nice work on the psu!! given the number of them, wouldn't it be almost as cost efficient to get a custom toroid with multiple secondaries? something like that in my experience of high quality would run about 130usd and it'd be a lot easier to lay out. thats what i would be doing and you could probably get that price down with a GB. Richard Sumner would be my recommendation. i actually recently got a potted toroid running about the same current with 8 secondaries at 4 different voltages and currents for 140, so i would think this would be slightly cheaper even as a one off.
Although stray fields can of course be problematic compared to toroids, without shielding between windings the split bobbin 'formers have quite a bit lower interwinding capacitances, which assists the isolation. On that topic, the common mode chokes that JG shows are usually essential, given all the crud on the mains these days.
Thanks for the IV resistor tip (would never have thought of it)
I thought I got the VRef thing, but now I'm confused again..... can't I just take Vcc from the DAC itself (or rather the shunt) and use a voltage divider to set VRef (and use that same VRef for every 4 channels) ?
i'm not sure connecting the most noise critical point in the whole circuit, loading it and exposing it to as much noise as you can at every corner of the board is such a good idea? plus do remember that each circuit phase is floating so what would you reference it to? solvable problems i guess,m but not worth the effort imo. no you would be much better off generating it. myself i'll probably be using ltc6655, opa1641 and a couple of nice resistors, perhaps with a buffer. starting with a bulk foil trimmer to get the right value.
definitely using the chokes is essential, which should provide all the isolation i would need anyway
If the chokes are really well-balanced. Bruce Hofer told me that it took AP a while to find a good-enough one for the new line of instruments, which are the first to have offline switchmode supplies. Although he was dubious at the outset, he said the new machines are the first to have no measureable trace of 60Hz (in North America) in the spectra, unlike the System Ones and Twos that use mains-freq toroids (with always a bit of radiation from the leadout wires).
Thanks Joachim for the support.
> wouldn't it be almost as cost efficient to get a custom toroid with multiple secondaries
Not IMHO.
If I would use a transformer at all for the CEN/SEN, I would definitely only use one transformer per channel.
IMHO this is essential if you want good stereo image, and hence as little coupling between channels as possible.
If you are using balanced, then it is still OK to use one Tx for both phases, as they are 180° antiphase.
Patrick
> wouldn't it be almost as cost efficient to get a custom toroid with multiple secondaries
Not IMHO.
If I would use a transformer at all for the CEN/SEN, I would definitely only use one transformer per channel.
IMHO this is essential if you want good stereo image, and hence as little coupling between channels as possible.
If you are using balanced, then it is still OK to use one Tx for both phases, as they are 180° antiphase.
Patrick
Good transformers are not that expensive anyway. Less then 10,-€ buys you a BLOCK Spit Bobbin that i use all the time for sensitive phono stages. The two secondaries could be put in parallel. It is very important for lowest hum to have that transformers as far away from the circuit as possible. I would put the whole pre regulator in a separate box or compartment.
I just measured two Telema common mode chokes i had around. One measures 108.5mH plus 108,5mH, the other 104,3mH plus 104,2mH. So they are a bit different but the windings on each are similar. DC impedance is 1.5 Ohm on each. That is similar, uncritical anyway because i use 10 Ohm in series additional to get better hum rejection in the deeper regions.
sorry yes i did mean balanced, i guess i mostly assume that people are going balanced. batteries for me too, but i was just saying that if i were to do such a thing it would seem to me neater to get one unit with multiple secondaries or it could be quite a massive sprawling IV stage based around quite a tiny signal portion with the power supply orders of magnitude larger and more complicated. kit going dual mono for completeness you would use 2, but with stereo balanced it would seem no big deal to me.
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