Vintage DAC White Elephant repair(Assemblage 3.1)

I wanted to start a thread on a project I decided to take on. A little while ago @Mr.Sneis acquired an Assemblage 3.1 DAC that was DOA. Details of what he went through are in the following thread:

https://www.superbestaudiofriends.org/index.php?threads/dreamwhisper.4155/

I inherited this DAC minus the opamps a bit overconfident in my DIY skills. But honestly my reason to be in the hobby is to build and repair stuff, not so much to listen to music (although I enjoy that too). At the absolute worst I can repurpose the enclosure and power supply and then harvest the 4 x pcm1704 multibit chips to sell to scavengers at diyaudio. But that just feels wrong, so I'm going with rehabilitation.

 

First thing I need to do is to replace the missing op-amps. Even though I acquired manuals for the Assemblage 3.0 and 3.1, some of the important details are kinda missing. So I did a bit of research and found the opamps I needed:

• 4 x LT1357 (I/V Conversion)
• 4 x OPA134PA (VAS)
• 4 x BUF634 (output buffer)

I need 3 opamps for each phase so 12 total*. At Arrow this cost $55 with free overnight shipping which isn't so bad, so I ordered it with a couple regulators I needed for another project. Now let me show you the box they used for 12 op-amps + 2 linear regulators.



Here is the packaging for one regulator, which cost 36 cents:



And this is just absurd:



Going to update my progress from time to time

*There seems to be a premium upgrade using OPA627 op-amps everywhere. At $33 each this is ends up being a $400 upgrade!

 

I should probably send these buf634's your way, I have them stashed away somewhere sorry.

I bought another working 3.1 recently. It came with opa627 and I tried few others (namely lt1028, ad797, lme49710ha) but I still haven't found anything I like a whole lot yet. Doesn't help the damn dac uses 4x opamps for iv and 4x lpf.

 

One of the nice things about these TOTL DACs is that they tend to be modular



Left and right boards are the analog outputs which includes the DAC chips while the digital board in the center is for digital processing and filtering. In exchanges with @Mr.Sneis it seems like the problem is the pmd200, which would actually be a BIG problem since this chip is basically unobtanium. I somewhat assumed that these older multibit chips were like modern chips and took an I2S signal directly and the pmd200 was just some optional money component. This is not the case:



The I2S stream needs to be converted to separate left/right digital streams before the PCM1704. So I'm left with these options:

1. Salvage the PCB and pmd200 the best I can (ideal scenario)
2. Find a df1704 decoder board or make my own PCB. df1704 is actually still availible
3. Convert to NOS mode by using a project I found at diyaudio

Here is the state of the pmd200 linked from the other thread:




It's a misaligned flux covered mess. Good news is it seems the pmd200 is probably in good shape and I won't need to source another one. Bad news is that if the PCB is toast, then it doesn't matter.

Already got the BUF634's but I'll take all your OPA627s

 

Looks like a fun project, hope you get it working!

 

Here is another option for a digital board

http://tech.juaneda.com/en/projects/digitaldecoder.html

Unlike the board by iancanada, this is a complete hardware implementation instead of the software domain. At 95€ for the PCB alone, it's a bit steep. It's also a bit of a hassle since I need to use a jumper to set the resolution (16, 18, 20, 24). Still something to consider.

 

Any news?

You made a good call on the opamps BTW.

I rolled all sorts of combinations and broke down and bought the stock opamps from Arrow, same nutjob packaging. Hey free next day delivery, how can you say no?

The stock opamps do not offend and I mean that in the best way possible. I think the other opamps I've tried oscillate and it shows as peaky or brittle treble or over-done bass. They likely need bypassing or other circuit changes to work properly.

 

Thanks for the confirmation

Haven't advanced much since my workstation has a stack of boxes from family members that have decided to commandeer the space for storage. Also just bought a bunch of items at an audio garage sale and got super excited and was pre-planning a 2a3 amp.

I read that the output buffer is likely not necessary as it's just to lower the output impedance. There's a mod where they wire the sockets so that the opa134 is essentially the output. I'll try both ways if I get it up and running.