you've got to allow for mains fluctuations, transformer regulation etc. Vac x 1.2 is a more typical loaded voltage.... you may get more than this but never rely on Vac x 1.4142 as this assumes zero supply resistance, zero mains fluctuation and infinite capacitance.... none of which we are able to obtain. secondaries around 38-42V are likely to be more suitable. 2V for diode drop is standard.
I am getting a custom toroid made for my aleph 4 that I am working on, would 1000VA 40+40 suffice? Would that account for the losses, or would that be too voltage on the rails? I was planning on getting 35+35. The guy said he could make any voltage necessary, with the power rating that I want.
Also, is it better to get two seperate 500VA toroids for the aleph 4, and have a seperate one for each channel? What is the optimal VA rating for each channel?
i would tend to use secondaries between 38 and 42V for +/-48V so 40V should be fine... separate transformers/rectifiers/caps for each channel is certainly better. most ppl consider average DC x 2 a good minimum for transformers so 500VA per channel would be the minimum .... personally i dont like anything below average DC x 4 in highly biased or class A amps which would equate to 1000VA per channel but you could get away with 500VA just with more voltage drop.
That should be perfectly fine and no i wouldnt go for a lower voltage.... also, i'm assuming that you measured the rail voltage @ no or very low load so some additional voltage drop will occur under full load conditions
You may wish to trying Duncan supply simulation software to check voltages, currents and ripply (see previous thread - Duncans amp tools PSU 11)
Prior to downloading the simulator I emailed my local toroidal manufacturer Harbuch Electronics Pty Ltd in Sydney for the specification to suit the Aleph 2 and their response was :
For 2x45V/3A with +/- 0.75V P-P ripple with +/- 90,000uF you require a 625VA transformer 2x36V/8.7A part # J2423 freight
I then ran the simulator and and after sorting out how to use it, the simulator result is very close to their spec and suggest you try it.
For a no load voltage of 36 volts (ac) @ 8.70 amps the secondary impediance is about 0.20 ohms with 5 % regulation(there is a handy help menu with formulae to workout no/loan and impediance).
You then select the bridge, capacitor and loan resistance(15 ohms is correct for 45 volts DC @ 3 amps)
The rms voltage is 45.09 volts and the transformer sags to about 35 volts AC.
I ran several values of capacitance, 40,000 uf @ 20 milliohms ie two large can in parrellel gives a ripple of 750 millivolts.
Parrelleling more capacitors to say 90,000 uf @ 7 milli ohms reduces the ripple to 312 millivolts and increased the DC rms Voltage slightly to 45.29.
For the Aleph 4 @ 48 volts DC the simulator suggests about 37.75 volts AC (no Load) for the same conditions except I have allowed for the reduced bias current of 2.5 amps for the Aleph 4 (240 watts or 120 watts per bank @ 48 volts @ 19.20 ohms load ).The transformer sags to about 37 volts rms.
The ripple for 90,000 uf was 271 milli volts and increases to about 650 millivolts for 40,000 uf.
I reality of course the standing Dc voltage under load could be a bit higher for either the Aleph 2 or 4 without any penalty, and the master himself has indicated the design a quite tolerant of supply variations.
I though this was fun and sort of makes progression through the project more interesting as you get to undertand more about various aspects of the amplifier.
Contrary to hints above, I do not know what voltages Nelson used ..... however, I can tell you what I used ..... and what I get under load
Remember I found my toroids surplus, so "took what I could", I did not choose according to theory!
They are 35-0-35VAC, the cap bank is 4x 80,000uF in a Pi filter arrangement (there is a 1.5V drop across the chokes, which are large, gapped iron core types wound with braided wire .... don't know the exact value as they were attached to the heat-sinks when I bought them).
And the amp sees .... 44.5VDC
This doesn't bother me, as I don't need the 100W anyway and the amp is not worried by minor variations in rail voltage. I could remove the chokes and get another volt or so, but I'd rather keep the filter.
I do seem to recall Nelson using the x1.2 "rule-of-thumb" in some piece of correspondence, just don't ask me to remember where !
Wow, how are you going to put 70 1500uf capacitors into the amp? Are they very small in size? What value of film capacitors are you using?
For Aleph 4 that I am in process of building slowly( well, because $$$- MOSFET MATCHING-thinking of 80 pcs). I thought of using wima for C103, C12, C4 and black gate or panasonic for C11. The rest, I will be using panasonic from Digikey.
Because they don't have 25000uf, I am thinking of dual mono power supply using 8 2200uf capacitors. They are the cheapest I can find, but still expensive compared with yours.
Presently, I have bought only the transformer and the heatsink, The transfomer arrived in spoilt condition (cut by the fixing hardware, and Avel send me another one). I hope this time it will arrive in good condition.
70 pcs for $40 is real cheap? What is"NOS"?
Have you done your layout yet?
What preamp are you planning to use for your Aleph?