NOS Burr-Brown PCM56 DAC with Tubes!!!

[L0rdGwyn]

You've done a really nice job on that DAC and it actually measures quite respectably for what it is. Well done.
WRT not identifying with audiophiles, 100% agree.
WRT PCM63, those DAC's can sound *really* good. I think I actually have at least 2 x original K grade floating around used in old projects here.
WRT best I-V for PCM63, my recommendation is run DAC into a grounded base stage BJT with collector cascoded into cathode of a tube. Run *lots* of current
and it can sound staggeringly good.
Lastly, if you are using Jlsounds III for PCM63, this may be a slight compromise because LE is reclocked inside CPLD. Might be worth trying to re clock LE OP
directly from master clock but I have not done this or looked into it, don't use PCM63 anymore.

Terry

Thanks, I appreciate it. I'll give your I/V stage some thought. What chip are you using these days?

 

[TCD333]

DAC's -> a/ Moon 430HA for it's flexibility of multiple IP's (sounds OK) b/ Whatever is coming through for repair or upgrades c/ Current project PCM1794 for a 'musical that measures' project d/ Sabaj A20D 2022 will be here shortly for a 'blameless measuring' constant or yardstick.
The PCM63 can sound really good, there are many little tricks to help it. I just wanted to move on to DS or native DSD type converter.
TCD

 

[L0rdGwyn]

I finished designing and assembling the boards for my NOS PCM63 DAC. Running two PCM63 in parallel per channel, output of each is the parallel Iout pin of the two chips. Planning to again use Sowter SUT I/V with some sort of tube output stage, likely a buffer with the SUT wired 1:10. Put the Talema transformers on the boards this time around to save space.

 

[Doodski]

I finished designing and assembling the boards for my NOS PCM63 DAC. Running two PCM63 in parallel per channel, output of each is the parallel Iout pin of the two chips. Planning to again use Sowter SUT I/V with some sort of tube output stage, likely a buffer with the SUT wired 1:10. Put the Talema transformers on the boards this time around to save space.

View attachment 246273

Wonderful. I like! Very pretty layout. (If a layout can be pretty...lol. geeking out on your stuff.)

 

[Alexium]

That is indeed a very pretty layout, as well as a very pretty mask.
May I ask what software you're using for designing the PCB layout?

 

[_thelaughingman]

That is some PCB pron if i've ever seen one.

 

[L0rdGwyn]

That is indeed a very pretty layout, as well as a very pretty mask.
May I ask what software you're using for designing the PCB layout?

Thanks! I use EasyEDA, free software of the board house JLCPCB.

 

[L0rdGwyn]

Did some testing of my parallel PCM63 DAC.

I configured the JLSounds I2SoverUSB module for PCM63, made connectors from the module to its power supply PCB and the left and right channel DAC boards.

Then I connected everything up in a janky way for the sake of testing. Since these PCM63 chips are pretty old, I put them in one at a time and tested along the way to ensure I would know if one was bad. Thankfully, they are all good to go.

Here is my sloppy test setup on a box. My desk is a disaster from so many simultaneous projects, but it is what it is.

I dialed in all of the 5V supplies, everything is working as intended.

For the sake of initial testing, rather than rigging up the Sowter I/V transformers, I just soldered in 68ohm I/V resistor dummy load. This would represent the primary impedance of the transformer in theory.

Here is a 1kHz FFT at -6dBFS output from the DAC at 96kHz sample rate.

Pretty happy with that result.

So with all of that tested, I am going to move forward with ordering parts for my tube buffer stage. I have a healthy stock of NOS E182CC, planning to use them in a circuit like the below.

The output is a transformer coupled cathode follower. Both sections wired in parallel and biased by the DCR of the transformer. Each triode has a 15mA/V transconductance, so it should perform well. I am planning to use Lundahl's LL2753 interstage transformer wired 1:1. It has a primary and secondary DCR of 150ohms, so I am looking at around a 350ohm output impedance from the DAC with this setup.

 

[L0rdGwyn]

Well I finished my PCM63 DAC! It sounds excellent, to me, but it isn't winning any measurement awards given the output stage I used, so I think I will refrain from making a dedicated thread for it. Even if I sing its praises and say it sounds better than the better measuring DAC of this thread, I'm not sure anyone will believe me

Here are the highlights of the design:
- 1:1 isolation transformer with the secondary wired for balanced AC, as such the DAC circuit is running off +/- 60VAC balanced mains
- Dual mono boards, one DAC PCB per channel with two PCM63 run in parallel
- Separate cascaded series regulated power supplies for all four DAC power supplies (+analog, -analog, +digital, -digital)
- JLSounds I2SoverUSB digital front end, also powered by cascaded series regulated power supply
- Sowter 1465 I/V 1:10 step up transformers for converting the current output of the parallel PCM63 into small signal voltage
- HEXFRED solid state rectified B+ supply with transformer snubbing circuit and current-limiting resistance
- Maida style regulated B+ supply with separate regulators for left and right channels
- E182CC / 7044 / 5687 output stage, battery biased and loaded with Sowter 9705 4:1 line output transformers
- Ultra low noise regulated DC heater supply for the E182CC / 7044 / 5687 output tubes

Here is a look at what I did for the tube output stage. Between the higher voltage swing needed due to the step down transformers, and the transformers themselves, THD follows a typical single-ended tube 2nd harmonic dominant pattern with THD around 0.12% at -6dBFS. Low frequency bandwidth is down 2dB at 20Hz. I haven't measured it directly yet, but theoretical output impedance is roughly 350ohms. I don't think anyone here will be impressed with those numbers lol but for a tube guy, those are perfectly acceptable specs.

