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Old 8th May 2006, 07:49 AM   #71
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this is what I found better for my needs.

voltage at fets- 16V
bias 10A
number of fets-16
ac current gain 50%( this is good?)

question how I can up the bias? lowering source resitors or some other ways?
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Old 8th May 2006, 10:51 AM   #72
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50% current gain is "standard" - most people shoot for that.

If you look at cell B33 of the spreadsheet it says you want a 0R4 source resistor with 16 FETs - you just increase the resistance of the pots in the current sources until you get .5V across them.

You will need heat sinks that give you an R theta of 0.1K/W or less. I suspect that your current sinks are nowhere near up to the task.
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Old 8th May 2006, 12:25 PM   #73
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hi Bob thanks again for help. in excel calculations I can not change b33 cell value(LOCKED) ONLY B34
AS MUCH i CHANGE B34 VALUE i GET DIFERENT VOLTAGES ACROSS SOURCE resistors and ampers.

which resitors I need to change to increase bias. I have just bought bunch of pretty 50k ohm multiturn pots want to buy new. can I just put in series some resisitanse for them and with what values I should play? or I can change some resistors in schemtic.
BTW I tried to rise input sensitivity and changed 100kohm resistor to 130khom. result 15V ofset

another trouble(for impatience) I already buy 1500w 5x 20V secondaries transformers. how I can lower voltage of trafos I think I would nedd avout 12-14V secondaries (maybe aply inductor before caps LCCC?)

regarding my heatsinks. they dissipate with no problems 250W channel but 300w after 3 hours are to hot to touch more than 2-3sec.
I desided play safely and for prototype build huge enclosure with heatsinks- height 220 lenght 600 fins 40pieces of 4mm, base from 9mm. that woul let me dissipate arround 650-700 single or 1300-1400w both channels. without problems. I could use bigger rails witht his heatsinks but I do not need hundreds of wats. 80-100W for 8ohm and 200w for 2.5ohm just be fine.
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Old 8th May 2006, 08:19 PM   #74
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Cell B33 is the spreadsheet telling you what source resistor you need to achieve the desired bias. You adjust the variable resistors in the aleph current sources (not the one in the ccs that feeds the inputs.) As you increase the value of that variable resistor your source resistor voltage should increase.

Cell B34 is where you input the standard value closest to B33. You then see the power dissipated there in the cells below.

An L input filter would definitely lower the rail voltage. Find Duncan Amplification's PSU designer and simulate your chosen arrangement.

If you can hold your hands on the heat sinks for 2-3 seconds they are probably at around 55C. With standard isolators (Use .7 in B68 and B69) you should be OK. You can always use a small fan to help cool.
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Old 9th May 2006, 06:24 AM   #75
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yap I understand it after worked more with this program. lets say I want to up tje bias with parareling devices packs(mosfet, source resistor, 221ohm resistor) from 8 to 16. that means that I am already getting more bias with standart values. do I need change any other components in schematic? I am studying at the moment grolins and kristjan schematic differences. grolins has only 4 devices but highly biased and kristijan 8devices with looks like also 5 amps bias. pots are 100kohm to grolins and 50kohm to kristijan schematic.
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