Valve DAC

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Ray P
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Valve DAC

Post by Ray P » Sun Sep 15, 2019 11:53 am

I mentioned this project over on the 'Digital' section so I guess it's time to start a thread here.

The terrible thing though is that I'm guilty of breaking my personal pledge to not start another project until I got the other ones finished but you know how it goes, I got into an exchange with the designer over on DIY Audio, I read the design paper, it all got me enthused and before you know it...

Anyway, the DIY Audio thread is here;

https://www.diyaudio.com/forums/digital ... -13-a.html

The core of the design is a valve-based delta-sigma decoder and the reference voltage is also using a valve. Obviously the digital aspects, such as clock an data distribution uses modern technology. The DAC cores need to be 'fed' SDM data (sigma-delta modulation - delta-sigma and sigma-delta seem to be freely iinterchangeable - SDM is the basis of DSD, which is Sony's proprietary derivative) so the original uses an FPGA module that essentially operates as an ASRC (asyncronous sample rate conversion).

As some of you will be aware I've been experimenting with DSD decoders for a while and I use HQ Player software to derive SDM and stream it to the decoders over the network. That meant I don't need the FPGA module and Marcel, the designer of the Valve DAC kindly reworked his design and, most importantly, the PCB to eliminate the FPGA aspects and to implement an SDM input interface and it is this derivative that I've started working on.

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Ray P
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Re: Valve DAC

Post by Ray P » Sun Sep 15, 2019 12:00 pm

So, I've buddied up with another DIY Audio member and we shared the cost of getting some rather expensive PCBs made up. They're expensive because they're large 4 layer boards but they also required a custom layer stack, specified by the designer. Here's an image of the PCBs with a CD for scale;

Image

There's quite a lot of fiddly surface mounted componentry for the digital elements but I have a friend who is very experienced at hand soldering this stuff so he's taking care of it for me. I'm hoping to get my board back in a week or two and I'm hoping to be able to test/measure these elements before building up the through-hole Valve based delta-sigma cores.

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Ray P
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Re: Valve DAC

Post by Ray P » Sun Sep 15, 2019 12:07 pm

To get the SDM data to the valve DAC I've built an HQ Player network appliance based on a Beaglebone Black and equipped with an isolator/reclocker. The appliance takes the SDM stream from HQ Player and passes it to the DAC input - it works at up to DSD512.

Image

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Ray P
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Re: Valve DAC

Post by Ray P » Sun Sep 15, 2019 12:16 pm

The PCBs I picture above aren't the full story, in time I'll need an output board equipped with the necessary reconstruction filters.

Also, whilst the main PCB has power supplies on it for the B+ and 5V digital requirements, as well as AC heaters for the DAC core valves, I'm not going to populate them and instead I'll use a Neurochrome Maida HV regulator for the B+ and a Salas Reflektor-D shunt reg for the 5V. Not sure about the heater supplies yet but would prefer DC to keep noise to a minimum - that'll require 6.3V @ 1.8A so not to difficult with a mainstream LT1084 regulator circuit with some careful planning of input voltage and heatsinking.

I've already got the Maida and Reflektor regulators and I have a suitable PCB in my spares box for the heater supply.

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Ray P
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Re: Valve DAC

Post by Ray P » Sun Sep 15, 2019 12:26 pm

Valve complement is 6 x E88CC (each delta sigma core uses three) plus an 85A2 for the reference voltage.

I've got a Mullard 85A2 but I'll probably get some Russian 6922s for the cores, at least to start with.

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Thermionic Idler
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Re: Valve DAC

Post by Thermionic Idler » Sun Sep 15, 2019 8:15 pm

Off-topic but just noticed the CD was Pachelbel's Canon, so I have to share this...

https://www.youtube.com/watch?v=JdxkVQy7QLM
Deck: Garrard 301
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Phono: Whest PS30RDT
Pre-amp - Scratch built AMB α24/LCDuino
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Speakers - FF225WK Metronomes

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Nick
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Re: Valve DAC

Post by Nick » Sun Sep 15, 2019 11:31 pm

Here is another one

Resistance isn't futile it's V / I.

Wolfgang
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Re: Valve DAC

Post by Wolfgang » Mon Sep 16, 2019 2:30 pm

I think Ray's new project would deserve better than two off topic comments about funny Pachelbel sketches. I wonder what would have happened if he had used a CD with Bach music for showing the size of the PCB?

When I read the original article about the tube DAC in Linear Audio many months ago I was very impressed that somebody would even touch this topic and build a DAC only with tubes. B. Rozenblit started something much less demanding last year and failed.It's not so easy. But it would have been a multibit DAC.

The problem with this tube DAC is that it gets really complex - at least IMO- if one wants to build a complete DAC with tubes only. Ray's approach to use his DSD DAC concept and use tubes for the last stage only is really good. That he could pull off to make the developer redesign the output stage and present these PCBs is also not nothing.

