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Old 9th February 2002, 05:17 PM   #51
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Well then... should I be worried about the length of wiring from the front end to the final stage? In my original plan I'd have to use leads as long as 8 to 10 inches. Would that cause a problem?

I have also settled on a solution to accept the offer on Fairchild IRFP240/9240, unless something extremely surprising turns up.

Can I just assume that the lot size of Fairchild IRFP240/9240 is 30, or is there something to gain in requesting that multiple tubes be from the same lot?

Then, where did I saw someone, I am not sure was it NP or not, comparing lateral mosfets and hexfets and exploring the transconductance issue in detail?

-Kimmo Sundqvist
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Old 10th February 2002, 03:44 AM   #52
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Quote:
Originally posted by Musher
Can I just assume that the lot size of Fairchild IRFP240/9240 is 30, or is there something to gain in requesting that multiple tubes be from the same lot?
-Kimmo Sundqvist
Definately request that all the mosfets are from the same lot.
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Old 11th February 2002, 01:32 PM   #53
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About transformers....

I will idle both channels at 100 or 120W. Do you think one 545VA transformer could take that and live long?

Also, if I put a 2x18V smaller transformer to the end of it, will that make enough voltage for the front end? Main transformer will have 2x24V secondaries and I will end up with +/-32.4V supplies. That 18V would give +/-49V supplies for the front end. Enough or too much, or what would you recommend?

-Kimmo Sundqvist
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Old 11th February 2002, 02:13 PM   #54
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Quote:
Originally posted by Musher
About transformers....

I will idle both channels at 100 or 120W. Do you think one 545VA transformer could take that and live long?

Also, if I put a 2x18V smaller transformer to the end of it, will that make enough voltage for the front end? Main transformer will have 2x24V secondaries and I will end up with +/-32.4V supplies. That 18V would give +/-49V supplies for the front end. Enough or too much, or what would you recommend?

-Kimmo Sundqvist
Dont do that.

If your going to use separate transformers for the front end then make sure they have the correct voltage rating to work on their own ... you shouldnt have the secondaries of seperate transformers connected in any way as it will almost certainly cause a number of problems. Generally speaking, the front end is usually given rails that are approximately 15V higher than the output stage. Also, you should probably expect that output stage rail voltage to sag somewhat under load.
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Old 11th February 2002, 06:19 PM   #55
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Actually, tagging secondaries from one transformer onto another works quite well. Just make sure that the phase is correct and that you don't exceed the current draw of whichever transformer is smaller.
Two channels idling at 100-120W would be 200-240W total--quite safe for a 545VA transformer.
The math on the 24V secondaries yielding approximately 32.5V rails looks okay, but I'm not quite sure how the 49V rail was calculated for the front end. If I understood the original question correctly, I'd expect the rail to be more along the lines of 57V, thus:
24VAC+18VAC=42VAC
42VAC*1.414=~59.4VDC
59.4VDC-(diode losses, etc.)=~57.5VDC

Grey
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Old 12th February 2002, 05:50 AM   #56
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If you chose to do that, you'd want quite a good regulator for the front end cause the output stage would quite a bit of modulation the front end power in such a configuration.
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Old 12th February 2002, 11:00 AM   #57
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ok i got done by the time limit

what i meant to say was that using such a configuration would cause the output stage to modulate the front end power supply quite a bit and so you would need to use quite a good requlator / filter to ensure a clean supply for the front end.
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Old 13th February 2002, 02:50 AM   #58
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If you're pushing the circuit hard enough that the rail collapses--or at least bends a bit--then I can see your point. I don't load my transformers that heavily. I lean towards really stiff power supplies. Those who build otherwise will need to take into account the possibility of saggy rails. I suppose you could send 'em to a plastic surgeon for a bit of a nip 'n tuck, but I'd rather have the young smooth ones.
Even so, with roughly 57 volts to play with and a target somewhere in the 40s, you've got plenty of headroom for a regulator. Just watch the heat dissipation in the pass device.

Grey
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Old 14th February 2002, 01:34 AM   #59
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agreed
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