Valve DAC

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Ray P
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#16 Re: Valve DAC

Post by Ray P »

Here's the relevant paragraph from the introduction (Section 1) of the full article;

"Although I believe that excellent audio equipment can be made using solid-state technology, I considered it a nice intellectual challenge to see how far I could get with building a DAC that has all critical analogue and mixed-signal functions realized in valve technology. I also saw it as a nice excuse to experiment with valve oscillators and switching circuits, rather than the usual amplifiers. This article summarizes my findings..."

Which gives context to this excerpt from Section 2;

"The actual digital to analogue conversion, reconstruction filtering, time reference and voltage reference must therefore be regarded as critical analogue and mixed-signal functions. Because of the reasons explained in section 1, they must be realized in valve technology."

So my reading is that "must be realized" is simply the constraint set for this specific project.
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Nick
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#17 Re: Valve DAC

Post by Nick »

Yes, that gives a different context than Wolfgang's post did. I will try and find time to read the article again tonight.
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#18 Re: Valve DAC

Post by Wolfgang »

"The actual digital to analogue conversion, reconstruction filtering, time reference and voltage reference must therefore be regarded as critical analogue and mixed-signal functions. Because of the reasons explained in section 1, they must be realized in valve technology."
If the word “critical” in this sentence refers to the difference between the input of the DAC(interface etc ) and the analogue and mixed signal functions then Ray’s interpretation is correct. In this case it means that the purely digital part is done with typical parts like in other DACs and only what is considered as “critical analogue and mixed signal functions” is done with tubes.

I interpreted the word “critical” as referring to the quality depending on whether these “critical” functions are done with tubes or solid state like in typical DACs. I had reason to believe that this was meant as a 85A2 glow discharge tube for the voltage reference and a tuned plate crystal grid oscillator for the time reference add both typical tube qualities. Tricky business.
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Ray P
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#19 Re: Valve DAC

Post by Ray P »

A couple more excerpts from the overview section of full Linear Audio paper;

"Digital signal processing is essentially just a bunch of calculations that need to be done. As long as
the correct calculations are done and as long as they are done correctly, the result is independent of
whether the number crunching is done by an FPGA made of deep submicron CMOS transistors, by
a valve computer or by a team of human computers. It would, however, be very hard for normal
thermionic valves or for human computers to do the required calculations in real time. Hence, the
most practical solution by far is to use deep-submicron CMOS.
"

So by implication, it is sensible to take care of things like the digital clock and data lines with modern silicon based technology and focus the application of valve-base technology thus;

"The actual digital to analogue conversion, reconstruction filtering, time reference and voltage reference
all have a direct impact on the sound quality. This is obvious for the actual digital to analogue
conversion and reconstruction filtering. Variations in the voltage reference and time reference modulate
the output signal in amplitude and phase, respectively, thereby creating undesired sidebands.
If noise shaping is used, fast undesired variations in the voltage and time reference (noise and jitter)
frequency-convert out-of-band quantization noise into the audio band.
"

As its not my intellectual property I'll limit the quotes to these, all from the introduction sections of the paper. If you're interested in reading more the full paper only costs 2.99Euros and is available here;

https://linearaudio.net/article-detail/2284

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Nick
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#20 Re: Valve DAC

Post by Nick »

Read it last night, Ray's interpretation is the one I think the author means. He sets himself a (possibly intellectual rather than performance) goal of doing all the non DSP parts with valves, so those mentioned parts have to be implemented using valves to meet that goal.

Its an impressive body of work and I will be interested in the results, but even the "valve" sections are including valves instead of done using them only, so the pedant would say its still a DAC with valves rather than a valve DAC. But it does indicate how complex it would be to replace the remaining silicon, just implementing a couple of the J/K flip flops would start looking like a 19" rack mounted solution.
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#21 Re: Valve DAC

Post by Ray P »

Nick wrote: Fri Sep 20, 2019 9:21 am Its an impressive body of work and I will be interested in the results...
I'll try not to be too much of a malingerer like I normally am with my builds!

I concur about the DSP processing, it's scary to think of how many valves, all the power requirements and just how physically large it would be if it only utilised valve technology!
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#22 Re: Valve DAC

Post by Ray P »

Nick wrote: Fri Sep 20, 2019 9:21 am He sets himself a (possibly intellectual rather than performance) goal of doing all the non DSP parts with valves...
Just to add that I have liaised a few times with Marcel, the designer, over the last couple of months and one of the influences to my decision to have a go at this project is that he still uses his original build as his own DAC.
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#23 Re: Valve DAC

Post by Ray P »

I got a message earlier to say my chum has done all of the smd soldering apart from two parts I forgot to order :roll:

Should get the board back on Monday when I see him at work.
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Ray P
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#24 Re: Valve DAC

Post by Ray P »

So the build is officially underway!

Picked up the PCB from my mate today - he's done a really good job with hand-soldering all those tiny smd parts.

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Image

I placed an order this morning for lots of through hole parts...
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#25 Re: Valve DAC

Post by IslandPink »

Something to keep you busy when you come back from bike rides there Ray !
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Ray P
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#26 Re: Valve DAC

Post by Ray P »

The main PCB is coming together;

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#27 Re: Valve DAC

Post by Ray P »

I fancied a bit of bling so I've been experimenting with mounting 3mm light tubes through the holes in the centre of the valve sockets. This is a 5mm purple LED, attached to the light tube with heatshrink tubing. If I keep it I'll get some adaptors 3D printed to make the arrangement a bit stronger.

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#28 Re: Valve DAC

Post by Cressy Snr »

Cool.
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#29 Re: Valve DAC

Post by Ray P »

A better picture

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Ray P
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#30 Re: Valve DAC

Post by Ray P »

I made some good progress with the Valve DAC project this morning, installing and testing the power supplies and starting to assemble the chassis. I hpe to get all the power supplies tested before the end of the bank holiday weekend.

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