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Old 2nd January 2004, 08:13 PM   #21
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Default MOSFET need to use mica insulator?

One more common question, does Q1 ~ Q3 IRFP9610 need to add the mica insulator at all?

From the schematic, it seems no; but I saw a few saying it must, or the bias will be hard to adjust! Are they right or not?

Thanks,

Thomas
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Old 2nd January 2004, 08:31 PM   #22
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All three mosfets must be electrically insulated from each other. thermal coupleing makes no difference in bias. Thermally coupleing the diff pair will make the dc offset more stable, but most people don't bother. Most people don't even use sinks.

I bought 2 bars of copper 3/16" x 2" x 36" on Ebay for something like $18.00. Made nice busses
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Old 20th February 2004, 08:45 PM   #23
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Default Help with large filter caps

Hi, I'm thinking about using 4 x 27,000 micro F at 63 Volts for +- 45 volt rails. It's for my very first Aleph project. Question is if this is enogh per channel.
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Old 20th February 2004, 09:37 PM   #24
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It'll do but more would be better. Also make it a CRC to get the most smoothing possible from the ufs you have.
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Old 20th February 2004, 11:40 PM   #25
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If I do a CRC layout, Do i have to change the secondary voltage of the Transformer ? What would the Resistence value and power rating be? Should I keep the same voltage and up the capacitance?

Thanks.
SAAD//
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Old 21st February 2004, 02:41 AM   #26
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This is why people recommend L instead of R...it doesn't change (much) the voltage coming through. With resistance, it's a balancing act. The more resistance you use, the better your filtering will be. The flip side of the equation is that the more R you use the more voltage drop and heat you'll have to deal with.
Inductors are superior but cost more, and are bulkier & heavier. Bulk and weight aren't a problem to DIY folks, but cost is a nuisance to those whose wallet has been on a diet. (Hint: Wind your own.) If you go with resistors, you can calculate the heat (I squared R), and voltage drop (E over I) easily enough, just plan ahead and you'll be okay. Unless you want to get really nutso, stay under 5 ohms; don't be ashamed to go under 1 ohm.
Don't get confused when you read the specs on an inductor and see that it has, say, one-tenth of an ohm resistance. It's apples and oranges. It's the inductance, not the resistance, that makes an inductor worthwhile. Inductors have the added benefit of increasing filtering as the frequency increases. Resistors have a flat response. This translates as inductors being better at taming incoming RF and such.

Grey
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Old 21st February 2004, 04:47 AM   #27
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We have a Metal Supermarket here in SLC. I love those guys over there...not the cheapest to buy from but they also do any shearing or cutting for free while many metal vendors charge anywhere from 2.00 to 30.00 per cut. They also will order in anything you might need that they don't normally stock and you only have to buy what you want of it.

Mark
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Old 21st February 2004, 10:45 AM   #28
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Also beware with the CLC. The inductor is not able to dissapate more than about 8 watts or so without getting hot enough to melt the insulation in the center of the coil. My first try was around 2 ohm, 2.5 mh. the extra resistance made the PSU sim look great, but at 6A the inductor dissapates 18 watts and gets too hot for comfort. I'm resorting to mounting a big *** pair of 3 ohm 100 watt wire wound vitreous ceramic type resistor on the outside of the amp in back because I need to dump a few volts.
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Old 23rd February 2004, 04:58 AM   #29
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I think I'm going to try both CLC and CRC filters.

Mr, Rollins

What would the inductor value be if I'm trying to shed couple of volts. The Tranformer I'm looking at has a 35v+35v secondary and If I 'm looking to get +- 45v at the rails, according to Mr.pass my Transformer secondary should be 33v+33v.

Mr, Donaldson,

I was wondering if the total bias current is my "I" and voltage I want to drop is my "E" when I'm calculating for the resistor.


All the help is greatly appreciated.

Regards.
SAAD//
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Old 23rd February 2004, 09:03 PM   #30
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SI,
If you're wanting to drop voltage, it's generally better to use resistors. One of the reasons to use an inductor is that it keeps voltage losses to a minimum.
As for a resistor, one or two ohms should do the trick. Make sure the resistor has sufficient wattage capability.
Of course, you could also use both inductors and resistors.
On the other hand, the Aleph 2 would be perfectly happy with that extra couple of volts.

Grey
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