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Old 12th January 2004, 07:11 PM   #1
JanDH is offline JanDH  Netherlands
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Lightbulb Aleph-X Some Thoughts

As a novice I have looked at a lot of amplifier designs and decide to go for the Aleph-X. Iíam going to build a three way active cross-over and of course three times an amplifier per channel. Iíam thinking of : 20 Watts for the tweeter, 30 watts for midrange and 150 watts for the woofer. I noticed that multiple IRFP044N are placed parallel in order to obtain a certain capacity.

- Well why donít I see the IRFP064N or IRFP1405 applied.
- Why are there in the power stage relatively many resistors used. Would eg the MP930 series from Caddock not do very well. High power and low induction.
- Does it make any sense to separate the power for drivers and power stage? (Regulated power for driver?)
- I noticed a relative low rail voltage. Why not going for a higher supply in combination with eg IRFP1405

Pls any advice is very welcome in order to make the right decisions for the project I have in mind.

Thanks!!
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Old 13th January 2004, 03:31 AM   #2
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" Well why donít I see the IRFP064N or IRFP1405 applied."

I suppose you could if the capacitance, transconductance and other parameters are similar to IRFP240.



" Why are there in the power stage relatively many resistors used. Would eg the MP930 series from Caddock not do very well. High power and low induction."

Many have used the Caddock and other similar type resistors in the Aleph amplifiers. I doubt that you would hear much of a difference between them and high grade 1% wire wounds though. The Caddocks also may need to be heat sink mounted in order for them to be able to operate at max dissipation. That is somewhat of a disadvantage along with would amount to a very marginal gain in sound quality if any at all. They are also very expensive.




"Does it make any sense to separate the power for drivers and power stage? (Regulated power for driver?)"

Actually there is a constant current source for the input stage, and in these Alephs ther is only an input and an output stage anyway. There is not really a driver stage in there per say.




" I noticed a relative low rail voltage. Why not going for a higher supply in combination with eg IRFP1405"

An X amplifiers rail voltage requirement is dependent on a number of things including output stage bias, output power requirements, and power required into low impedances, say 4 ohms. It would be nice to hear from Grey Rollins on this one


Also keep in mind that all the semiconductor devices need to be hyper matched, or even better, matched in place in the operating amplifier circuit you intend to use. This is necessary to achieve the lowest possible DC offset at the output terminals. So be ready and set up for doing that!!

Mark
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Old 13th January 2004, 11:11 AM   #3
JanDH is offline JanDH  Netherlands
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Thumbs up Aleph-X Some Thoughts 2

Hi Mark,

Thanks for your reply.

My comment starts with '-'

" Well why donít I see the IRFP064N or IRFP1405 applied."

I suppose you could if the capacitance, transconductance and other parameters are similar to IRFP240.

- Yes I think you right however devices parrallel also double the capacity, at least I learned so.

" Why are there in the power stage relatively many resistors used. Would eg the MP930 series from Caddock not do very well. High power and low induction."

Many have used the Caddock and other similar type resistors in the Aleph amplifiers. I doubt that you would hear much of a difference between them and high grade 1% wire wounds though. The Caddocks also may need to be heat sink mounted in order for them to be able to operate at max dissipation. That is somewhat of a disadvantage along with would amount to a very marginal gain in sound quality if any at all. They are also very expensive.

- Oke but we are working on these projects because we want it all. Some folks think that parallel resistors (or evices) create e 'foggy sound'. The prices from mouser.com are acceptable.


"Does it make any sense to separate the power for drivers and power stage? (Regulated power for driver?)"

Actually there is a constant current source for the input stage, and in these Alephs ther is only an input and an output stage anyway. There is not really a driver stage in there per say.

- Almost every amplifier has a current source. People like http://www.borbelyaudio.com/ and http://www.lcaudio.com/ go for this theory.


" I noticed a relative low rail voltage. Why not going for a higher supply in combination with eg IRFP1405"

An X amplifiers rail voltage requirement is dependent on a number of things including output stage bias, output power requirements, and power required into low impedances, say 4 ohms. It would be nice to hear from Grey Rollins on this one


Also keep in mind that all the semiconductor devices need to be hyper matched, or even better, matched in place in the operating amplifier circuit you intend to use. This is necessary to achieve the lowest possible DC offset at the output terminals. So be ready and set up for doing that!!

- Yes as I noticed from the excel configuration sheets. It is difficult to order all info from this site and filter only this information that is important.
However I get the strange feeling that this amplifier is a potential dangour for my valuable speakers isn't it.


Regards,

Jan
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Old 13th January 2004, 01:48 PM   #4
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As stated in some other threads around here: if possible, avoid using TO220 devices as output transistors. Though the IRF1405 looks impressive from the datasheet, you will run into problems if you have to get rid of serious amounts of heat. I have cooked lots of them in high power dc motor driving applications at only 10 watts of power losses.
And, those extremely high power FETs indeed have an enourmous amount of input capacitance, you will need serious driving current to feed them.
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Old 13th January 2004, 04:16 PM   #5
jwb is offline jwb  United States
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The IRFP044, 140, 240, etc. series of TO-247 transistors really are the best. They can dissipate quite large heat loads without much trouble.

