Valve DAC from Linear Audio volume 13

[leadbelly]

Hi, just noticed this thread. This is very interesting to me, but the pcb prices seem so high compared to Chinese manufacture. Marcel, if you have Gerbers I can run a group buy. I ran one here quite a few years ago, but still remember the in's and out's.

[MarcelvdG]

It indeed uses sigma-delta (or delta-sigma, that's the same) modulation. In fact you can choose between three sigma-delta algorithms, a seventh-order chaotic sigma-delta and two fifth-order sigma-deltas with a kind of embedded pulse width modulator with randomly rotated pattern. These last two have the advantage that they are dithered according to dither theory, which eliminates the tonal behaviour that many sigma-delta modulators have. You can also do a blind ABX test, I've included a state machine for that.

The complete design database including Gerber files, KiCAD database and FPGA files (source code and synthesized + implemented) can be downloaded for free from the downloads section of the Linear Audio website, Downloads | Linear Audio. You have to scroll down to "Design data package for Marcel van de Gevel's Valve DAC from Vol 13".

The article that explains the ideas behind the design can also be downloaded, but that will cost you almost half a euro, see Linear Audio | your tech audio resource

I rather like buying things that are not made in China every now and then, but of course you can have the PCB manufactured in China or wherever you like. Some component values may have to be adapted when the prepreg thickness is very different from the 360 um that Eurocircuits uses, but that can be done.

[leadbelly]

Originally Posted by MarcelvdG

I rather like buying things that are not made in China every now and then, but of course you can have the PCB manufactured in China or wherever you like. Some component values may have to be adapted when the prepreg thickness is very different from the 360 um that Eurocircuits uses, but that can be done.

Thank you very much for this contribution!

Even if you have made the package freely available, it would not be ethical for me to even start a group buy without your agreement. I would plan on Chinese manufacture of the boards, looking for best price and also previous experience on this forum. I would start by opening a thread in the group buy forum to gauge sufficient interest. Let me know your thoughts.

[MarcelvdG]

Don't worry, I'd consider it an honour if you started a group buy for this. I would like to warn you, though, that the PCB is not the only expensive part, see also posts 8 and 9.

[Trajan]

But how does it sound?

[leadbelly]

Originally Posted by MarcelvdG

Don't worry, I'd consider it an honour if you started a group buy for this. I would like to warn you, though, that the PCB is not the only expensive part, see also posts 8 and 9.

Yes, of course. That is the beauty of a group buy, if there is enough interest even parts other than PCB's can be sourced at lower cost. It really just depends on interest more than anything else.

[MarcelvdG]

Originally Posted by Trajan

But how does it sound?

See post 235 of this thread:

DAC gallery

[leadbelly]

Marcel, I am starting to get some questions back from the PCB houses, and I have some of my own:

The filter PCB gerber files do not include silkscreen or solder mask.

What about PCB specification itself: substrate, thickness, surface finish, etc.? What are they for your prototypes, can we use this, or do you have suggested improvements?

You mentioned that there is a error with the transformer solder pads. Is this because the software library assumed a different Jensen form factor? This may be important if we seek a substitute supplier such as Cinemag.

You mention that the transformer is optional. The schematics in the design package don't show that option. Is it to be found somewhere?

[MarcelvdG]

Originally Posted by leadbelly

Marcel, I am starting to get some questions back from the PCB houses, and I have some of my own:

The filter PCB gerber files do not include silkscreen or solder mask.

True, the filter PCB was targeted for the Eurocircuits naked proto process which has no silkscreen or solder mask.

Originally Posted by leadbelly

What about PCB specification itself: substrate, thickness, surface finish, etc.? What are they for your prototypes, can we use this, or do you have suggested improvements?

FR4, about 1.55 mm total thickness, single-layer filter board, four-layer main board with 360 um prepreg thickness. I'll look up the other data soon.

Originally Posted by leadbelly

You mentioned that there is a error with the transformer solder pads. Is this because the software library assumed a different Jensen form factor? This may be important if we seek a substitute supplier such as Cinemag.

I simply made a stupid mistake entering the footprint. That mistake is fixed in the layout data on the Linear Audio site.

You mention that the transformer is optional. The schematics in the design package don't show that option. Is it to be found somewhere?

There is a pdf file in the zip with the design package that shows the various filter options.

[leadbelly]

Originally Posted by MarcelvdG

FR4, about 1.55 mm total thickness, single-layer filter board, four-layer main board with 360 um prepreg thickness. I'll look up the other data soon.

Thanks! It would be appreciated if you could look up the other data as well. Always better to copy a successful prototype.

I was surprised at 1.55mm thickness. With this large a board, 4 layers, and the need to survive multiple tube socket insertions and removals, I would have thought that thicker would have been the way to go. Any comment?