what resistor to use in Aleph-X

[joke]

O.K. here I am again with one question that I have asked before , but I cant find document where I saved answer to this question so ... here it is ...

I 'am building Aleph-X and I am planing to use 2 pair of mosfets per side , or total 8 per chenel ... I have transformator with 2x 15,20,25,30-40V 1000W ...and I am planing to use +/- 20V for suplay...and hopeing to get something like 50W per chenel...
So my question is , what value to use for mosfet ( IRFP240 ) source resistors ( R5,6,41,40) ???
For 15V people use 0.22 Ohm and I see that for 18-20 thay use 0,47Ohm or 0,68Ohm , so what should I get ?

And one more thing ... Is R21 3 Ohm resistor realy important ( in revision 1.0 )??? can I use some other value ??? Or can I jumper it ???



Thanks
Josip

 

[grataku]

Joke,
didn't you buy some mills from me last year?
Anyway, if you are not going to parallel mosfets, I wouldn't go higher than 0.33 ohm. 0.47 would really only work for a 8ohm load. If you parallel mosfets it's a whole differrent game.

 

[joke]

Hi Grataku...Thanks for your replay...
Yes I did..but I missplaced them , lost them, oll 50 peaces ( with some other parts ) ...aggghhhhh...
Whell when I was moving from one place to another ,I just....don't know....
So I 'am back on start...at least I build housing for aleph...

I am planing to order rest of the parts that I am missing so....

Paolo, you wrote me once that I should use 0.68 Ohm for 20V , as I remember you say that with 0.47 Ohm I would have more disipation per mosfet ...0.47 Ohm would be bether for 18.5 V ...
Whell I belive that under load 20 V would be litle less so maybe I should stick to 0.47 ...
I would paralel 2 mosfet and load would be 8 Ohm , min 6 Ohm...so what should I order ???

Thanks

 

[jarek]

And one more thing ... Is R21 3 Ohm resistor realy important ( in revision 1.0 )??? can I use some other value ??? Or can I jumper it ???

You can ommit R21 (3 ohm resistors). I do not use them, just short a jumper.

 

[Netlist]

Quote from Grey in the AlephX thread:

"To anyone who wants to really shave this circuit down, you can remove R21 entirely. Also D2-5. You'll be exposing yourself to the possibility of ground loops (not really a common problem) and opening the door to static discharge blowing a hole in your front end MOSFETs. But if you're careful when hooking and unhooking the signal connections, you don't really need them. Note that the A-75 doesn't have them, and there are a number of those cruising along happily even as we speak."

Good luck in building these beauties

/Hugo

 

[joke]

Thanks for replays...
Any toughs on source resistor ?

 

[Netlist]

I think Grataku pretty much answered your question but if you want the original, here it is:

"To calculate the Source resistors, divide the total bias current in half to get the amount for each side. Since I'm assuming one pair of devices per side (four total for the channel), we'll end up biasing the pair at 1.9375A (3.875A/2 sides). We'll set the voltage at .5V across the Source resistors (don't forget that you can adjust this a bit with the pots). From here, it's a simple Ohm's Law calculation: .5V/1.9375A= .258 ohms."

And:

“There's nothing magic about .5V--you're free to set it where you like.”

/Hugo

 

[joke]

Thanks everyone...



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