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NOS Burr-Brown PCM56 DAC with Tubes!!!

L0rdGwyn

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Hey ASR,

Showing another DIY project of mine, I said I wasn't going to post anymore but I am proud of my monstrosities :D I designed a NOS DAC based on the vintage Burr-Brown PCM56. It's a 16-bit chip. This is my foray into digital design, so I thought simple is best. I know the NOS R2R thing isn't the most popular here, but I think there is something to the sound that goes beyond the measurements. I used to be a "by-the-measurements" guy when it comes to DACs, but I heard a NOS R2R design that really blew my mind and bought into it, so thought I'd build one.

It's a dual mono design with separate 4-layer PCBs, dedicated ground layers, separate power supply rails for the +/- analog and digital supplies of each chip. Power supplies are cascaded series regulators. To optimize for HF mains noise, using a 1:1 isolation transformer in balanced configuration for +/- 60VAC between mains and the DAC supply transformers. Digital front end is a XMOS-based USB module from a company called JLSounds configured for PCM56.

The output signal is taken from the Iout pin of the PCM56. I/V conversion is performed by a pair of Sowter SUTs wired in 1:5. There is a tube output stage to reach 2Vrms line level using the Russian 6E6P-DRY. This is a high gm tetrode strapped as a triode. It is extremely linear as a triode with a relatively low plate resistance of 1.2K. I am loading it with a IXTP08N100D2 / DN2540 cascode CCS. The tubes are battery biased with a 1.2VDC battery between the SUT secondary and ground. I am using a high-performing HV regulator for the B+ supply. Also using a DC heater regulator for the 6E6P-DRY to keep coupled AC noise as low as possible.

Here is a look at the I/V and output stage. Have some anti-parallel zeners on the output to limit DC transients on startup / shutdown as the output caps charge / discharge. As you can see, the output tubes / CCS are setup as a hybrid mu follower with the output taken from the source of the DN2540. This gets the midband output impedance of the stage down to around 135ohm.

6E6P-DR IV Stage.png


Here is a look at the 6E6P-DRY triode curves from my curve tracer. Doesn't get much more linear than this for a triode.

6E6P-DR_Triode (2).png


So that's the design at a high level. Here are some pictures of the final build. Need to shorten and tidy the digital input wiring.

Phbj93a.jpg


jXY7ODV.jpg


e7S6V2N.jpg


xC0E9JG.jpg


hFTyaRp.jpg


g6JCdnL.jpg


Here is a FFT of the left channel, 1kHz -6dB undithered sine wave. Did well to minimize LF mains noise, even a bit better in the right channel. Worth noting that the PCM56 is rated for THD at -94dB, the -87dB H2 is due to the output tube.

UMwsPMY.png


Here is the frequency response. The blip at 3.5kHz is an artifact of my soundcard measurement setup using the MOTU M4, along with the "waviness" of the FR from 10kHz-20kHz, seen in all of my measurements.

iia95K3.jpg


Would you believe me if I told you this DAC sounds incredible? Maybe not, but to me it does! Really happy with the sound :D
 
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L0rdGwyn

L0rdGwyn

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Here is a look at the tubes in operation. When I see the tube glow, I can hear the "tube sound" better ;)

The upper CCS FETs are dissipating around 750mW, I have some little heat sinks on there now, not pictured here. I have ferrites on all of the 6E6P-DRY electrodes to discourage parasitic oscillation.

PXL_20211010_024945225 (1).jpg
 

Jimbob54

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Nice work. Of course, being fully indoctrinated into ASR my instinctive response is "yeah, but it will sound no different to a Topping D10 paired with an Atom"- which may be true , but rather missing the point of what you have done.
 

groovybassist

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L0rdGwyn

L0rdGwyn

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Nice work. Of course, being fully indoctrinated into ASR my instinctive response is "yeah, but it will sound no different to a Topping D10 paired with an Atom"- which may be true , but rather missing the point of what you have done.

Thanks! I used to be indoctrinated. I have a Topping D30 here, right next to me, all else being equal the difference in sound quality between the two is knock-you-over-the-head obvious in favor of the PCM56 DAC. It is of course much more expensive to produce, BUT also measures more poorly. Probably no one will believe me, and that's okay! I have made peace with that :D
 

DHT 845

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Pcm56 and tubes its almost holy grail (i.e. philips 1541 and tubes) :)
Nice job!
 

Jim Taylor

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Fantastic iteration of a tube circuit! Great work! Thanks for showing us! Jim
 

Alexium

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Great build, very clean! What's your source for getting the PSU transformers? I could use a couple of those.

I have tried to hear a difference between DACs for a long time, never succeeding. I'd love to listen to your DAC, maybe this one would convince me DACs can sound differently from each other :)
 

MakeMineVinyl

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Hey ASR,

Showing another DIY project of mine, I said I wasn't going to post anymore but I am proud of my monstrosities :D I designed a NOS DAC based on the vintage Burr-Brown PCM56. It's a 16-bit chip. This is my foray into digital design, so I thought simple is best. I know the NOS R2R thing isn't the most popular here, but I think there is something to the sound that goes beyond the measurements. I used to be a "by-the-measurements" guy when it comes to DACs, but I heard a NOS R2R design that really blew my mind and bought into it, so thought I'd build one.

