"Sound" of IRF240 bias point setting - diyAudio
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Old 21st January 2004, 12:30 PM   #1
Klaus is offline Klaus  Germany
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Question "Sound" of IRF240 bias point setting

Hi,

I am running a pair of Aleph4 mono blocks. One question about the optimal bias current through one IRF240/244. Many guys utter that the Aleph2 shall sound better than the Aleph4 – assuming equal build up of PSU etc. The main difference is that in the Aleph2 the power FETs biased to 0,6A each, at Aleph4 just to 0,4A. Has anyone tested it – does the bias point 0,6A sound better?

So why not just reduce the Aleph4 to 4 Fets with 0,6A each? What disadvantages would the loss of dampening cause? Appreciating you comments…

Regards

Klaus
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Old 21st January 2004, 03:01 PM   #2
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Default Klaus

When I built my Aleph 1.2's I replaced R19/56K2 (R26/121K in Aleph4 if I'm not wrong) with a 200K trimmer just to see what happens with bias changes.

Now, each amp has 10*IRFP242R output mosfets;
when biased @ 330mA each it sounded dry in mids, relatively thin and a bit grainy at the top end, whereas @ 430mA each these phenomena were gone.
Finally I set the bias at 4.5A and the amps sound wonderful.

If your heatsinks are large enough bias the hell out of it and you won't regret it, even if you'll have to change the mosfets in 4-5 years. Just keep a full set of replacement mosfets aside.

Tschuess,

Nickolas
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Old 21st January 2004, 03:28 PM   #3
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Default IRFP242R

And for anyone interested here is some data about IRFP242R
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Old 21st January 2004, 03:43 PM   #4
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Default IRFP242R

more data:
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Old 21st January 2004, 05:06 PM   #5
Raj1 is offline Raj1  United Kingdom
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Hi,

quick question
when a multitude of output devices are used in parallel, does this not affect the sound quality negativley too, all that extra input capacitence for each mosfet must roll off the sound somewhat removing air and fine detail?

Thanks

Raja
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Old 21st January 2004, 06:56 PM   #6
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It is my experience as a first approximation, that it is the
total bias which is the most important number. Within
limits, how many devices that is shared by is not as important.

I agree too, lots of bias. Hard to have too much.
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Old 21st January 2004, 07:59 PM   #7
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Default each device has an ideal bias point so...

How can the total amplifier bias mean anything? Each device has to be biased so it is operating well within its linear region with no chance of leaving it (Class A) during musical peaks so how is the TOTAL bias meaningful?
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Old 21st January 2004, 08:07 PM   #8
jam is offline jam  United States
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A non-believer!
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Old 21st January 2004, 08:25 PM   #9
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Default Re: each device has an ideal bias point so...

Quote:
Originally posted by GringoAudio
How can the total amplifier bias mean anything? Each device has to be biased so it is operating well within its linear region with no chance of leaving it (Class A) during musical peaks so how is the TOTAL bias meaningful?
That's pretty much just how it is. While each device becomes
less linear at a given current, each experiences less signal
current, and you get a cancelling effect.
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Old 21st January 2004, 09:17 PM   #10
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Default So at what point....

...does more bias just become wasteful? I see that unless musical peaks push the current into the outer maxima of the FETs' current handling abilities then you can just increase the bias ad infinitum. With enough devices sharing the load this is unlikely tohappen unless bias is crazy high. How do you decide how many devices to use and how high is high enough for bias? And BTW I will be bragging that Nelson Pass answered my DIYAudio questions alreadys o if anyone else can help me please do so too. Thanks!
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