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"Sound" of IRF240 bias point setting
"Sound" of IRF240 bias point setting
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Old 22nd January 2004, 12:41 AM   #11
GRollins is offline GRollins  United States
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Decide how far you want to push each device thermally and go from there. The bias current per device x rail voltage will give you the wattage dissipated for each output device. At that point you simply use enough devices to get your target bias, which you determined beforehand by either looking at the lowest load you intended to drive or how crazy high you wanted to get.

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Old 22nd January 2004, 06:54 AM   #12
Klaus is offline Klaus  Germany
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Default cooler...

Thanks for all the answers!

Well, I regret a little bit to have not bought laaaaarger coolers… Now I have somewhat like 30K warm up delta. In summer 60°C at the coolers. I will test the easy R19-tweak and test the sound change to maybe 0,47A per FET, equals +35K.


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Old 22nd January 2004, 03:30 PM   #13
GringoAudio is offline GringoAudio  Canada
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Default WOW Grey too!!

Thanks to the gurus for your bias answers!!
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Old 22nd January 2004, 07:16 PM   #14
Nelson Pass is offline Nelson Pass  United States
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"Sound" of IRF240 bias point setting
Default Re: So at what point....

Originally posted by GringoAudio
...does more bias just become wasteful
At some arbitrary point, yes.
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Old 22nd January 2004, 07:33 PM   #15
Bakmeel is offline Bakmeel  Netherlands
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I am wondering... would there be a physical explanation for the improvement of the sound quality? Could you estimate "sound" if you had some data of a fet used and its bias setting?

It would surely be interesting to know, it would be an aid to selecting fets and bjts...

More Power Igor! More Power!
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Old 23rd January 2004, 03:26 AM   #16
grataku is offline grataku  United States
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I thought there was a limit to how much bias/how many dev in parallel. Besides the SOA there is the input capacity of the mosfet. Too many devices in parallel and the amp will start to sound "dark". NP mentioned something to that effect starting to happen in the biggest aleph.
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Old 23rd January 2004, 04:21 AM   #17
GRollins is offline GRollins  United States
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Aye, but that will vary with the choice of device. Want a heavier, darker sound, perhaps for subwoofers...go for higher Gate capacitance. Or go for less capacitance for a tweeter amp.
I'd place that in the "voicing" category rather than the "heat" category when it came time to make decisions.

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Old 23rd January 2004, 05:28 AM   #18
moe29 is offline moe29  United States
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is there a relationship between number of parallel devices and
rail voltage? Higher voltage spread over more devices is equivalent
to lower voltage over fewer (eg. 2 devices for Aleph 3, 6 for Aleph 2).
Equivalent in that more voltage in more devices overcomes increasing gate capacitence?

(i feel like a 1st grader in Algebra class again!)
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Old 23rd January 2004, 05:55 AM   #19
Brian Donaldson is offline Brian Donaldson  United States
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All of the rail voltage is dropped across each fet. you use more fets on larger alephs to decrease current/fet so you don't exceed max wattage per fet. Aleph 2 is .5A at 45V per fet= 22.5Watt. Smaller Alephs have smaller voltage and maybe slightly more current over fewer fets while still not exceeding 25 watts/fet.

I had the bias cranked a little too much and had a fet and source resistor melt down. I think the faital blow was about 4 months ago when I was putting them through the ringers and enjoying too much bias. and they sounded great. I let the sink temp reach 175 deg before I knew it. It seemed to survive, but I bet that weakened the chips bond to the tab and let one of the current source (pos rail) fets short and the active set tried to fight it. I'm hell on fets. 80 volts at 25 amps and 160000 ufs can make a lot of magic smoke very quickly. The 8 source resistors looked like little halogen lights on a dimmer.
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Old 24th January 2004, 08:19 AM   #20
Samuel Jayaraj is online now Samuel Jayaraj  India
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Nelson says that it is the total bias and not the bias of each individual mosfet that matters. These may well be true for Vertical Mosfets running in Class A with a relatively high bias current!

One question in this regard is, what if one were using Lateral Mosfets of say, the Hitachi type; these devices require an optimum bias of 100mA. Hence, when parallelling these devices, would one multiply the number of output devices with 100mA or would TOTAL BIAS be set without taking into account the actual current flowing through each mosfet? In the latter case, would these devices still exhibit their negative TC characteristics?
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