Valab NOS DAC - subjective listening and modifications

[Bill Allen]

Gentlemen,

Lets put the coffee cup down then go back and re-read this entire thread........its only 11 pages log ..... "so far".

The answers to almost every recent question has previously been posted. Come on guys, lets do "a little" research before asking anymore simple questions.

The last thing we all need, is another 89 page Valab Dac thread full of fluff!

(No response is required just a friendly reminder)

 

[zzodhi]

Agreed.

 

[Bill Allen]

Gentlemen,

If you are serious about modding your Valab Dac it's time you draw out your very own circuit schematic. Get your reading glasses on, follow the solder traces and draw it just the way "you see it". I drew my schematic three different times, each time adding new details and footnotes.

Once you have all the part numbers on paper you can go online and download the PDF files for all the chip sets. There free to download ... it doesn't cost you any money! Take a good read of these Data Sheets. You might not understand everything at first, but slowly things will come to you.

Don't be afraid to keep going back and re-reading this thread. After a 2nd or even 3rd read you be surprised what you have learned. Eventually the light bulb will go on and you see how simple this Dac really is.

Modding a Valab Dac is the perfect learning opportunity to enter the world that is DIY Audio. Soon buying and trying audio gear will be a thing of the past. The DIY Audio Hobby is a blast once you get hands around the basics. Have fun!

 

[Bill Allen]

Ok with that said; here's the deal with the analog output section of the Valab Dac.

The output current is completely derived from the 8 Dac chips. The Dac chips are all wired in parallel, imagine them all stacked on top of each other. In fact this is how the earliest versions of this Dac where assembled. Yep, there were serious overheating problems with that design. Laying them flat against a heat sink, like on the Valab Dac was a much better idea.

Anyway all 8 legs of both the left and right analog outputs are tied together. (Draw out and study the solder traces on both sides of the tiny Dac board to see how that was done). Looking carefully you will see this collective analog "current" first touches the small blue I/V resistor. This simple resistor is connected directly to ground. Then by PFM (pure fricking magic) this passive resistor converts the Dac output current to voltage. That's how we obtain our 2.1 Volt output spec.

Ok, looking very carefully at the board you will see the positive signal now travels through 2 little magnets, these RF noise blockers are also connected to ground via a very tiny surface mount cap. From there our positive signal reaches the passive coupling cap that everyone is so excited to change. The caps outputs to a solder trace then to white copper wire which hooks to the RCA Jack.

Simply put the positive analog signal path only touches the I/V resistor, passes though a little noise filter, and finally exits through a capacitor which in turn is really nothing more than a filter (A filter that blocks DC power). What remains is the positive A/C voltage that is our pristine musical signal.

Looking underneath the little Dac board the ground side of the I/V resistor follows a solder trace to the white wire that hooks to the ground of the RCA jacks. (Only by drawing this on paper can you see how this really happens on the board). Draw your schematic! Pretty Please, With Sugar on Top!!!

Ok not so fast, there is a ground trace that returns to both I/V resistors via a pair of tiny 47K ohm surface mount resistor. Some people call these idle resistors. If the Dac is on but no music is playing, your upstream gear may send a small amount of DC offset back towards the Dac. This is simply shunted to ground with these little resistors.

So you ask .. Are these idle resistors required on the ground circuit? Is the little ferrite filters required on the positive circuit? Well, in most cases probably not.

So how simple could we make this analog output section?

Well you could literally solder the negative wire of an RCA cable to the output of the I/V resistor, and your favorite coupling cap to the positive side of the I/V resistor. Last step; solder the positive wire of the RCA cable to the output of the cap. You’re done. Can you imagine that in your head and/or on a piece of paper?

Well that's it!

I introduce you to the real genius of the Valab DAC.

 

[Bill Allen]

Amertuollig

Nice Job on your DAC.

We have all been curious on the sound of the DAC after removing the AC transformer form the Board.

I am currently working on a project to add battery power to the Valab Dac. Initial experiments have been positive and I am currently playing with different types of batteries. It appears that much more than 3000uf capacitance is required when using batteries. Your picture reveals to me that removing the stock A/C transformer will allow the room for the larger caps. Thanks.

My friend Tony and I are also looking into using Paul Hynes replacement 3 pin voltage regulators.

3 Terminal Regulator Replacements


This review of a custom phono amp kind of sums up my trials and tribulations concerning gear modifications and the subsequent subjective listening results.

AudiogoN Reviews: Paul Hynes Design Phono Stage Tube preamp

 

[r8833]

Thanks Bill...its starting to make sense now


Rich

 

[Amertuollig]

Thanks Bill for the details.

I made a big mistake with my ohmmeter. I do not verify that the contact between I/V resistor and 47 kOhm resistor was connected to the ground : it is the reason i found the 2 resistors were parallel to the 10uF cap causing trouble in my mind.

Sorry for this.

 

[Amertuollig]

First, the stock Valab dac is a very fine sounding dac but with a minor shrill character (i am very demanding with respect to this). It was clear to me that minor modifications would be benefit to the sound not for the pleasure to modify the dac but because i found that the subjective listening performance of the dac could justify to spend some time.

