The ValveDac

[acg]

Linear Audio 13 had an article by a fellow Marcel van de Gevel about an interesting DAC concept.  Seems like Marcel perhaps designed this thing as an intellectual exercise to see if it could be done and certainly he seems to have spent a great deal of effort, thought and time in getting it to not only work, but by all accounts to work well.  What he ended up with is unique in the audio world...digital to analogue conversion using vacuum tubes.  Lots of dacs call themselves valve dacs but in reality they use SS dac chips or resistor ladders with a valve I/V stage and perhaps, very perhaps, some valve rectification in the power supply.  Marcels ValveDac is different.

 

For a start the voltage reference used in the dac cores and the clock circuit makes use of and 85A2 voltage stabiliser vacuum tube.  Nothing new here, voltage stabiliser tubes are common-as and I use them in the bias circuit of my power amplifiers.  Using them in digital clock and dac circuits is likely novel.

 

The clock reference uses a pair of EF80 pentodes and an ECC81 double triode.  Prima Luna uses a "mini-triode" in its dac clock circuit, probably in a similar way, but seems to use SS component rather than the EF80's as current switches.

 

Then there are the dac cores with each channel full differential using a trio of E88CC double triodes or equivalent.  The upper two E88CC triodes drive a passive differential reconstruction filter which of course is balanced output.  If single-ended is your thing (like me) then add a 1:1 output transformer to drive your preamp/amplifier with the same voltage output as balanced.  Surely this is a new thing.  A dac using valves in those three areas is surely a new thing.

 

There is an FPGA on-board which runs the show, an upsampler, multiple source selection, plays all the PCM and DSD rates (apart from DSD1024), internal asynchronous reclocking, digital filter selection and other important and useful things. 

 

So, as a guy currently with 40 vacuum tubes in my playback and on the lookout for another high-end dac my interest was picqued especially because of its curiosity value but also because another 10 tubes in my playback will take things to a neat half century of tubes, and that is some milestone!  It is still Marcels daily listener, and two guys that built a DSD only version have talked it up.  As a start I am going to use it for playback from my PC, from a RPi source, from the AppleTV and play the DSD stream coming from my Pioneer UDP playing SACD's.  Could probably think of some other things as well but that is a good start.  I've ordered some pcb's and a bucket load of parts most of which have arrived in the last week or so.  Another dac project sat in my cupboard for a long time (years) and I recently gave up and sold it all to another SNA'er because I thought I would have no time to get the project done...well...things change.

 

There is a thread at diyaudio with the design documentation, schematics, FPGA programming, Linear Audio article, quite a bit of banter about various parts of the design and build where Marcel is very active.  Should anybody be interested in the project I do have spare pcb's available at cost and can provide a BOM from Mouser.  Would be nice to see more than just me have a go at this thing.  It is not an inexpensive build but they never are with tubes.  The more I look into the thing the more impressed I am with Marcels thought processes and priorities when designing and prototyping.  He is a very clever dutchman and the whole project is probably something that not many people would have been able to successfully get to a working prototype let alone something that other people can build.

 

Did I say the pcb's are big?   See the photo below with what used to be my sons favourite movie for a reference...a masterpiece.  Mostly through-hole soldering with some large 1206 and 0805 smd on the underside along with five fine pitch smd components which will be more difficult.  I'm going to start populating the smd side tonight.

 

 

 

And somebody elses build also showing the reconstruction filter pcb on the left...

 

 

 

And Ray from the UK built the DSD only version (which does not have the trio of tubes for the clock circuit)...

 

 

Very nice build Ray!

 

 

I'll be sticking the tubes out the top of the box as well.  Hopefully the build goes to plan and I can get it performing and sounding how I need it to...

 

   

[sir sanders zingmore]

these guys claim to do upsampling via tubes:

 

 

[acg]

Well, ain't that something!  Could get rid of the upsampling chip in the ValveDac and add another 8 tubes to the count in the process...soon it will be a Collosus dac 

[davewantsmoore]

2 hours ago, acg said:

Then there are the dac cores with each channel full differential using a trio of E88CC double triodes or equivalent.  The upper two E88CC triodes drive a passive differential reconstruction filter which of course is balanced output.  If single-ended is your thing (like me) then add a 1:1 output transformer to drive your preamp/amplifier with the same voltage output as balanced.  Surely this is a new thing.  A dac using valves in those three areas is surely a new thing.

