Tuesday, October 30, 2007

Dual balanced NOS DAC AD1865 board

I've finally constructed my DAC board (with "I2S" line input and without I/V stage since I want to experiment). Power supply regulation is made of precision voltage regulator + fast opamp (AD826). Top side of the board is ground plane.



SPDIF is "crap". Really! It is simple but to have low-jitter at the DAC it requires quite complex circuit (PLL w/ FIFO). Ideal: slave transport w/ very good clock, get I2S directly and reclock at DAC requires tweaking CD player. No go... but USB is quite a good solution. Digital from computer (where I have lots of music; also it can provide more accurate bit stream since it does not require real-time reading from CD), PCM2707 (not very low jitter), using external Tentlabs 12Mhz low jitter clock. Nice.
The USB board is DDDAC1753 mkII. See picutre(s).

USB board output I2S but I need AD1865 compatible format (18-bit, MSB, standard). I do not like using a digital filter like DF1704, since I will have to oversample to 96Mhz. So I built (PCB in construction) a circuit using shift registers and flip-flops. I learned Eagle (and Eagle 3d).

Note: I also take a loot at gates jitter (and also CPLD, FPGA) and found out that HC is good enough (having not bad PS).


DAC I/V stage

It seems that transformer I/V is not so simple. Well it is, but I was not sure it will work and to test it I would need to but transformers that are very expensive. Too expensive to play with. OK, so I started to study solid state I/Vs. Op-amps are no go. I/V should have ideally 0 ohm impedance to DAC and be very low noise. After a few days of studying transistor topologies, current mirrors/sources, I came up with idea (not new): common base + current mirror + resistor (+ buffer). AD844 (used as common base; trans. current. amplifier) is very good and easy start, making a discrete one (see Pedja Rogic circuit). I did simulations and I will try those... :)