The replacement of the 10 uF caps and the I/V resistors were the prior goal to achieve.

The choice of Obbligato caps is a pure question of personal taste.

Concerning the need to removing the AC transformer : it is more a question of coherency of modifications approach than the certainty that this modification would give an objective sound improvement.

The stock AC transformer generates heat and vibrations : are they desirable ?

To my point of vue, i think vibrations have sound subjective impact : harshness and shrill character.

In addition, excessive heat can accelerate the aging of electronic components.

Further in my country the voltage oscillates between 230 V and 240 V certainly not 220 V. I think that the the ratio of secondary coil turns to primary coil turns is defined to get the proper voltage 0-9V with respect to primary 110 or 220 Voltage.

Effective 230 V AC transformer was for me required.

The subjective feeling in the improvement seems to be more pace in the sound and less shrill character. But i have done many modifications quasi in the same times and my feeling would be a little bit misleading.

The noise is reduced when the dac is switched on but without playing music and when my amplifier level is set at its maximum.

To finish, the dac is now less affected by voltage artifact when i switch on my neon in my garage.

The use of the battery that you are working is probably the ideal solution but not for the WAF.

Good research !

 

[Pars]

What is this "Rule of 5" that you mention throughout this thread? I'd asked a few pages back but must have gotten buried

 

[PatOMalley]

Bill,

Any more on the regulators?
Should we replace both or just the DACs power regulator?

Quote:

Originally Posted by Bill Allen /img/forum/go_quote.gif Amertuollig My friend Tony and I are also looking into using Paul Hynes replacement 3 pin voltage regulators. 3 Terminal Regulator Replacements

 

[PatOMalley]

Regarding voltage regulators swaps ...

Looks like the LT1084 regulator in the DAC has a pinout looking like this:
gnd - out - in

A lot of regulators out there are "7805" drop in replacements which have a pintout like this:
in - gnd - out

There are some regualtors which are easier to rewire than others. Looks like the NewClassD UWB can done easily by using hookup wire:

Paul Hynes, though, makes his regulators by each order and it looks like he can/will adapt the regulator to your use. Meaning making the pinout LT1084 compatible. There is a 3 week lead time on orders and custom orders may take longer.

C'mon. I know where to get hookup wire. I get to swap a regulator AND rewire the thing myself. I'm in. The NewClassD go by the name of DEXA at Partsconnexion.

reference: DEXA UWB

Thanks for the incentive, Bill.

 

[mattzack]

I just got a reply back from Kevin on replacing the output caps and the 2009 Valab Dac. He has some caps for $5.00 each that he recommends. His reply :

Dear Sir:
You may try to use film caps to replace these two output tantanium metal can caps. In my store, I have 10uf/100v type. It is great caps to upgrade this dac.
Metalized Polypropylene Capacitor 10uf 100V PIO OIL - eBay (item 300303632221 end time Apr-26-09 09:39:46 PDT)

Sincerely, KEVIN

I sent him a reply, asking him if these caps would do a better job tha the Obbigato film caps. He has not yet replied back.

I also asked him, and I'll ask any of you, do you know of a great usb cable that will improve this Dac even more? I run a Mac Mini into the dac.

Thanks

 

[krisno]

My god I look forward to get rid of those tantanium caps. I got this brand new amp today, thought it was my other amp which created the unbearable brightness. But it is the damn dac. Those output caps are really a disaster....

K

 

[mrdon]

I purchased a used 2008 model late last year and it has sat in my system unmodded until this morning.

I am currently unemployed, so I had to dig through my bag of tricks to see what was available. I settled on two areas - replacing the output caps and ala Bill Allen adding a 220uf power cap to supply the LM084CT regulator.

For the output caps, I replaced the (2) blue Phillps 47uf/25V caps with Elna Silmic II 47uf/25V - I would have liked to use some nice films caps or BG N's but my budget is very slim. The only 220uf caps I could find was a used Elna Cerafine 220uf/16V, so in it went.

My first reaction to the sound of the modded VALAB Dac was that a layer of congestion had lifted - I could hear much more detail in the music but if I am being honest the highs were a little harsh - a typical 'lytic in the signal path issue. Nevertheless, after a few hours of burn-in the Elna Silmic's have started to settle down. In comparison to the somewhat "bloated" bass in the stock VALAB, the bass is now nice a tight and full. In this early stage, I am very happy with the results so far.

When I do get some $$$, I really want to replace the Silmics with some nice film caps like a Soniccap or Auricap. A step further would be to replace all stock 'lytics with the same value Nichicon UHE's.

 

[r8833]

My three sons

all three dacs modded with rubycon ZA caps, 390ohm caddocks I/V and panasonic FM for power filtering

DAC1 - Mundorf supreme 4.7 uf and .22uf Silver and oil bypass

TeraDak - Obilgatto 4.7 and .22 Vacuum tube valley PIO

Dac3 Obilgatto 4.7uf

[/IMG]

Rich