 

For the clock, I think so...but not the others (Lampizator).

 

DSD is good... but I say why stop there, and not use high rates AND more bits.

 

[acg]

3 hours ago, davewantsmoore said:

 

For the clock, I think so...but not the others (Lampizator).

 

DSD is good... but I say why stop there, and not use high rates AND more bits.

 

Dave,  my understanding is that the Lampis are SS chip dacs that have a DHT I/V stage.   The top of the line Pacific dac is a pair of DHTs and a valve rectifier... no other tubes.   

[davewantsmoore]

1 hour ago, acg said:

Dave,  my understanding is that the Lampis are SS chip days

 

Not all.....  they made a model which converted DSD to analogue using values (and did PCM to DSD conversion).

[acg]

9 hours ago, davewantsmoore said:

 

Not all.....  they made a model which converted DSD to analogue using values (and did PCM to DSD conversion).

 

I think you mean their "DSD only" engine which I think is more or less an RC filter on the DSD signal, but somehow managing the noise.  Others like the Signalyst DSC1 do this discretely.  Not sure where Lampizator used tubes in that process, but perhaps they did/do.

 

This ValveDac is actually a PCM converter with all DSD input converted first to PCM.  The DSDonly version of the ValveDac does away with the clocking and FPGA and sample rate converter and like the Lampi and Signalyst stuff only deals directly with DSD.

[oohms]

I was going to try the simple way, by using an AK4137 board to convert pcm to DSD and then filtering it to analogue but it's in the DAC project pile for now..

[davewantsmoore]

On 30/01/2021 at 8:56 AM, acg said:

This ValveDac is actually a PCM converter with all DSD input converted first to PCM.

 

Well .. it's a SDM converter  (it takes PCM as the input format, yes).

 

 

... but I did think (like Lampizator) they would have built a "single bit" SDM converter..... but I actually went ahead and read the thing (geez long) .... and see they built a 9 level quantiser.  

 

[acg]

@davewantsmooreThere is a switch on the thing to select which sigma-delta modulation to use:   single bit;  five level PWM; and nine level PWM.  Apparently the nine level PWM is where it sounds best.

 

Tis a long read but  I think worth it if for nothing more than a layman (such as myself) to get a handle on some of this digital theory and what goes in to building a dac from the ground up as opposed to whacking a chip in a standard circuit on a pcb.  

[acg]

On 30/01/2021 at 12:28 PM, oohms said:

I was going to try the simple way, by using an AK4137 board to convert pcm to DSD and then filtering it to analogue but it's in the DAC project pile for now..

 

Yeah, that's the really simple way, but from what I understand noise is difficult to get under control.  I'm only aware of a couple of commercial products that manage to get a decent result...the aforementioned Lampizator DSD modules for one. 

[acg]

A couple of days soldering and all the parts I have are on the pcb.  Just a handful of power supply caps are yet to arrive.  Many of the through-hole components have been moved from the top to bottom of the pcb so that I can maximise the height of the valves protruding from the top of the chassis which leaves a pretty bare topside...

 

 

...cause it is all party underneath...

 

 

 

Even wound 24 potcore inductors for two reconstructiom filters.  The intention is to experiment with type, slope and frequency of the filter to come up with the best fit for my system.  First up I will test 85kHz '0.05deg Linear Phase' vs 'Gaussian to 6db' and then perhaps trial lower orders and perhaps corner frequencies as low of 44kHz.  An enjoyable build with a little bit of everything from fine pitch smd to using silicon adhesive to attached oversize dc blocking caps to the pcb.

 

[Jventer]

@acg

I am very interested in what you have been doing and to date I have been an observer.

My interest has massively increased   and I am thinking of doing same.

For a start, if you don't mind : what is the $ and Hour outlay to build this Dac ?

