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Old 9th October 2006, 06:25 PM   #21
MikeB is offline MikeB  Germany
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Hi John, it looks reasonable, but nothing special. And it contains the standard mistake, the vbe mutliplier is built the way that the amp will blow when pot-wiper looses contact...

But, it will never give 100watts into 8ohm. Power output is quite similar to the symasym.
Also, bass might be weak with the caps chosen.
The IRFs will work.

Mike
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Old 9th October 2006, 06:34 PM   #22
mlloyd1 is offline mlloyd1  United States
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you CANNOT (well, I suppose you could , but you SHOULD NOT, if you want to be successful ) substitute vertical/hexfets (IRFP240 and IRFP9240) for lateral mosfets (2sk1058, 2sj162) unless you do a fair amount of redesign. They have different thermal characteristics. To keep it simple, the hitachi parts have a zero tempco bias point around 100mA, the hexfet/verticals have a zero tempco bias point of several AMPS!!! Since we will not be biasing them at that current (well , maybe Nelson will ), you need a bias regulator of some sort. The Vgs or Vbe multiplier mounted in thermal proximity to the output devices suggested by some folks is one way to do that.

the biasing arrangements is one area where the lateral fets are simpler/easier to use. Set your circuit to provide Vgs bias on the output devices to set quiescent drain current to apprx 100mA per device (somewhere around 0.5V Vgs). since this is the zero tempco point, you can use a simple variable resistor to set the current. this is not perfect, but it works and is simple.

there is more but let's keep it simple ...

slone's amps use lateral mosfets ...

mlloyd1

Quote:
Originally posted by jmateus
luisxssj3

In fact this schematic makes much more sense, is this something that
could be implemented with IRFP240 and IRFP9240? I really don't
know the difference between vertical and lateral mosfets,
...
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Old 9th October 2006, 07:00 PM   #23
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Quote:
Originally posted by ilimzn


The thermal drift of MOSFET threshold voltage is not subject to 'some beliefs' but to physics. So, unless you try it with zero bias, suggesting to those that are trying to learn that VMOS, DMOS, HEXFET and other similar MOSFET topologies are direct drop-ins for lateral MOSFETs as used in most of these schematics, without any other changes is irresponsible, to say the least. You are lucky I am not a moderator here because IMHO this would be cause for a ban. To make it worse, posts like this continually perpetuate this disinformation.
Is this a flame on my part? Not really - I would suggest that you try yourself what you are suggesting to others, and there will be flames enough.
It is not the first time when you perform an attack on personality when you disgree with some technical solutions. I'd suggest you to take some courses in colledge, it helps.

Building amps with FET outputs I've discovered that they work well on microcurrents, so high bias is not needed at all. It is the fact that can't be denied by any moderators.

When I mentioned a belief, I mean belief that class AB is better than class B. It was true for tube amps without feedback (or with low gain in the feedback loop) long - long tme ago, when class AB assumed a common cathode resistor for output tubes that introduced a negative feedback at least for DC current. In case of symmetrical transformerless transistor output, no matter is it FET or BJT output, it is completely different story, and class AB means more idle current and LESS stability of a working point without audible gain in the sound quality.

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Old 9th October 2006, 07:09 PM   #24
SY is offline SY  United States
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Well, I didn't take any courses on electronics when I was in "colledge", but the differing tempcos look like a serious issue, at least to my admittedly limited level of understanding.

With micro idle current, what does the distortion spectrum look like?
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Old 9th October 2006, 07:27 PM   #25
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Quote:
Originally posted by SY


With micro idle current, what does the distortion spectrum look like?

Try 10 MA VS 100 MA and compare. Also, current damping was very popular in England in the beginning of 1980'th, when class A driver with N times of voltage amplificsation of the overall amp supplied output through resistor of N times of a load impedance. The result was excellend sound with near zero idle current of output transistors.
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Old 9th October 2006, 07:29 PM   #26
MikeB is offline MikeB  Germany
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Current dumping (not damping) topology is a completely different story and does in no way correlate to the actual subject: Using IRFs without thermal compensation.

Mike
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Old 9th October 2006, 07:35 PM   #27
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Quote:
The result was excellend sound with near zero idle current of output transistors.
But that's a totally different circuit. And bipolar, as well; that doesn't mean I can drop the idle current of (say) an Adcom 555 to 1mA and expect it to sound "excellend."
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Old 9th October 2006, 07:37 PM   #28
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Quote:
Originally posted by MikeB
Current dumping (not damping) topology is a completely different story and does in no way correlate to the actual subject: Using IRFs without thermal compensation.

Mike
Gentlemen, calm down and read my original posting!!!

I proposed LESS idle current WITH thermal compensation! Stop flame please and read carefully, not WHO writed, but WHAT writes!

Current dumping IS corellated with biasing of output transistors DIRECTLY.
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Old 9th October 2006, 07:45 PM   #29
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Quote:
Originally posted by SY


But that's a totally different circuit. And bipolar, as well; that doesn't mean I can drop the idle current of (say) an Adcom 555 to 1mA and expect it to sound "excellend."
It is the totally different story. If I drop current in some control circuit of my Nissan Armada I can't expect any good result as well.

The question was, do we need 100 MA bias of output complementary FETs IRFP240 / IRFP9240, or not. The answer is, we don't, despite there is a belief about "class AB sounds better than class B".
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Old 9th October 2006, 07:46 PM   #30
MikeB is offline MikeB  Germany
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I can't help it, but the actual subject was using IRFs without thermal compensation... You should have noted that you changed subject to idle bias in current dumping amps. This way it sounded like you proposed using the IRFs in actual discussed circuit would be absolutely fine, and using zero bias there also. No wonder Ilimzn exploded...

The only one upset right now is you.

Mike
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