Selasa, 13 Desember 2011

Battle of the old NOS legend? Part 1

I don't want to miss writing in December :), there are some design things in the pipeline, but first my current DAC adventures. I have managed myself  a piece of equally legendary AD1865-N-K DAC. It is one of 'a must have' DAC chip, I'm always a fan of multi-bit DAC, next in my list is the expensive PCM1704, may Allah The Great permits.

Me not buying ready made PCB for the AD1865 :), not challenging and not fun enough :),  so the usual PCB layout-ing took some time, and to my surprise, this time designing is a breeze compared to TDA1541 board!. The AD pins are very layout friendly. I use both NOS schematic with 74HC04 hex inverter and the other with 74HC02 NOR, why?. Well, blessing in disguise really.

First I made CS8414 - 74HC04 - AD1865 pcb. After powering up the damn thing just gives noise, all effort to resolve failed, check and rechecking everything, tried other connections etc...very frustrating.

So then come the other schematic, now instead of 74HC04 I use 74HC02. I always think less is better. Using the 04 you use only 1 logic, while with 02 you use all 4 logic's.

The 04 however has 1 data line for both left and right channel, i.m.o. not so elegant, both AD1865 inputs are continuously bombarded with digital data, while in the OFF state, I can be wrong of course :). One way to try is to slit apart data trace with a cutter, and listen to your DAC, it still managed to get through!.

The 02 in contrary have 2 separate data lines for each channel. Mind you though, both NOS digital glue with 04 and 02 introduce data phase shifts between channels and clock (SCK from CS8414), with 02 comes worst. I know this from DIYaudio and by simulating the digital input-output data stream. But the shifts are small, and people says this is similar with moving your head 8mm during listening :), or is it?

This can be fun to check, because TDA1541A NOS with CS8414 data stream is exactly in phase! Ok, here are the plan, I use mediocre power supply (ex. Pioneer LD player), measure all 3 DAC boards : the original hardwired TDA1541A DAC, AD1865 with 74HC04 and AD1865 with 74HC02, all analog stage are my favorite single ended -DC coupled 2SK170 jfet, All board will use the same CS8414 about that?..

How about the HC02 board?, still no luck bringing it alive!, must be the DAC...even a Vout mod from Iout did not help..

Frankly, I have listened to all of them :), I got my AD1865 replacement (many thanks Doc!....) , the AD1865-HC02 is still using Voltage output. This is preliminary listening though, I'll let them for a couple of weeks playing...

Here are the pics, AD1865 PCB's are CNC'ed, I tried as far as possible using same brand and same values component.

From bottom left : the 'Red' shoebox power supply, TDA1541A DAC, AD1865 with 74HC04 and SEJfet out, AD1865 with 74HC02 and Vout, note the coupling capacitors.

Now, back to listening he he he...

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