L'Fake - A Cheap- A** Expedient to Replace the Elusive SIT - Page 13 - diyAudio
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Old 9th July 2013, 04:25 AM   #121
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BTW, for those hung up on light bulbs, there was a Heathkit push-pull amplifier circuit a long time back using light bulbs for emitter ballast resistors. One of the virtues claimed was a bit of inherent current limiting. Given all the stuff on the internet these days, it might not be too hard to find that circuit and launch a separate thread for discussion.
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Old 9th July 2013, 05:23 PM   #122
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wrenchone, the attached picture was a paper exercise in which I converted your earlier schematic into a push-pull amp using complementary symmetry Mosfets throughout.
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Last edited by Antoinel; 9th July 2013 at 05:28 PM.
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Old 9th July 2013, 07:18 PM   #123
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May I correct an error in the schematic above. I called Q4 and Q4' 'depletion" devices. Now I see they are actually enhancement Mosfets connected [e.g. Q4 per wrenchone] as modulated current sources.
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Old 9th July 2013, 08:03 PM   #124
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If you want a good symmetrical amp, you should look at "Le Mutant" (do a search in the Pass forum). There's no Schade involved, it's all about modulated asymmetric current mirrors. It also sounds very nice, and has little to no output offset with no servo. This requires selecting the input diff pair rather closely, but thar's the breaks.
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Old 9th July 2013, 09:53 PM   #125
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Quote:
Originally Posted by wrenchone View Post
If you want a good symmetrical amp, you should look at "Le Mutant" (do a search in the Pass forum). There's no Schade involved, it's all about modulated asymmetric current mirrors. It also sounds very nice, and has little to no output offset with no servo. This requires selecting the input diff pair rather closely, but thar's the breaks.
Thanks wrenchone.
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Old 13th July 2013, 10:53 PM   #126
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Default Another Kernel of Fake-SITness

Attached is the fake SIT kernel of the previous circuit, all on its own and fitted with a static current source load. It acquits itself well, with a pretty clean-looking 0.4% THD, almost all (2nd and 3rd), for ~1W output. Remember, that first Watt is important... Gain is 7.2X, so the amp will need a preamp with gain to sing loudly.

I used an IRFP360 in the simulation, as it was the lowest voltage fet in my collection of models with a size 6 die. Any monster fet with voltage rating of >60V would likely do as well, though some tinkering would probably be needed to get the bias centered.

I would use one of the IXYS depletion mode fets for the current source load for a minimum parts implementation (three active devices - what's not to like?), but a ring-of-two current source with any respectable bipolar transistor and an IRFP140/240 would work just as well.

Food for thought, anyway.
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Old 14th July 2013, 07:28 AM   #127
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For the ring-of-two current source, I meant an IRFP9140/9240 - easier to bias...
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Old 14th July 2013, 01:24 PM   #128
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Quote:
Originally Posted by wrenchone View Post
Attached is the fake SIT kernel of the previous circuit, all on its own and fitted with a static current source load. It acquits itself well, with a pretty clean-looking 0.4% THD, almost all (2nd and 3rd), for ~1W output. Remember, that first Watt is important... Gain is 7.2X, so the amp will need a preamp with gain to sing loudly.

I used an IRFP360 in the simulation, as it was the lowest voltage fet in my collection of models with a size 6 die. Any monster fet with voltage rating of >60V would likely do as well, though some tinkering would probably be needed to get the bias centered.

I would use one of the IXYS depletion mode fets for the current source load for a minimum parts implementation (three active devices - what's not to like?), but a ring-of-two current source with any respectable bipolar transistor and an IRFP140/240 would work just as well.

Food for thought, anyway.
I shadowed your work in this thread with interest by another I entitled "Other Uses of Vaccum Tubes" in the Everything Else forum. I reported in post#104 above a curious sonic improvement I stumbled on. It led me to propose a transient experiment which is depicted in its attendant attachment. It is another feedback loop from the Drain [post coupling cap] to the Source of the output MOSFET by using the loudpeaker as the feedback element. I hope that you consider doing this experiment and share your results.
Best regards
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Old 16th July 2013, 08:04 PM   #129
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I grabbed the latest version of the L'Fake (the two-stage version) and tried to audition it in my living room using a bench supply. One side played, the other took off with some horrible 60-Hz tinged racket that I suspect is either oscillation or a ground loop (or maybe a combination of the two). It didn't display this sort of antisocial behavior in the basement, but then it was being driven by an isolated Discman. I may try that in the living room to see if that clears up the problem. My living room outlets are fed with ungrounded post-and tube wiring, which can't help matters. My basement circuits are properly grounded.
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Old 31st July 2013, 01:27 AM   #130
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Post script - the amp played fine in the basement (same type of bench supply, too), so there is some kind of arcane grounding thing going on.
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