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PretentiousFood

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About PretentiousFood

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    The Ginger Avengers Sidekick

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  1. PretentiousFood

    Egmont/Tubecad with CCS

    I would probably skip the feedback and cathode Rs (which reduce gain and increase rp), and use a lower mu, lower rp tube like a 12AT7. If you do want to stick with the 12AX7 and nfb, they really do benefit from a plate load several times greater than rp, which shouldn't be too hard to pull off with an lnd150-based gyrator. The DN2540 only really shines at quiescent currents above 10mA or so. Something like a BSS126/DN2540 cascode or LND150 cascode may be better for the tail of the LTP, but note that the LND150 usually cuts off around 700uA if you try to cascode it.
  2. PretentiousFood

    Egmont/Tubecad with CCS

    Neat! I'm not sure the feedback works there; the two outputs are summed through R22 and R23 and terminate to a common-mode node. But the two stages are cap-coupled, so common-mode feedback won't do much. Perhaps add some resistance between the two cathodes? Or connect the feedback resistor to the plates rather than cathodes for some trendy schade feedback? What's C2 for?
  3. PretentiousFood

    What are you EATING right now?

    Don't do it Brent!
  4. PretentiousFood

    Part Sourcing Assistance/Advice Thread

    Mouser's finest! I'm not sure about the hfe grade as I can't find the receipt. The package says FCE. FCE is not an hfe grade, but E is the highest and sounds like something I would buy. Shoot me a PM if you'd like.
  5. PretentiousFood

    Part Sourcing Assistance/Advice Thread

    I think I have about 100 of each (ksa992/c1845) that I'll never use if anyone needs. Can check tonight.
  6. PretentiousFood

    The Menace

    If you are lucky, it gets a little bit better if you literally reverse the polarity, input to the plate, and take the output off the grid.
  7. PretentiousFood

    What Are You Building Today

    @cetoole was visiting, so we made a slightly absurd amp using parts from the bin. I had originally milled the chassis with the intention of building a 6E5P triode -> 4P1L triode SE; then realized that would be a bit awkward, since the 6E5P is rated for higher dissipation than the 4P1L. I may have also forgotten that 4P1L filaments need power supplies when I bought my power transformer, too, so that was a non-starter. Not wanting to leave two unused holes in the panel, the obvious solution was to throw some 0C3s in. Glow tubes are pretty; it follows that any amp that uses them will sound better than one that does not. A normal person might be tempted to string them in series and use them as a reference for a regulator. Instead, each channel gets its own shunt regulator with a voltage reference made up of a zener stacked on top of the glow tube, and a SiC shunt device. R7 ensures that the tube strikes. And the current source (which is nothing inspired) The regulator terminates into the cathode rather than ground, keeping the cathode regulator out of the output current loop. It also forces the PNP to run at constant current regardless of cathode current, meaning its impedance will also be constant. It sounds like a single ended triode (i.e. terrible). But it was an awfully fun build. Slightly tempted to parafeed or otherwise upgrade the iron. It's surprisingly silent when nothing is playing.
  8. PretentiousFood

    Blue Hawaii BJT Build thread

    No access to Spice right now but I've had great luck with this circuit and a variation using two PNPs and two DMOSs. If base current starves the zener, use a Darlington or replace the LND150 with a BSS126 or BSP135, I think the reason why the DMOS circuit is so good is that it does not have a bias string that runs parallel to the CCS. The way it's cascoded also holds the lower device's Vce or Vds constant rather than holding Vbc constant, as most PNP cascodes do. Using a BJT as the lower device almost always results in more transconductance, and thus higher impedance.
  9. PretentiousFood

    Blue Hawaii BJT Build thread

    What kind of noise? I've had good luck filtering switchers with a CLC with -3db a decade below fosc (so ~10khz), to pick up where the regulator starts to drop off. Gives values in the low mH and uF range.
  10. PretentiousFood

    What Are You Building Today

    It's going into a Parasound D/AC 1600. It's still a work in progress-- I needed to replace the DAC power supplies because the stock ones were remarkably derpy. Pulling the I/V stage causes the regulator output voltage to go up. I put in some shunt regs and ripped out approximately a million bypass caps that were to far from the load to do anything. I don't like GICs/FDNRs. That's what the Parasound had. The big problem I see is that the op amps don't have sufficient gain at frequencies that we care about (352.8KHz and multiples) to actually work. The PCM1702 specs this fellow, which has just 30dB of gain that rolls off very quickly at 300KHz+. Not only is the impedance driving the shunt op amps going to be sort of high, it will also change with frequency. I like Sallen-Key filters because they can be built around an emitter/source follower. Those are much more dependable. By cascading it with an RC pole, you can build a third order filter with just a current sink and a BJT as active devices, and it seems to work well into the lower MHz without doing anything weird.
  11. PretentiousFood

    What Are You Building Today

    It's my own, but very similar to @cetoole's. It's a folded cascode, with differential filtering and a discrete Sallen-Key filter. Trimmers instead of an op amp servo, and a nickel sandwich on the output. Using some nice TO220 parts that let me bias it very hot.
  12. PretentiousFood

    What Are You Building Today

    For my Parasound. Getting it to bias was like resistor sudoku.
  13. PretentiousFood

    SEN I/V stage for BIII

    Maybe I'm missing something, but the Pass(ish) I/V stages seem a bit silly. The input impedance of a JFET is always going to be higher than that of a BJT past a few mA, because its transconductance is lower. The matched JFETs are necessary for input offset, but a JFET running at IDSS makes a lousy current source. Just about all of them use at least one output coupling cap, if not two. If you haven't come across it already, highly recommend checking out the Hawksford paper on I/V stages. I would try running a common base BJT very hot, without feedback.
  14. PretentiousFood

    current feedback electrostatic amp

    Just popped it into a Spice. I don't have models for the output devices, so used BC337/327s everywhere (they would blow up, but Spice doesn't care. ) Bandwidth doesn't look too hot, even with the outputs unloaded. May have made a mistake, but the input stage looks like it's starved for current? Replacing the emitter degen Rs with 1k and bumping the ouput stage Rs accordingly (~3k) helps a lot. Not sure how to attach the .asc, here are some screenshots.
  15. PretentiousFood

    The Headcase Stax thread

    Absolutely. Those are feedback resistors, and they eat up a whole 1.5mA each during peaks (~400V / 250k), independant. This may be more academic than anything else for reasons Jim brought up; there's really not a whole lot going on at 20kHz, and little reason to push the output to full swing. At the risk of being lynched, I took them out and run it open loop...
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