4 ch 312 power transformer choice SOLVED 30-35VA safest

[pacemaker]

Hi Guys,
sorry for the dummy question,
but i didn't find a precise answer after A LOT of searching
throught the threads,googling, and scanning print version of the threads with Ctrl+F.

I want to power up 4 channels of 3nity Access312 boards
and APP2050 and Avedis Audio 1122 opamps,
with PeterC green PSU board with

i bought a 50VA 230V primary 2x18v secondary transformer from my local shop,
it is a little bit to High to fit a 1RU case (38mm+metal disc+2 plastic discs)
i was looking for a smaller ones and my local shop only has 6VA and 10VA 230v primary 2x18v secondary
or 15VA 230V primary 2x15V secondary

i saw in the API312 thread that fabios recommend a 35VA for a 4 channels Unit:

i 've tried to calculate power drawing ,
with these threads in the power supply and 312 META :
http://www.groupdiy.com/index.php?topic=3178.0
http://www.groupdiy.com/index.php?topic=8347.msg99593

API2520 seems to be 20mA per rail, so 40mA for  a card,
4 cards= 160 ma x 36V = 5,76VA
Is my 40mA per channel value wrong and should I find a 30 or 35VA ?
Or can i use a 6VA , 10VA best for headroom, power transformer ?

 

[pacemaker]

is the question too dummy  :'(

 

[pacemaker]

That's what i thought ,
but i am not sure of the 20mA value power drawing for the Access 312,
and Fabio recommend 35VA power transformer in the 312 thread,
and the AMVECO 62075 from the API 312 PSU partlist Rev 1
is also a 35VA transformer
Can the fellows who had allready built 4 channels 312 pres
post wich transformer they used ?
3nity , do you know how much current  your boards draws ?
Or does anybody knows ?

 

[Harpo]

If you also want your DOAs to work and drive some load, you might run out of beef quickly with a 6VA or 10VA transformer for 4 channels.
This 20mA is quiescent current per rail. (Depending on DOA and rail voltage about +/-15-30mA). There are DOAs that allow for max.250mA load current, probably never needed for a mic preamp, but expecting 40-60mA per rail driving 600 ohm isn't that far off. YMMV.

 

[pacemaker]

Hi Harpo,
you are the man !
so can confirm that 50VA is way too much for such 4 channels preamps,
and when Fabio recommends 35VA , is it too much headroom ?
I looked at my local shop, they have 15VA 2x18V ,
it's a bit more than you say but it will leave plenty of headroom right ?

 

[Harpo]

Fabio's 35VA seems spot on for 4 channels at full load, only leaving a little headroom.

some crappy math giving
(36V*1/root(2))/600R=42mA load
+2* maybe 20mA quiescent current=40mA
+current for phantom LED maybe set for 10mA
sum=92mA * k-factor 1.8 for fullwave bridge=166mA
48V phantom/(6k8||6k8)=14mA * k-factor 4 for villard doubler=56mA
166mA+56mA=222mA * 4 channels = total 888mA
ignoring current draw from resistor network at your voltage regulators
36V*888mA=32VA

You probably won't draw full load from all channels at the same time, your DOA might not swing rail to rail and 14mA for phantom is the worst case current draw of a shorted cable, leaving nothing for your mic, but 15VA might be on the low side. YMMV

 

[pacemaker]

ok,
so i will follow your advice on the "low side" of a 15VA,
and Fabio 's advice ,35VA for a 4 channels API312 pres.

i will give my 41mm high 50VA power transformer ,
back to my local shop and trade it for a smaller ( 37mm ) 
30VA power transformer, just for safety headroom.
i want my 4 channels to be as best as possible,

Thank you very much HARPO