To be precise, my transformer is a custom 100VA part with 6 secondary windings. The ones used to supply the UltraBiB with power are rated 14VAC @ 1A for a total of 28VA. The transformer does run very cool.
If 28VA rated windings (though in a much larger toroid) run cool, would regular 25VA toroid work? It seems from the current draw specs in the manual that even triple the power consumption wouldn't reach 25w at all. At the same time Salas recommended a 50VA, which is probably the safer (and better if you're indifferent otherwise) option...
Thanks for the detailed recommendations! Looks like I'll still have to use the trimmer and maybe swap it for a fixed value resistor later. I am actually planning on getting some mumetal shields around my toroidal transformer, even though people seem to be unsure whether it's necessary. I'm glad to hear a stronger opinion on this. I could get an Antek but it doesn't seem to be any fancier than Talema, a bit bulkier but also cheaper. Do you think it will do to have a few layers of mumetal sheets around the toroidal?
I'll order extra resistors as you suggested, not sure if I'll get to the point where I can experiment with fine-grain differences but definitely a good idea to get more of these resistors that I could potentially need.
I did misunderstand "elevated" earlier. I'll probably get the chunkier ones then if for nothing but to remind myself later on that other configurations might need higher wattages.
If I go slightly more conservative on CC estimation then maybe standard 470r R9 works well? I'm not super sure now what CC to use. Dam1021 is speced to typically consume 0.18A + and 0.06A - (at 10V for a total of 2.4W). I think Soren mentioned that the power consumption is pretty stable. However, the max power draw is marked at 5W to be safe. At typical levels (+100mA margin for each rail), the total power for the transformer comes out to a bit over 15W. I really prefer the 25VA transformer for its smaller footprint... I didn't leave any safety margin in my build in terms of space which is kind of ridiculous and as it is the 35VA transformer sits on a wooden block just to not hit the other boards... But it runs very cool so I wasn't very worried. It's funny that the situation was caused by the sigma22 derivative being advertised at 110mm L when it's actually 127mm. But this makes things easy for me now to upgrade to UltraBiB 1.3, since I know it would fit almost perfectly.
If I work backwards from a hypothetical 25W transformer (divide by 3 for overhead, by 12V AC then by 2 for two dam1021), I think I have about 347mA max for both rails for each dam1021, before accounting for the CC overhead to the peak. Again, Soren said typical usage is 180mA positive, 60mA negative. Calculations break at this point since it means that I would have only 50mA overhead for each rail to the typical (not peak) usage. But is there a chance that things could still work out? For instance if I go a bit less conservative on the 3x factor? Thanks a lot!
About C1, 10.000µF is a good choice for 850/900mA current. Does the "quality" of the cap impact the sound? In a reverse way, the bad quality (general purpose cap) of C1 can't harm the final result, can it? If so just look for a long life one... even if these two output audio cap won't last for ever...
Thanks for your patience Salas!
Thanks for your patience Salas!
A quick edit. I misunderstood the 50-150mA overhead. 50mA per rail on dam1021 (x2) would amount to 100mA per rail on the UltraBiB. So no problem there. I also checked the thread on dam1021 and it seems that the 180mA and 60mA +/- rail power draw on dam1021 is really constant, no bursts. I asked Soren about it and hopefully I can get a more official answer soon.
Edit 2: Don't know how I missed it but Soren's response to this earlier was right in front of my eyes... Reference DAC Module - Discrete R-2R Sign Magnitude 24 bit 384 Khz
Going back to my previous calculations... 347mA-100mA(overhead for two rails on each dam1021) gives me just enough for typical usage. Soren is recommending 50% extra for max and by his tone I think the estimate leans towards safety (though safety implies quality here...). So if this were to work with a meager 25VA little toroidal, using Soren's estimate for peak current, I would have to cut corners on the 3x factor by about 25%. Salas, is this okay? I would assume that Talema has a little bit of safety margin on the 25VA and also things won't completely burn down immediately if the 25VA limit is exceeded. Assuming that we don't see Soren's peak estimate very often in practice, maybe this would work?.... Thanks and sorry for trying to cut corners in the specs of an excellent work of engineering!