The interior circuit.

And the DAC itself fully assembled.

 

[Alexium]

Awesome, thank you for sharing another of your creations!
I think the use of a transformer for I/U conversion is the weakness of this design, it will affect the linearity of the DAC. But if you like the result then it's good enough, and I don't know if the degradation can be large enough to be audible anyway.

 

[L0rdGwyn]

Awesome, thank you for sharing another of your creations!
I think the use of a transformer for I/U conversion is the weakness of this design, it will affect the linearity of the DAC. But if you like the result then it's good enough, and I don't know if the degradation can be large enough to be audible anyway.

Ahh who knows, but the result is excellent (subjectively speaking), I am extremely happy with it.

Listening in my system, it is a transformative piece of gear. It amplifies that "at the venue" listening experience for live recordings where you feel you are listening into the recording space. The realism, clarity and spatial cues are so precise, my cats were frightened, they thought someone was in the house!!

 

[DSJR]

Late diving in here, but wonderful build standards. I wish I'd been half as good when I tried for a few years to do it professionally (less said about that these days the better ). Cases look fab too (I don't have a pillar drill set up to accurately drill case holes, so I'm stuffed really as regards amp and other kits )

Forgive me saying and forgive the naivety, but I do worry about the entire nos 'thing' going on in many circles. Best not dig my hole any bigger though, but I suspect it's what's going on *over* 22kHz that's the worry even if we can't hear that high..

Anyway, I'm green with envy on the builds and nice circuit boards etc...

 

[L0rdGwyn]

Late diving in here, but wonderful build standards. I wish I'd been half as good when I tried for a few years to do it professionally (less said about that these days the better ). Cases look fab too (I don't have a pillar drill set up to accurately drill case holes, so I'm stuffed really as regards amp and other kits )

Forgive me saying and forgive the naivety, but I do worry about the entire nos 'thing' going on in many circles. Best not dig my hole any bigger though, but I suspect it's what's going on *over* 22kHz that's the worry even if we can't hear that high..

Anyway, I'm green with envy on the builds and nice circuit boards etc...

Thank you! Yes, what can I say, I am buying into the voodoo. And that is based on my personal experience hearing these types of designs. Five years ago, before I was heavily into DIY audio, I would have balked at owning a NOS DAC, I firmly believed the measured performance of a DAC was all that mattered. Then I demoed a boutique NOS DAC in my home, and I questioned my outlook. I'm glad I did, I am getting much better sound now, measurements be damned. For what it's worth, the DAC does have two stages of passive filtering from the I/V and line output transformers. There is no way for me to convince anyone over a forum, but I don't think if anyone heard this DAC site unseen they would expect it to measure how it does or use a NOS topology.

Here's a recent experience I had - I went to a Head-Fi meetup at a hifi shop in Pittsburgh, Pennsylvania, USA. With me I brought a system that consisted of a 6528 tube OTL headphone amp, the NOS PCM56 tube output DAC of this thread, and the ZMF Atrium headphone. I had multiple people tell me it was the best sounding system at the meet, and that included the system featured at the shop itself, which consisted of the dCS Lina Stack (a $30,000 USD ultra high end headphone DAC/amp system) and the Focal Utopia 2022 headphone. This system of course measured better than mine, and certainly would be considered more "accurate". But I don't design for accuracy, I design for my personal enjoyment and how I like to hear the music, others seem to enjoy my taste.

That anecdote might mean nothing to people, but I have gotten enough positive feedback from multiple sources on my designs that I feel confident I am producing enjoyable sound, even if it isn't strictly by the book. That is all I can say to defend my monstrosities lol.

 

[DSJR]

Just quickly, my 'epiphany' for HiFi came in a late 80's visit to a little country pub in Nth Wales with an uncle and 'cousin in law' and as well as beer straight from the barrel there was a jazz trio playing and the dynamics and basic 'rawness' of a sax at ten paces, as well as the dynamics of a drum kit heard live. I ended up with speakers the same as my current avatar which did a great facsimile of that in a domestic room and which I still grieve over having to sell them for size and weight reasons when I married twenty seven years ago now

Each to our own and glad you feel you can share your excellent DIY builds with us. It 'all' sounds wonderful to me now, so there ya go

 

[L0rdGwyn]

Here's one more pic of the DAC next to another tube headphone amp I designed that uses a choke-loaded parallel section 6SL7 input stage capacitor coupled to a transformer-coupled cathode follower EL34 output stage (I don't think I shared that build on ASR) and the ZMF Caldera headphone.