clean psu design attempt

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hi

i've build an headphone amplifier (http://headwize.com/projects/showfile.php?file=szeke1_prj.htm). It's a class A amp, i'm now shure about the power consuption, but **should** be less than 10W (2 chan).

now, since it is very sensible to the power supply quality, i decided to build myself one to get rid oh the hum

i tried to design it myself, basically it's a transformer+recitifier bridge and filter, followed by an lm317 regulator (as taken from the datasheet), a capacitance multiplier (http://sound.westhost.com/project15.htm) to reduce ripple and a couple of capacitors for energy reserve

the schematic is attached

i'd like to build a nice unit, with a voltmeter panel on the front, and regulate the voltage with a pot (R1, under the lm317), so I can use it for different projects
i'd like to have from +12V to about +20V on the load

now i need help with calculations, here's what i did:

from the datasheets of lm317:
to have high current out (typ 2.2A), Vin-Vout <= 15V
for the lm317 to work, Vin-Vout >= 3V, let's have 4V
so, 4 < Vin-Vout < 15

now i want 12<Vout<20 (i know i loose something on the capacitance multiplier, but i'll survive ...)

4 < Vin - 12 < 15 => 16 < Vin < 27
4 < Vin - 20 < 15 => 24 < Vin < 35
the most restrictive are
*** 24 < Vin < 27 ****

let's have 25V ...
to have this, i need a transformer with 25 / sqrt(2) = 17.67V
=> the transformer should be a 18V


is all this right???
are there any big errors on the schematic?

dc output shouldn't be more than 20V @ 1A, so, considering losses in the psu, how many VA should the transformer be?

i've never designed a psu before, so I just combined different elements and did some math ...

please help
thanks in advance

Nicola
 

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