(if you wish to discuss via PM please let me know)

[acg]

Good to get it out there @Jventer

 

 

$$ are roughly as follows:

$100 - pcb's - they are big and four layer

$500 - Parts for dac pcb including valve sockets and decent resistors/capacitors but excluding dc blocking capacitors

$250 - reconstruction filter excluding output transformer which is only required for single ended output

$375 - for high quality output transformer pair if going for single ended output.  You can get the specified Jensen transformer or like me a custom wound pair on a 80% nickel core from Dave Slagle for that price

$200 - Power transformers

$400 - FPGA pre-programmed

$300 - Vacuum tubes.  All types can be found quite cheaply or you can try the uber-regarded varieties for a lot more $$.  Basic is all you need.

-------------------------------------------------------------------------------------------

$2125 for single ended output

$1750 for balanced output

 

+ plus DC Blocking caps (x4) which are minimum 10uF - I've used 18uF - of the best quality you wish to try

+ plus a chassis or case

+ plus input boards to suit your system (I will use a JLSounds USB to I2S input, an HDMI input as well as optical).  JLSounds was about $125 delivered.  Other two were perhaps $50 for the pair.

 

 

As stated earlier, this is not an inexpensive build.  I have a bit of soldering experience and it took me two days to put everything on the pcb's.  Then there will be time to mount the gear in a case and sort out the inputs.  Luckily, the designer is very helpful and very responsive on the diyaudio site which makes me think that I will actually get this dac operational. 

 

I will build a chassis from scratch and am happy to pass on the design and cutting files to anybody that also wishes to go that route, but beware, my stuff is always heavy, over-engineered and rarely the 'minimum required size'.

 

Like anything high quality and tube related the $$ cost is high compared to SS, but in overall terms this dac will cost a whole lot less than my valve preamp, valve phonostage and valve amplifiers so in those terms I am happy to bend this to be a "good value" project...haha.  

[davewantsmoore]

3 hours ago, acg said:

@davewantsmooreThere is a switch on the thing to select which sigma-delta modulation to use:   single bit;  five level PWM; and nine level PWM.  Apparently the nine level PWM is where it sounds best.

 

Yes.... so it isn't a PCM DAC (in the sense of TDA1541 or BB1704)  .... it takes PCM format input, yes .... but this is run through digital filters, and internally it is a SD architecture.

 

[Jventer]

1 hour ago, acg said:

Good to get it out there @Jventer

 

Thanks a lot Anthony.

The cost IMO is not that daunting.

I am awaiting your progress - if it sounds good I will most likely follow.

 

Please allow me to again thank you for your contributions here - very interesting and much appreciated.

[davewantsmoore]

22 hours ago, acg said:

 

Yeah, that's the really simple way, but from what I understand noise is difficult to get under control.  I'm only aware of a couple of commercial products that manage to get a decent result...the aforementioned Lampizator DSD modules for one. 

 

It's very sensitive to jitter .... although using extremely high rates can usually out-run that.

[acg]

All the outstanding parts have arrived bar one tiny little package which has found the distributor this morning.  Let's see if AusPost Express can manage to get it from Melbourne to me in two days so I can work around my sons 13th birthday (yay, I've got a teenager - with first moustache traces already in the making, wears the same size shoes as me) this weekend to either fire up the magic or release said smoke...

Edited February 17, 2021 by acg

[acg]

It is alive!

 

 

Sounds pretty decent too straight off the bat.

 

Had all kinds of problems getting it going properly...well not really, it was only one kind of problem...connectors!  Some ordinary soldering on one of the 80 pin 0.5mm pitch FPGA connector that was easy enough to resolve and then a deformed molex connector on a cable that allowed the wire to rotate and lose connection.  So much time spent to diagnose that last little issue.  But look at those ten vacuum tubes aglow in the breadboard...so pretty.  Am going to play around with with a few things to see if I can squeeze something more from it, but it certainly does sound good right from the get go.

 

 

[Valvomatic]

Hi acg,  It's been a while for this thread... do you happen to still have any extra PCBs?

[acg]

6 hours ago, Valvomatic said:

Hi acg,  It's been a while for this thread... do you happen to still have any extra PCBs?

 

I do.

[Valvomatic]

and the price is... ?  :-)

[acg]

6 hours ago, Valvomatic said:

and the price is... ? 

 

The pcb pairs cost me $105 each.  So that plus postage to wherever you may be.   FPGA are hard to come by though...

[Valvomatic]

ok, let me do a little FPGA research and I will come back to you.  I'm in Sydney.

[acg]

All good...I did have a spare but sold it recently.