I think you'll find that if you are using parallel FETs in the output stage, you probably don't need to use huge power resistors. In my Aleph-X I was able to use BC 1W metal film resistors for every place in the power stage. They are far less bulky than 3W types and non-inductive.
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Old 13th January 2004, 04:56 PM   #6
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Please note that both the Aleph and Aleph X amplifiers
require that the negative rail of the front end requires
the same voltage connection as the output stage.

The only thing you can gain with a separate + supply for the
front end is perhaps a bit less noise, but even then you can
achieve this with some decoupling to the supply rail which
powers the current source for the input diff pair.
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Old 13th January 2004, 09:31 PM   #7
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There's nothing sacred about the '044. If you've got some other device you'd like to try, feel free to do so. Just keep an eye on the SOA for whatever you decide to put in.
I use Caddock resistors in mine. I've got/had several versions of the Aleph-X drawn up (found some schematics that seem to have survived the recent hard disk adventure--thank goodness for floppies). As I said in the Aleph-X thread, the circuit I run isn't quite the same as the one I published. Given the high cost of Caddock and other boutique resistors, I felt that the circuit would reach a wider audience if it didn't call for comparatively rare, mega-buck parts. It's the values that are important. If you want to use Vishay, Caddock, Mills, etc. and your wallet is thick enough, then go for it. Consider heat sinks for the Caddocks, as they're only good for a couple of watts without them. The internal solder connections come unstuck if you overdo it.
Regulate the entire rail if you really want to max the thing out. I'll eventually go that route, but am not doing so yet. The regulator circuit is ready and tested and I've even got the parts, what I lack is the time to do a circuit board.
I'm not sure why people are so startled by the low rails; it's a bridged amp, and low rails/high current go with the territory. If you want to use higher rails, you'll end up with a higher powered amp. No problem. But...you'll have to increase the bias in order to keep from current limiting into lower impedances, and that implies lots and lots of heat. You'll need big heat sinks. Or you could consider going water cooled. Broadly speaking, the rails determine the output wattage, and the bias current determines how low an impedance you can deliver that wattage to. If you're not careful, you'll end up with an amp that can deliver 500W...into a 100 ohm load. Clearly not a practical amp for general use.

Grey
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Old 28th August 2004, 08:12 AM   #8
macka2 is offline macka2  United States
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Grey & Nelson,

At the moment I am looking to scale several X Alephs to bi amp and eventually quad amp

I have done a search but not found anything specific regards low supply rail limitations.

I recall Nelson at one point saying idealy the X Aleph is best with 25 volts. Is that per rail or rail to rail.

Just curious.

Ian
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Old 28th August 2004, 04:50 PM   #9
jwb is offline jwb  United States
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Mine are working with ± 14V DC rails, and I find it perfectly fine. Of course it will depend on the load you want to drive. If you are going to biamp or even quadamp a speaker, you can obviously afford to build smaller, less powerful amps.
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Old 28th August 2004, 05:24 PM   #10
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Ian,
At the moment, I've got an Aleph-X running at roughly 20V rails (a little over 60W output) and a bodacious amount of bias current. No real game plan there, it's just that I had a pair of power transformers on hand that seemed lonely.
Regarding distortion:
Some preach the Gospel of Bias Current as though it's the only thing going. Ain't so. Rail voltage is important, too. Somewhere in the original Aleph-X thread, I posted some numbers relating rail voltage to distortion. No, I don't remember where, but it's near the back of the thread, so start at the end and work forwards. The upshot is that increasing rail voltage decreases distortion up (or down, depending on your point of view) to a certain point. Obviously, you'll never get to zero. Where's optimal? Well, that's a complex question. You've got to balance rail voltage (higher is better), against bias current (higher is better), and heat dissipation (less is better), the capability of your chosen power transformer, heatsinks, the thickness of your wallet, etc. etc. etc.
It's enough to drive a man to drink.
Hmmm...interesting idea, that one.
There's no one right answer, just as there's not really a wrong one. The 15V rails I showed in the original schematic are there because I developed the Mini-A first, and it ended up having 15V rails because my target power was 10W. The Aleph-X is essentially two Mini-As back to back. There's nothing sacred about 15V rails. I ran up the Aleph-X in its posted version just as a demonstration of concept, not something to be slavishly adhered to. Feel free to adapt the critter to whatever rail voltages/currents you have available. As long as you attend to a few part values, it'll take pretty much whatever you throw at it.
Note that I don't take credit for the thing. Nelson just left a few building blocks on the table and I got to playing with them, seeing whether thay would fit together. They did. But they're his building blocks. He was gracious enough to let me play with them.

Grey
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