Toroidal secondary voltage for Aleph

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Jmertz

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Joined 2002
#1
Hello,

Can anyone give me an idea what secondary voltage I need to get 42 volt rails with around 6 amps bias current on the output stage? I know you can approximate it by multiplying the secondary RMS voltage by 1.41 (to get peak) then subtract a diode drop or two, but how good is this approx. in a real amplifier???

Say, I have 30-0-30 volt RMS secondaries and
50,000uF storage on each rail. Can I expect to get 42 volt rails??

Last time I biult an amp, I got a 38-0-38 toroid and when I measured my rails they turned out to be 55-56 volts. I figured they would be around 52.

Can anyone clear this up for me??

Thanks

Jake
 
#2
Jake,
A 30-0-30VAC transformer will get you pretty close. If you stay below about half the rated VA, the rail won't collapse, so the calculations will be about right.
That's assuming that the transformer is exactly 30-0-30VAC. Which isn't always the case (probably what happened with your 38-0-38). For instance, the Plitron 30-0-30VAC transformer I've got on my bench right now gives +-42VDC rails. Technically, it should probably be about 40.5 to 41V, but the Plitron secondary voltages run slightly over, so it offsets the diode drop through the rectifier.
If you're looking to have a pretty solid rail, I'd go for a 1000VA transformer, so as to have plenty of current on hand.

Grey
 
#3
Theoretically a 30V secondary transformer will get you 42VDC, not accounting for rectification losses here and there.For real, you will get a much lower figure say at about 37-38V after subtracting rectification losses and voltage sag during current draw. If your intention is to built an Aleph 2, get a transformer with a sec. voltage rating of 35V, that should give you plenty of room for the drop in voltage.

For better understanding, you should read thru page 7 of NP's Zenlite article.
 
#4
Thanks guys that helps.

I think I may have figured out some more of the mystery......
1) the primaries are rated for 115V on the Plitron standard transformers. I think here in the U.S. we get more like around 117V out, therefore, the secondaries will be 2 volts higher just from that.

2) I looked on the Plitron page and found that the secondaries are rated at full load, this could add another few volts when no load is present. (I measured with no load)

Anyway, just though I would mention that in case anyone else could use the info.

Now Grollins, I am intereted in the 30-0-30 Plitron transformer that you have......

did you measure the rails under full load??

I plan on purchasing a 30-0-30 standard toroid from them also. I would like my rails to sit at 42 under load so that I can minimize power dissipation in the output stages. (the original aleph2 has 45V rails, mine will have 42 or as close as I can come anyway)

Is it very noisy under load?? (can't have any noise messing up my sound ya know) 😎
 
#5
1) a 2V AC increase on the primary of a 115V to 30V transformer will increase the output by about 0.5V AC which when rectified means an increase of around 0.707V DC.

2)As you increase the load on the transformer, the transformer heats up and it's resistance increases which will lower the rail voltage and increase noise on the rails...
 
#6
The Plitron I mentioned is the 117017201. Actually, it's not 30-0-30VAC; it's two independent 30VAC secondaries. Rated at 1kVA. Yes, I've tried it on my Alephs, with the caveat that I was using one per channel (i.e. monoblock [otherwise it'd be an Aleph 4]--roughly 3.5A load instead of 6A). There was absolutely no sag on the rails at all at that current draw, and yes, it came out at about +-42VDC as I recall. Hardly warmed up at all, but Plitron uses cores that are heftier than what other folks seem to use. Sounded great.
Or, as a fellow I knew used to say:
If some is good, then more is better, and too much is just enough...

Grey
 
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