L'Fake - A Cheap- A** Expedient to Replace the Elusive SIT

[wrenchone]

So am I, but I usually reserve that enthusiasm for tubes. The L'Fake (non-incandescent) was the best working option at the late date I got everything working the afternoon before. The supply toroid I chose (out of a limited selection - some provided by you at the first BA) was a little voltage-shy for the L'Fake Lite. I should have followed my initial inclination, which was to leave off working on the power supply and tuck my bench supply into the chassis, which would have had enough capability to power either amp at its preferred rail voltage (enough room inside, too).

[Antoinel]

Originally Posted by wrenchone

This amp, and the L'Fake Lite made it to BA - only the L'Fake got played. Nelson was much more interested in the simple light bulb-fed version that didn;t get played.

Thanks wrenchone. SIT appears to be scarce. Q3, Q4, Q5 and the associated passives in your schematic are the Triode Characteristic Generator [TCG]. Integrate them on a 2-sided PCB the size of a TO-247 package [4 leads] and sell them as L'Fake SIT. Clones of it will no doubt be made; but your original has an intrinsic power and prestige to be coveted and sought after by DIYers.

[Zen Mod]

it's not that simple

R11, R6 must be in game ....... and they're not so un-correlated to rest of circuit

[Antoinel]

Originally Posted by Zen Mod

it's not that simple

R11, R6 must be in game ....... and they're not so un-correlated to rest of circuit

Thanks Zen Mod. The challenge and objective continue to convert a cheap MOSFET [voltage controlled current source] to a voltage controlled resistor like a SIT. I am not skilled to determine whether an ideal voltage source power amplifier is a voltage controlled current source or a voltage controlled resistor? There are two R6s in the schematic of wrenchone.

[Zen Mod]

Originally Posted by Antoinel

.....There are two R6s in the schematic of wrenchone.

that important one - connected with one side of R11

[wrenchone]

Here's a corrected schematic, so there's less confusion.

If you want to discuss the true kernel of fake-SITness. The simpler light bulb-loaded schematic posted earlier in this thread would be a better start. The circuit currently under discussion has a modulated current source load and a global feedback loop among other complicating factors. It's playing right now, driving a pair of so-so Alesis monitors (4 ohms, no less) and doing a good job of it - Beethoven's Violin concerto with Anne-Sophie Mutter in the lead...

Note - the simpler circuit is shown in post #14.

[Antoinel]

Originally Posted by wrenchone

Here's a corrected schematic, so there's less confusion.

If you want to discuss the true kernel of fake-SITness. The simpler light bulb-loaded schematic posted earlier in this thread would be a better start. The circuit currently under discussion has a modulated current source load and a global feedback loop among other complicating factors. It's playing right now, driving a pair of so-so Alesis monitors (4 ohms, no less) and doing a good job of it - Beethoven's Violin concerto with Anne-Sophie Mutter in the lead...

Note - the simpler circuit is shown in post #14.

Thank you wrenchone. Will a circuit of the schematic of post#14 generate the triode-like characteristic curves; for example by using the setup of Rothacher in his first article on SIT? Is it appropriate to refer to your amplifier as " A Solid State Triode"?

[wrenchone]

I'm irreverent at best - I prefer to call it "L'Fake"...

The idea of Schade feedback to impart triode-like characteristics to pentode-ish devices like power mosfets is not new, and has been discussed at length elsewhere on this site. My added twist was to put a follower inside the feedback loop to allow use of a high value feedback network and still be able to decently drive the mosfet gate capacitance. This makes the whole setup easier to dive with normal sources. I did this all unawares back in '07 with my "Half-Nelson" amp, not knowing anything about this "Schade" business then. I'm still sussing out the implications...

The basic circuit should give triode-like characteristics with a curve tracer setup. Perhaps the best way to try this would be to unhook the lower resistor of the feedback voltage divider and apply the variable bias there. It might be a useful exercise to try. Let me ponder that for a bit.

[Antoinel]

Thank you wrenchone. I'll do more reading on Schade feedback, and connect with your schematics.

[Antoinel]

wrenchone and Zen Mod: BJT is absent from the vocabulary of Schade Feedback. It has pentode-like characteristic curves like FEts. Why is it excluded from this application?