Posted on Tuesday, 19th April 2011 by Auw Jimmy
There are not many options to go with NOS DAC. One is TDA 1541, and the other is AD1865. Some says, the TDA is better (at least the S1), meanwhile some other say the AD1865 is better (perhaps because it’s used by Audio Note?).
I’ve listened both (TDA 1541 S1 vs AD1865) and I decide to go with AD1865. It’s a matter of choice actually, there is no absolute winner.
The problem with AD1865 is the data input format, which is not I2S common but 16 or 18 bit LSBJ. So basically we need to find way to convert I2S to AD1865 LSBJ format (I will use the I2S output of QLS QA-550).
I prefer to use I2S instead of SPDIF out of QLS QA-550. If we use SPDIF, then we will need to upgrade the SPDIF transformer again (another cost). I think the I2S is better solution at this moment (but we still have problem converting I2S to AD1865 LSBJ).
After digging around, I have concluded two possible approach. First, we can use SRC chip to convert the signal (theoretically, this is possible). The SRC chip also not expensive and not difficult to design.
Second approach is by shifting the data register with the help of 74HCT shift register. I find a design from pavouk and (theoretically) his approaches is a big YES!
Then I layout the simple dual layer PCB, with the help on digital ground section from my good friend, ATP “Creno”. So this is the PCB.
Basically it’s a dual layer 1 oz PCB. I’ve ever thought to make it thicker 4 oz silver plated, but this probably later.
Each supply rail will have its own L-C-C filter. The L should be hand made toroidal one, then filtered with Os-Con or Black Gate, then finally bypassed with ceramic capacitor. The shift registers will share the L-C filter, but also will have their own bypass capacitor.
The AD1865 output shall be the current out, converted to voltage by one I/V resistor each channel (I have Shinkoh tantalum on hand). The output shall be connected to Lundahl or Sowter step-up transformer (counting budget…) to make sure the current is stable on any load… or directly feed to high-mu SRPP stage like 12AX7.
The converted voltage output probably only 0.3-0.4V. Definitely a step up transformer or high mu tube stage (or combination between step up transformer + low/medium gain tube stage) shall play an very important role here.
I’m still in the middle of the road considering Amorphous or Mu-Metal one. Damned, why this world can’t be simplify with lesser choice?
The result? Not quite sure…