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Good quality reservoir is always preferable. If you you will get some type with good DF characteristic from a premium industrial brand like Panasonic, Epcos, Vishay, Nippon Chemi-Con, Nichicon, etc. you can't go wrong. Lesser quality caps for loss factor and high frequency impedance can have a somewhat inferior subjective impact also.
Ah man I may have not been precise enough on my message, sorry Salas, here I go again: let's say audio grade Nichicon KG (NKG-160 - 10000uF 35V Nichicon KG Type, Gold Tune, snap-in | Hifi Collective) short life vs industrial Cornell-Dubilier SLP (SLP103M035C7P3 | Condensateur electrolytique aluminium 10000μF, -20%, 35V cc, Traversant, +105degC, serie SLP, 25 (Dia.) x 40mm | RS Components) No way to put cheap chinese brand 
As it's the same price league it's hard to choose, and you're the after-sales' man... oh sorry, I meant the designer
As it's the same price league it's hard to choose, and you're the after-sales' man... oh sorry, I meant the designer
Dual DAC loads, common regs, common transformer scenario:
(180x2)+100=460mA CC target (positive side). Thus R1=1.2R 1W-2W
(60x2)+100=220mA CC target (negative side). Thus R1 2.4R 1W-2W
Should pull near 10W in total. Derate 40% due to full bridge and give times two for safety against too hot core (10x1.4)x2=28W i.e. 30VA core choice should be logical.
Thin sheet, one layer, no gap at the seam, solder along there to make it continuous. You can also ground it with a short wire to chassis for electrostatic draining. If its already a magnetically shielded model don't do anything extra.
Ah man I may have not been precise enough on my message, sorry Salas, here I go again: let's say audio grade Nichicon KG (NKG-160 - 10000uF 35V Nichicon KG Type, Gold Tune, snap-in | Hifi Collective) short life vs industrial Cornell-Dubilier SLP (SLP103M035C7P3 | Condensateur electrolytique aluminium 10000μF, -20%, 35V cc, Traversant, +105degC, serie SLP, 25 (Dia.) x 40mm | RS Components) No way to put cheap chinese brand
The first one
(Unless you will have thermal radiation problem on it, but better have bigger sinks)
That's an awesome explanation!! Exactly the details that I was hoping for!
By 10W do you mean the loads or the shunt inclusive? Dam1021 currents seem to be rather voltage independent and I was planning on staying with the 9v dc that I'm currently feeding it. If the waltage you were describing was for dam1021 alone, then it's around 6.12W. I really don't want to push it but my options I think are 25VA or 35VA(current) Talema toroidals. The 35VA currently runs pretty cool, but it has to sit on a small wooden block due to size issues (I designed the layout with an incorrect measurement of the regulator by the seller, though coincidentally the actual size is almost identical to the Salas V1.3 making an upgrade rather easy). The 25VA would most definitely be able to sit flat on the chassis where the "cooling vents" are. In addition, I should have space to install a steel enclosure over the toroidal transformer, with an extra layer of mumetal inside. It really appeals to me functionally and aesthetically, but I wouldn't go against other technical reasons. Is it possible that with a more precise and non-conservative estimate that a quality 25VA transformer could work well long-term?
Thanks again for all the help!
The first one
You meant you're not the designer but just that guy Joe from the complaint department?!
'You're the boss =) And as massive heat sinks lower thermal voltage drift that's just great.
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Hopefully it doesn't get too hot. The mosfets will use the chassis as heatsink so one less heat source in the box, kind of...
Would there be a loss in EMI insulation if I don't use the steel enclosure and just a layer of mumetal wrapped around it? Should I go for the thicker ones if I can afford it? (I have options from really thin to a couple of mm's) The enclosure won't cover the bottom and there are vents underneath the transformer. Also the chassis is not steel but aluminum, if that's relevant... Thank you!
You meant you're not the designer but just that guy Joe from the complaint department?!
Oooo! So mu-metal could be sufficient by itself? I thought steel is somewhat good for shielding too, though having different characteristics from mu-metal. Also, should I go for the thickest possible mu-metal that can still bend around my transformer? Thanks!
Okay
I'll go for the thickest one that bends well. And you don't think that I need a steel enclosure on top of that? Thanks!