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Old 11th January 2007, 09:14 PM   #21
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Lovan,
Three possibilities:
1) Use bipolars with enough gain that a variation makes no difference. In the real world, what's the difference between, say, 20mV of ripple on one rail and 30mV on the other? Unless you've got a really, really twitchy circuit, it doesn't really matter at that point.
2) Use another transistor/opamp/tube/horsehair/etc. to run the gain up. Same argument as above.
3) Match the transistors. I've got batches of matched devices that I haven't used yet and I'm sure the same is true for other people. Put the blasted things to work. It drives me nuts to look at something that isn't being used. I'll sit and stare at it, trying to figure out some way to make it earn its keep.

Grey
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Old 14th January 2007, 01:26 PM   #22
Manu is offline Manu  Europe
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It seems there are only advantages to use a capacitance multiplier in an Aleph assuming some volts loss in the supply is Ok? Less ripple, less caps, more space left, etc.
Any drawback still?
Could somebody post what could be the ultimate Capacitance-multiplier_PSU for an Aleph (5 for instance)?
thx
Manuel
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Old 14th January 2007, 03:07 PM   #23
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Ultimate? If you ask ten people for an ultimate design, you'll get twelve replies.
Look at some of Nelson's Zen articles. There's also a decent writeup at ESP about an "ultimate" circuit, although it seems more like guilding the lily to me. Any of them will work just fine, as the circuit is not complicated (or need not be).

Grey
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Old 14th January 2007, 03:07 PM   #24
Lovan is offline Lovan  Sweden
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The voltageloss isn't that significant if you don't use zeners and use a BJT as the passelement.
If you use a BJT then capacitance(C1) at the base acts like a big capacitor with capacitance=C1*HFE. This also acts like a slowstart and can take a while to get fully charged.
I don't know what happens when you use a FET though?
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Old 14th January 2007, 04:03 PM   #25
Manu is offline Manu  Europe
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Thank you for the replies.

Grey .
You're right of course. That's the problem, when a non-expert like me read the litterature on this subject : you get too much answers.
You're right, 2 : the best is certainly to follow Nelson in the Zen articles.

Nevertheless I would like to have your opinion about the eventual drawbacks of this kind of regulation (can one tell it so if one don't use a voltage reference?) compared to a unregulated supply.

lovan:
Do you have some rules of thumb to choose the right value (s) of this base cap according to the requirements of Aleph design?

manuel
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Old 14th January 2007, 04:25 PM   #26
Lovan is offline Lovan  Sweden
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I'm thinking of using 4*33mF per rail(mono). One of those will be used in a multiplier. With a 2N3055 with a HFE of about 20 it would be like a 660mF. Maybe have two 2N3055 in a darlington. That would be like 13,2F!
But it would probably take a halfhour to charge it
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Old 14th January 2007, 04:33 PM   #27
hacknet is offline hacknet  Singapore
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i was simulating these capacitance multipliers awhile back and the problem i came to was that the resistor feeding the base of the pass element and the capacitor being multiplier is very important.

too small a value and the multiplier goes out the window because it cannot cope with the current variation, to big and there isnt much RC filtering.

is there any way to make the multiplier more efficient? i've looked into having another transistor to buff up the gain and that helps abit but im not an expert at this, anyone has any other ideas? i was thinking about a multiplier on an aleph and it i thought it was alright because the aleph's current draw is constant, am i right?
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Old 14th January 2007, 04:36 PM   #28
Lovan is offline Lovan  Sweden
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Hacknet:

How much current variations are you talking about?
I'm very interested in seeing some number
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Old 14th January 2007, 10:14 PM   #29
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Manu,
I'm not sure I'd call a capacitance multiplier a regulator. A regulator, at least in my view, is something with a fixed reference--something which a capacitance multiplier lacks, although the longer the time constant the closer it approaches one.
My suggestion is this: Build one out of Nelson's Zen series. It's more important to just go ahead and do it rather than obsess over details. This is an an instance where there's a considerable amount of latitude. If you do something at all reasonable, it'll work just fine. You can also modify it fairly easily if the notion takes you.
As for capacitance multipliers taking a while to charge up...I wouldn't lose a minute's sleep over it. The Aleph circuits are marvelously tolerant of variations in the rail voltage. I had a pair on a variac and diddled the voltage to my heart's content. Yes, if you halve or double the voltage, you should consider making a few changes in the circuit, but 10% either way is a no-brainer.
Lovan,
The equivalent capacitance isn't real. It isn't really 13F. The only thing that has to charge is the little cap under the control element. That's fairly easy to calculate if you're so inclined--just look at the resistive divider and the cap.
hacknet,
The Aleph's current draw isn't constant, although it averages out over time. The Aleph-X is different, it does sum to something pretty close to DC. That's why you need capacitance after the capacitance multiplier--to handle the instantaneous variations. I'd suggest settling for a really slow time constant and not worry about it.
I'm not clear as to what you mean about making the capacitance multiplier more efficient. If you want to reduce the voltage drop or the heat produced, use a bipolar. If you want something that is doesn't draw from the cap, use a MOSFET.
MOSFETs differ from bipolars in that they don't require any significant amount of Gate current. One reason a capacitance multiplier based on a bipolar takes a while to charge up is that the base is siphoning off electrons. MOSFETs have an effective Gate resistance of approximately a gazillion Ohms (ain't science wunnerful?) and as such draw almost no current. They neither insert nor withdraw electrons from the node where the capacitor connects to the voltage divider--at least not enough to worry about.
Capacitance multipliers aren't the ultimate solution, just another tool.

Grey
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