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Old 14th March 2011, 09:34 PM   #1
lineup is offline lineup  Sweden
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Default Quick Dirty Blameless HIFI IRFP240 Class A

One IRFP240 for constant current
The other IRFP240 controlled by JFET feedback
True hifi and Class A

Limit just below 8 Watt into 8 Ohm

Need one jfet BF245B for correct operation (2N4416 can be used)
and one TL431 for 30 volt regulation
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Last edited by lineup; 14th March 2011 at 09:37 PM.
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Old 14th March 2011, 10:10 PM   #2
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Good way to Schade U3 JFET into triode emulation.
Excepting: Schade feedback to source isn't entirely DC coupled as it appears.
Some of it gets squirreled up and delayed into into M2's Miller.

I've played with that before, where U3 was an actual Triode. Mu 100, and R11/R3=100
to make a concertina splitter of equal impedance with +Mu/2 -Mu/2 gains. If you drive
two loads of similar capacitance, the pole and zero cancel. Feedback to U3's source
is not so shifted... You need equal plate and cathode or source and drain voltage type
feedbacks, I'm not sure you can arrange both at the same time with just JFET + PNP.

If you could increase impedance into that MOSFET gate somehow? U3 would behave
much more like a properly emulated triode.
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Old 14th March 2011, 10:17 PM   #3
lineup is offline lineup  Sweden
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THanks for tube talk.

What I have not done is compensation.
BUt it looks not to bad without.
THe gate resistor 220 Ohm does some effect.

THe current is set 1.46A.
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Old 15th March 2011, 12:23 AM   #4
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Dude, you the one that drew a Schaded JFET triode emulator. It is what it is...
I happen to like that circuit.
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Old 15th March 2011, 03:27 AM   #5
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This was the phase splitter I was talkin about. Same topology as yours
except there are voltage feedbacks of equal influence at both collector
and emitter (plate Mu is feeding back the emitter) of the PNP transistor.

Anyways, if this split was driving MOSFETs of equal Miller capacitance,
one would be a Pole, and the other would be a Zero. Phase shifts would
cancel each other out, and no phase shift seen by local feedback loop.
Useful tactic, I don't know??? Does the impedance top and bottom have
to match for driven Millers to cancel? Again, I don't know...

After I closed global feedback around whole amp, the impedance match
here was forced equal anyway. I went to a lot of trouble for nothing...

You might just be able to stick a small resistor up top, and a cap equal
to your MOSFET's Miller to B+ or GND... I forget, what was I doing again?
I need to take up hard drinking, so I'll have an excuse next time...
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Last edited by kenpeter; 15th March 2011 at 03:31 AM.
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Old 15th March 2011, 03:36 AM   #6
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Nevermind... You are feeding back from Source of Q2, not Gate.
I should have seen that earlier. Just ignore everything, forget I was here.

Thanks.
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Old 15th March 2011, 04:28 AM   #7
Bonsai is offline Bonsai  Taiwan
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nice simple circuit Lineup - I like it!
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Old 15th March 2011, 12:14 PM   #8
lineup is offline lineup  Sweden
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2 guys like the circuit
Make it 3 with me. I like it, too.

R5 (390) should be adjusted to get close to 12 Volt at the output
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Old 15th March 2011, 12:39 PM   #9
MiiB is offline MiiB  Denmark
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This is at fine circuit...voltage and current through the IRF device can be increased a bit..the top rail can be regulated..

Then if speaker is be referenced to the top rail..you take effectively take the PSU caps out of the loop...really like the use of the Jfet...
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Old 16th March 2011, 09:16 AM   #10
lineup is offline lineup  Sweden
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Thank you MiiB

As it is Class A I do not think it need to be regulated.
It will keep stable.
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