dac, which took about 2 years of tests and bare r&d in all these critical areas to be completed. It is a very compact design, due to the fact that all the features are built into the powerful Xilinx Spartan6 chip XC6SLX16. Measuring only 124 x 125 mm and including all ultra-low noise power management and all input conenctors on board, only AC power is needed to fire up the RD30.
Custom building blocks – FPGA :
- custom digital receiver
- asynchronous FIFO with digital PLL / DCXO, loop corner frequency 0.5 Hz
- Bytemorph 2.0 digital filter,60 bit internal, 384/768k , 1500 taps, -145 dB stopband att.*
- audio formatter and volume control , 256 steps
- user interface manager
*can be bypassed for native(NOS) mode
Digital inputs :
- AES/EBU : discrete lvds translator - 192/24
- S/PDIF : discrete lvds translator - 192/24
- USB : asynchronous, FX2LP interface – 192/24*
- I2S over HDMI (native LVDS or CMOS) – 192/24
*drivers available for Win,Mac, Linux
D/A conversion stage/supply/outputs :
- PCM 1704 x 4 multibit
- true balanced operation or single ended
- direct current output : 1.2mA x 4, 2.4mA x 2 respectively
- linear power supplies only, low noise regulators
- schottky rectifiers
User interface options :
- control buttons + led indicators
- control buttons + alphanumeric display, 4 x 20 characters
- remote control (rc5)
Typical setup with I/V conversion stage RD30IV & simplified diagram
RD30 Detailed connection diagram and board dimensions ; note that all four I2S outputs are available , for a possible external D/A stage.
Optional front panel board :
- type A : 4x20 alphanumeric display, IR sensor, control switches
- type B : leds for sample rate signaling, IR sensor, control switches
Remote control also works with T30 transport.
There are many product levels :
- digital front end only (no PCM1704 on board)
- digital front end + 4 x PCM1704
Each version can be ordered with or without front panel board, either led or alphanumeric version.
A custom toroidal transformers set is available as well.
Feel free to contact me about pricing & availability.
Folks who are expert at digital stuff have said about the made in China DACs that they are not so good due to layout and bypassing errors... I wonder about such things... have you got published data and graphs, like off an Audio Precision that shows technical performance by any chance?
our company is located in Europe - Romania and all products are designed and manufactured locally. There is no relation with China whatsoever.
The pcb is 4-layer with impedance control over certain signal traces. Routing BGA chips is not a trivial task, and we spent a lot of time optimizing the layout. I will post performance graphs soon.
Looking forward to seeing that...
Some technical questions on the RD-30:
- Are you generating the clock in the FPGA (soft PLL), or are you using crystal(s)?
- Is the Bytemorph DF oversampling to 768kHz or 384 kHz?
- Are you doing oversampling in integer multiples, or ASRC? I see only 384/768 kHz mentioned - for n x 44.1 kHz input I'd like to see 352.8 or 705.6 kHz.
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