transformer voltage needed for Aleph 4 or Aleph 2

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diy student

I was wondering what transformer output voltage would be suitable to create the +/- 48 volts needed on an Aleph 4 or the +/- 45 volts for an Aleph 2. Would a transformer with dual 30 volt secondaries with 4 37.5 volt taps work? Thanks for the advise.

Rob

GRollins

Rob,
The formula is 1.414*AC=rail for a capacitor input filter. The one caveat here is that you'll have to add a volt or two for diode losses in the rectifier. So for a 45V rail, you're looking at roughly a 33-0-33V secondary. For a 48V rail, about 35-0-35V.
Note that the voltage need not be exactly 45V or 48V. The cicuit will be perfectly happy with anything within shouting distance of that range. Just be sure that you've got adequate heatsinking if you choose to run the circuit a bit hot.

Grey

diy student

Thanks so much for the input. By the way, what does it mean to say that the transformer has 30 volt secondaries, but 37.5 volt taps? I thought secondaries and taps would be the same thing.

Rob

diy student

Grey the equation you provided is that for a single secondary or a dual secondary or does it even matter? So if I would use the transformer with the dual 30 volt secondaries would I get 42 volt rails before subtracting the volt or two for diodes in the rectifier? How will this affect the amps output power, and approximately to what degree? Thanks.

Rob

diy student

I think I was miss reading and looking at secondaries and taps for two different transformers. Sorry about that.

jduncan

The engineer at plitron was trying to tell me that I needed a different set for that...I'm building an Aleph 4 and want roughly 48V rails but he said that to get that I should use the one with dual 27V secondaries ....he said this would give me what I wanted....

I however, initially thought that I needed 35V secondaries too....

Could you please confirm that I really do need 35V instead of 27V?

GRollins

The 1.414*AC is per rail, i.e. a 30-0-30VAC transformer would give 42.42-0-42.42VDC, not including diode losses, capacitor leakage, and a bit of resistive loss here and there. The actual rail would work out about 40-41V or so.
Please note that a sufficient current draw will pull the rail down even further, but if you keep it reasonable (50-60% of the VA rating of the transformer or less), the rail will actually end up pretty close to the calculations. Don't sweat a half volt difference. We're also assuming a fair sized bank of caps. Try to save money on the caps, and the rail will collapse sooner, not to mention ripple problems.

Grey

jduncan

OK great, so for my Aleph 4's I have a 1000VA transformer with dual 35V secondaries and a capacitor bank of .2F (6 * 33,000uF) ....this should be great right?

cp642

right! The engineer at plitron has taken you for a ride..or.... he's assuming that you're building an aleph 5?

jduncan

nah, i even sent him the schematic!

GRollins

A 27-0-27VAC transformer would give you rails on the order of 36-37VDC after taking into account the diode drop, etc. The Aleph would work, but would not develop rated power.

Grey

blank527

Hello,
Should 600VA also be enough?
The aleph 4 consumes 500W so 600VA should be enough I hope!

jduncan

It actually consumes a bit more than 500W ....its closer to 570W if you have 48V rails....even though it is an SE design you should still also allow for significant headroom...I'm personally using a 1000VA toroid but you should at least use a 750VA one...they're not much more expensive than 600VA ones

Orgonom

Hallo jduncan,
because it is an SE design you have to use an 1000W
transformer. You are drawing permanently 570W. In other
design (class AB you need only peak power). An 600W
transformergets too hot. Better to take two 600W transformer
and stack them together. You can flip one and cancelout the
scalarvector. (my next try)

jduncan

Yeah, that's why I'm using a 1000VA transformer....sorry, I kindof ignored heat in the last post but you need it for heat reasons too

diy student

So if I use a 1010VA toroid with dual secondaries of 37.5 volts multiply that times 1.414 gives me 53.025 volts subtract 2 volts for diode capacitive and resistive losses and I end up with 51 volt rails. Am I correct in thinking that this is acceptable for an Aleph 4 which specs. 48 volt rails as long as I've got enough heatsinking on the output devices. Will it signifigantly increase needed heatsinking on the other semiconductors? What about an Aleph 2 which specs 45 volt rails?

Rob

jduncan

it's fine for either as long as you can deal with the heat.... it's not a huge increase, but it is an increase....

I'm personally using dual 35V secondaries but 37 or 38 volts is fine

cp642

Rob, I believe you'll use the toroids sourced from apexjr?
Don't worry about the slight increase in the voltage. Be expected to lose a volt or two under current draw, which will leave you with less than 51V.

grataku

Guys,
don't forget that the plitron and all transformers are rated for voltage under full load. At and above 500VA this voltage drop is about 4 %. So if one uses a 1kVA transformer for the aleph 4, which draws a constant 500VA, the and the voltage drop is assumed to be linear, one would end up with (35+(35*0.02))*SQRT(2)=50.48V, again without accounting for the diode drop.
When building class AB amps I tend to be conservative in the voltage rating meaning I buy transformer with a couple of volts less than they should have. Plitron makes good transformers, with low resistance large section wires, however, I sometimes bought transformers in which thin gauge wire is used and the drop is compensated by adding a whole bunch of extra windings that bring the voltage way above what it should be. Once the problem was so severe that I had to remove windings from the transformer.

[Edited by grataku on 11-28-2001 at 01:49 PM]

grataku

Diy Student,
by the way I am building the aleph4 myself I ordered the trasformer but I don't know what to do for heatsinks, where are you buying yours? I found some but they would cost me 85\$ each x 4 = too much money!!

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