It's a dual mono design with separate 4-layer PCBs, dedicated ground layers, separate power supply rails for the +/- analog and digital supplies of each chip. Power supplies are cascaded series regulators. To optimize for HF mains noise, using a 1:1 isolation transformer in balanced configuration for +/- 60VAC between mains and the DAC supply transformers. Digital front end is a XMOS-based USB module from a company called JLSounds configured for PCM56.

The output signal is taken from the Iout pin of the PCM56. I/V conversion is performed by a pair of Sowter SUTs wired in 1:5. There is a tube output stage to reach 2Vrms line level using the Russian 6E6P-DRY. This is a high gm tetrode strapped as a triode. It is extremely linear as a triode with a relatively low plate resistance of 1.2K. I am loading it with a IXTP08N100D2 / DN2540 cascode CCS. The tubes are battery biased with a 1.2VDC battery between the SUT secondary and ground. I am using a high-performing HV regulator for the B+ supply. Also using a DC heater regulator for the 6E6P-DRY to keep coupled AC noise as low as possible.

Here is a look at the I/V and output stage. Have some anti-parallel zeners on the output to limit DC transients on startup / shutdown as the output caps charge / discharge. As you can see, the output tubes / CCS are setup as a hybrid mu follower with the output taken from the source of the DN2540. This gets the midband output impedance of the stage down to around 135ohm.

View attachment 158744

Here is a look at the 6E6P-DRY triode curves from my curve tracer. Doesn't get much more linear than this for a triode.

View attachment 158745

So that's the design at a high level. Here are some pictures of the final build. Need to shorten and tidy the digital input wiring.

View attachment 158753

View attachment 158752

View attachment 158751

View attachment 158750

View attachment 158749

View attachment 158748

Here is a FFT of the left channel, 1kHz -6dB undithered sine wave. Did well to minimize LF mains noise, even a bit better in the right channel. Worth noting that the PCM56 is rated for THD at -94dB, the -87dB H2 is due to the output tube.

View attachment 158747

Here is the frequency response. The blip at 3.5kHz is an artifact of my soundcard measurement setup using the MOTU M4, along with the "waviness" of the FR from 10kHz-20kHz, seen in all of my measurements.

View attachment 158746

Would you believe me if I told you this DAC sounds incredible? Maybe not, but to me it does! Really happy with the sound :D
Wow, that's a really clean build. :cool:
 
OP
L0rdGwyn

L0rdGwyn

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Pcm56 and tubes its almost holy grail (i.e. philips 1541 and tubes) :)
Nice job!

Thanks! You know I had originally set out to build something similar with the TDA1541A, but that chip is a little more finnicky and expensive, so I thought PCM56 is a better place to start experimenting.

But I was very curious to hear a good implementation of the TDA1541A - I ended up purchasing a TDA1541A-based DAC from a small Serbian company called Audial, the DAC is the S5, designed by Pedja Rogic. Also NOS R2R, however no tubes (transimpedance amplifier I/V stage) and transformer coupled outputs.

It is a kick ass DAC, and I am happy to say my PCM56 design compares very favorably to it! In fact, the noise floor of my own is a bit lower. Two different flavors, one goes in my two-channel system, the other for my headphone system :)

PXL_20211010_000750026-2 (1).jpg
 
OP
L0rdGwyn

L0rdGwyn

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Great build, very clean! What's your source for getting the PSU transformers? I could use a couple of those.

I have tried to hear a difference between DACs for a long time, never succeeding. I'd love to listen to your DAC, maybe this one would convince me DACs can sound differently from each other :)

Thank you! And thanks to everyone else too, I am happy my design is well-received.

The isolation transformer is a medical grade model from Triad Magnetics, VPM series. The little 12V and 15V toroidals are made by Talema, 7XXXX series. Digikey has some models, but they did not have the ones I needed, so I had to order from the UK through RS Components. The tube B+ / heater toroidal is made by Antek, they have a great selection, many of which have dual HV secondaries, very flexible.

I felt the same way about DACs for a long time - tried many different delta-sigma DACs, all sounded exactly the same to me. I was adamant all DACs sounded identical...I then had the chance to audition some NOS R2R type designs where the difference / improvement was undeniable, completely turned my DAC world upside down, and I'm glad it did, I am getting much better digital sound nowadays having bought into it. Just my experience, of course.

I don't know if you are a headphone guy, but I am going to try to go to CanJam Chicago in June next year and bring my gear, I'm sure there will be an offshoot Head-Fi meet somewhere there. If so this DAC will come along!

Also, if you do go to CanJam, check out the ZMF Headphones booth. Assuming COVID doesn't derail, a type 45 headphone amplifier I am building for Zach Mehrbach should be featured at his table ;)
 
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