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Old 9th January 2002, 10:42 PM   #11
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http://www.passlabs.com/products.htm, the part about X150 has 50W quoted, and I can't figure out how could I have misunderstood it.

About the actual topic of the thread... I have bought about a year ago a 1000mm piece of Fischer SK110 heatsink, assuming it is the H-shaped 12x12cm big big one and I got the number right. It's 0.05K/W in that size, and I have plans to put it into four 10" parts, 0.2K/W each.

Now, into 25cm of the heatsink I only know how to fit 8 TO3s. An X250 has 32 TO3s, 16 for one channel. X350 1.5 times that, 24 per channel. X150 has 20, 10 per channel, though I am a bit lost on why that 10 is not a multiple of 4.

I don't want to use more than 25cm for a channel, since a piece that big can dissipate 150W quite well. And since I can easily get chassis made of 0.75mm galvanized steel sheet but not of aluminium, I more like the idea of having the heatsinks towering up from the amplifier (like in that A40 project in passdiy project gallery) than covering the sides of the chassis.

Also, if you know any MOS-FETs rated for more than 125W (of IRF9240 and 240) then please tell me. 150V of voltage rating would suffice, maybe even as little as 100V, but for that low it must otherwise be _really_ good.

-Kimmo S.
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Old 9th January 2002, 11:13 PM   #12
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generally speaking the lower voltage mosfets have less distortion so you should try to match them fairly close to the rails allowing for a safety margin ....... given that the rails for a X150 are less than +/-30V 100V devices would get plenty of safety margin so long as the wattage is large enough .... as Nelson has stated that the X150 has approximately 15W Class A per channel as we calculated and that the 50W stated is the total peak class A and that he will have this rectified on the Pass Labs site soon .... each channel should have a multiple of 4 mosfets in the output stage because there is 4 banks per channel so i dont know how it uses 10 devices per channel but you could also use 12 devices per channel giving 4 or mosfets per bank and a little more safety margin.
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Old 10th January 2002, 07:24 PM   #13
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The X150 has 40 outputs total, for 20 per channel,
arranged in banks of 5.
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Old 10th January 2002, 11:48 PM   #14
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well that explains it .... thanks Nelson.
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Old 13th January 2002, 10:16 PM   #15
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I can only get transformers with 18V and 24V secondaries easily. I think 18V turns to +/-26V unloaded rails with a 545VA Finnish transformer that has 6 percent of voltage drop. That maybe isn't enough for me, so 24V gives me 35V unloaded rails, and about +/-34V loaded.

This way I'd have output a bit more than X150 has, but a bit less than X250 has. What figure should I use for MOS-FET voltage drop at maximum output? I don't believe I can saturate them and still retain full quality.

Now everyone go and see http://www.fischerelektronik.de/fisc...Seiten/A67.pdf

I have one metre of that lower one, SK110. I will put it into 4 pieces, each 10". If anyone has suggestions how to mount the maximum amount of transistors on one 10" piece, please share your ideas. It should be a beautiful, yet efficient way, because the heatsink will be left visible when the thing is completed. One channel will have one 10" piece, and I am planning for two stereo units total.

I can have 150W of quiescent per channel, if the transistors can take it, since the heatsink can. A 10" piece will be 0.2K/W, if I haven't said it earlier. Maybe, just maybe, I should settle for 26V rails, since I don't really need the power anyway.

Tell me what you think, so I can go on obtaining many many IRF9240/240 and matching them. Or any better suggestions than IRF? Then I will have the problem of determining how the front-end is exactly made. With parallel JFETs? I will use current sources, but how about led as a reference?

-Kimmo S.
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Old 14th January 2002, 12:51 AM   #16
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you could use a capacitance multiplier or similar in the power supply to regulate it down to +/- 25V but anyway if the house wiring loads down 5% and your transformer loads down 6% and you'll lose ~1.5V in diodes then it will output about +/-28V DC so your pretty close to the mark.
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Old 14th January 2002, 03:29 AM   #17
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Kimmo,

If your speakers are 8 ohms or less, I'd use the +/-25V supply and power the front end from an elevated supply derived from a voltage doubler as in the Pass/Thagard A75 project. Using a doubler will give you a raw +/-50V, which you can knock down to a smoother +/-40V or so by adding a series R and shunt C in each leg of the supply. You can also regulate the elevated supply or add a capacitance multiplier as suggested above, but if you use current sources in the front end, I suspect that the amp will deliver very clean performance without it.

With this setup, you'd be able to bias the output stage up to 2.5A per channel, for 25W class A into 8 ohms, as well as drive the output stage to over 100W in class AB into 8 ohms. This strikes me as a pretty good compromise between having a pure class A amp and one that can drive moderately efficient speakers to realisitic levels without clipping.

OTOH, If your speakers are higher impedance, you might be happier overall with the higher voltage supply and a lower bias. Either way, using a separate front-end supply should let you drive the output to within 2-3V of the main supply rails. You can do this safely with MOSFETs as they don't saturate and "stick" the way bipolars do.
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Old 15th January 2002, 11:11 PM   #18
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It seems that IRF640/9640 are much better available here than 240/9240. Though I found 240 but not 9240, it was almost 6 euro apiece. 640 is 1.5 euro and 9640 is 2.7 euro. Last time I checked, $1 was 1.15 euro (and it's been a while).

640/9640 are TO220, but 125W, just like TO3 240/9240. How big difference that is practically? I think I should use double the amount of 640/9640. Or maybe 1.5 times. Anyway, is there anything or something I haven't noticed that would make 640 less suitable or worse for this project than 240?

-Kimmo S.
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Old 16th January 2002, 12:41 AM   #19
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I used IRF644s (also a TO-220 case) for my Aleph 2s. In addition, I'm running the bias current about 10% over stock. That works out to about Pd=25W per device. Nary a problem. Keep in mind that I'm running a water-cooled system, so I can dissipate heat easily; if you use plenty of heatsinking (a good idea, anyway) you'll be fine.

Grey
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Old 16th January 2002, 09:35 PM   #20
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Then to the problems with front-end

As a starting point I will use the schematic posted by Petter. Since I am not an electronics professional, I am left with gathering pieces of information from nice people, and none of these things are my own invention.

1. I think I know how to make current sources. And if I don't, I know how to see a book or two for assistance. Nobody has said much about using a led as a voltage reference, but I will try that out unless something better appears.
2. All current sources could have the same heatsink, or at least heatsinks of equal size.
3. How can I parallel JFETs and is there an improvement in using them somewhere in the front end? Really odd that this thing seems like "plagued" by DC drift problems... wonder how Mr. Pass did it. Maybe with trying and testing, since I think he had to find the right combination only once.
4. I will use either 26V rails with 120W idle dissipation (10W for each device), giving me maybe an X100 =) or 35V rails, because 545VA transformers are not available here with 18V secondaries, only 24, 30 and 38. For 18V secondaries I'd have to use two 210VA or even 330VA.
5. If I use a capacitance multiplier with the front-end rails and end up with +/-60V in case of 35V endstage rails, do you think that is enough?

-Kimmo Sundqvist
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