If it wouldn't need a powerful PC to do the DSD conversion and I would be into the sound of 1-bit DACs I would jump right on this project. But maybe this tube DAC will sound so different that I could even warm up to its sound. And maybe in the end it's not even the SDM but the rest (no tubes) that makes it sound - to my ears at least - rather cold and clinical compared to R-2R DACs.

I hope we will hear and see a lot more to this topic soon!

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Re: Valve DAC

Post by RhythMick » Wed Sep 18, 2019 7:34 pm

I for one would love to hear this.

If I can ask an honest and perhaps devils advocate question, what's the reason for using valves in the DAC cores? Curiosity? Or is there a genuine expectation that valves will bring something special to this?

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Ray P
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Re: Valve DAC

Post by Ray P » Wed Sep 18, 2019 8:01 pm

RhythMick wrote:
Wed Sep 18, 2019 7:34 pm
If I can ask an honest and perhaps devils advocate question, what's the reason for using valves in the DAC cores? Curiosity? Or is there a genuine expectation that valves will bring something special to this?
I've been waiting for someone to ask and the answer is very simple - because I can - so I guess essentially curiosity.

I love this hobby and the opportunity to do something truly off the beaten track was too much of a temptation.

I really don't know what to expect but it'll be fun to find out. At least I know that the designer's build was well received at a Dutch audio meet.

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Ray P
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Re: Valve DAC

Post by Ray P » Wed Sep 18, 2019 8:17 pm

While I'm waiting for the smd soldering to be finished on the main PCB I've been working out the input and power supply arrangements.

As I mentioned before B+ will be regulated with a Neurochrome Maida HV regulator.

Fir the 5V digital supply I've decided to, at least initially. use the onboard supply but will use a socket for the regulator device(L4940V5) so I can explore 'super regulators' or the Reflektor-D if the project works out well.

I'm aiming to use DC for the filaments and I also need to provide power to the Beaglebone and the isolator/reclocker so I've made up three simple linear regulated supplies (LT1085 & LT1084 based). The isolator/reclocker has an onboard high quality 3.3 supply.

The BBB and isolator/reclocker boards will stack nicely under the main board, as will the reconstruction filter output board.

I now need to exercise the virtue that is patience.

RhythMick
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Re: Valve DAC

Post by RhythMick » Wed Sep 18, 2019 11:55 pm

Ray P wrote:
Wed Sep 18, 2019 8:01 pm
RhythMick wrote:
Wed Sep 18, 2019 7:34 pm
If I can ask an honest and perhaps devils advocate question, what's the reason for using valves in the DAC cores? Curiosity? Or is there a genuine expectation that valves will bring something special to this?
I've been waiting for someone to ask and the answer is very simple - because I can - so I guess essentially curiosity.

I love this hobby and the opportunity to do something truly off the beaten track was too much of a temptation.

I really don't know what to expect but it'll be fun to find out. At least I know that the designer's build was well received at a Dutch audio meet.
Thanks Ray - keep us posted, please - watching with interest.

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Re: Valve DAC

Post by Wolfgang » Thu Sep 19, 2019 1:52 am

If I can ask an honest and perhaps devils advocate question, what's the reason for using valves in the DAC cores? Curiosity? Or is there a genuine expectation that valves will bring something special to this

Don’t know if that answers the question but this is in a nutshell the reason for using valves as M.v.de Gevel explains it:
A typical audio DAC consists of a digital interface, digital pre-processing, digital to analogue converter, a reconstruction filter, a time reference and a voltage or current reference.
The digital to analogue conversion, the reconstruction filter, time reference and a voltage or current reference have direct impact on the SQ. They must be therefore regarded as critical analogue and mixed-signal functions and must be realized in valve technology.

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Nick
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Re: Valve DAC

Post by Nick » Thu Sep 19, 2019 9:03 am

Ignoring the more contentious parts, what possible reason could there be for suggesting that valves are better in the "time reference and a voltage or current reference". Not saying there isn't a reason, but if there is one, its not one I know off.
Resistance isn't futile it's V / I.

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Ray P
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Re: Valve DAC

Post by Ray P » Thu Sep 19, 2019 5:12 pm

I don't believe the designer is saying that valves are better and I think there's a clue in the article summary on the Linear Audio page;

"Although Marcel believes that excellent audio equipment can be made using solid-state technology, he considers it a nice intellectual challenge to see how far he could get with building a DAC that has all critical analogue and mixed-signal functions realized in valve technology."

My reading of the designer's full article, in the context of this being an intellectual exercise, is that he simply set himself the objective of implementing the analogue and mixed-signal elements using valves. I have the full article available at home so will reread the relevant section to confirm (or not) my understanding.

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