PretentiousFood

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About PretentiousFood

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  1. 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.
  2. 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.
  3. For my Parasound. Getting it to bias was like resistor sudoku.
  4. 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.
  5. 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.
  6. 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...
  7. Is that the FR of with different amounts of feedback? If you keep feedback constant, I think it just clips, rather than rolls off gracefully. Here's a sim of an ES-X sort of deal. I stepped the source amplitude at 100mV, 1V, and 10V (some people have silly DACs). The EL34 (6CA7 in sim) is at 6mA and rails are +/- 400V, 120pF load. Happy to try different parameters. FR does not seem to change in the magical ideal world of Spice? http://imgur.com/a/2uglC Of course, the current requirement into the load increases with frequency and signal level. The blue line crosses at 10mA, and shows the frequency at which current limiting would occur for different amounts of voltage swing. http://imgur.com/qh0esa3 However, output distortion gets gross well before clipping. At 1 kHz: http://imgur.com/vJAiMyW This shows clipping into the rails. And at 10 kHz: http://imgur.com/u1sD8ze Waveforms: http://imgur.com/UYLhk9W This one shows current limiting.
  8. Parallel SE, they say. Balanced in with an input transformer, single ended gain stage, interstage transformer, parallel single ended output stage, floating secondary for "balanced" output.
  9. A shunt regulated power supply, with some daughterboards replacing the crappy voltage references I was using in the CCS.
  10. I think they're web-only, buy Satodenki had some last time I checked.
  11. These aren't mutually exclusive. I've used a choke input power supply into a shunt regulator for a dynamic HP amp to good effect. Excessive, but hopefully not inferior. Jim, aren't you doing the same for your SR-X? With a properly sized choke, you can get away with smaller power transformer and b+ caps.
  12. Oh hey there gorgeous
  13. Just another datapoint. I picked up a Metcal PS-800 about a year ago when they were selling refurbished units at a discount, and haven't enjoyed using it as much as the Hakko 936. I find the build quality of the pen to be a step down from Hakko, and, more importantly, the tips keep slipping out since they don't seem to have a locking mechanism. I also miss the Hakko's temperature control, as the default setting is a bit too cool to solder the joint on things like RCA jacks without heating up the entire jack. For day-to-day stuff it's perfectly fine.
  14. That's really cool! Kind of looks like an AD844. Would the cascode (Q1/Q2) be happier with more voltage drop than D1? The BC546/556 have some weird curves and don't saturate as quickly as something like the BC337/327.
  15. I was thinking of grabbing a few pieces for an input buffer, but haven't ever used them before. Any reason why it'd be a better bet than, say, the cheaper/current BF862 (datasheet PDF) if I don't need gain or particularly low noise?