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ALWSR Build Thread

Discussion in 'd.i.y.' started by timH, Apr 5, 2019.

  1. timH

    timH Senior Moment Member

    I'm sure we’re going to need this along the way so I thought I’d kick it off.
    With the ZA caps no more and I think ditto with the Stargets I’ve started looking at the Panasonic FC range. What should I compare though, ESR? Ripple?
     
  2. Andrew L Weekes

    Andrew L Weekes Senior Reverse Engineer

    I'll have a look at this today, I have a suggested BOM from another forum member which I haven't looked over yet.

    I can't remember whether I've ever tried OSCON's in these reg's, they could be beneficial for the reference filter, but ultra low ESR isn't necessarily a necessity, in fact stability margins can be improved with a bit of ESR in key places.

    Andy
     
  3. Andrew L Weekes

    Andrew L Weekes Senior Reverse Engineer

    I'm a bit puzzled by my parts list, it's been a while since I've worked with these reg's, but I'm not sure why C7 is listed as not fitted, since this reduces the noise gain of the regulator. Looking at a sample regulator I have here, it's fitted. If anyone has a better memory or can recall anything let me know!

    I also noticed R7 is shown as 1k, whereas originally it was 499R to match the impedance of the other op-amp input (500R). This isn't really important with the AD825, it was really for those brave enough to try the AD797 (which I don't recommend!) to keep impedance and thereby noise low. Raising the value to 1k, will improve the filtering effect on the reference although the unit I have here is fitted with 499R. Again, my memory fails me...

    I'll share a Google sheet with my suggestions in for people to comment on, I'm not sure what to choose on the electrolytic front - the Elna ROD (Starget) aren't readily available any more and ZA's are history. I think that is largely going to be a personal choice or based on suggestions from others. I'm looking at OS-CON for C2 where the low ESR will be advantageous in reducing any Zener noise.
     
  4. timH

    timH Senior Moment Member

    I’ve got an old unit here. No C7 and R7 is 1k. Can’t remember why. I was just following orders your honour :)
     
    Andrew L Weekes likes this.
  5. john.luckins

    john.luckins pfm Member

    Going forward I'm likely to be using either a 35v 220uF Panasonic FC or adding some 0.22 Ohms ESR to any other type of 100uF cap in the C1 position of my ALWSR's as I can't find any alternative. Here's why.

    Getting C1 right is quite important to noise performance of the ALWSR. Too low an ESR for the current being supplied will give you a noise peak on the output. There's an article on 3 terminal regs used as tracking regs on the Acoustica.org website which addresses the importance of significant ESR in the ALWSR Cap C1 (100uF). The LM317 tracking pre-reg has an equivalent output inductance which varies with current supplied. For a low 10mA load current this is 5mH and so for critical damping of a 100uF C1 of the ALWSR it will need to have a min ESR of 0.32 Ohms. Increase C1 to 220uF and it'll need to have 0.22 Ohms. ESR in Electrolytics is quite temperature dependant (it drops as temp rises) and it is quite hard to find high ESR 100uF Electrolytics. I got a mixed bag of Jamicon SK series from Maplin of all places that did the trick but they are hard to find now. Otherwise in most of my low current ALWSR builds I have added a 0.33 resistor to one of the legs of C1 and mounted it slightly raised above the board.

    I've just found the graph of the LM317's output inductance vs current. For load currents above 100mA the effective output inductance levels out at 1uH. This equates to a 100uF C1 minimum ESR of 0.142 Ohms. For a 220uF C1 this reduces to a min ESR of 0.1 Ohms. A Panasonic FC 35v 100uF has too low an ESR for this application but a 220uF Panasonic FC would be OK. Sticking with the need for at least 0.32 ohms min ESR for all likely output currents I looked at the Jamicon range in detail. It seems that their SK and TM 100uF 35v caps fit the bill for C1. Finding them in the UK is a different matter.

    John

    PS. I think C7 is listed as not fitted as it can make the AD797 version and even low gain AD825 versions unstable. It is only really helpful for higher gain i.e. higher voltage applications of the ALWSR IMHO.
     
    Andrew L Weekes likes this.
  6. martin clark

    martin clark pinko bodger

    Andrew L Weekes likes this.
  7. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Some great stuff there guys!

    I designed and built a shunt super reg using the AD797 and giving a regulated 250V for a valve phono stage and that was a complete twat (I've mentioned this before elsewhere) to get right.... I even ended up doing heating and cooling cycles on it to make sure it was stable under all possible operating temperatures after the previously stable circuit became decidedly unstable a few hours later, which had me virtually pulling my hair out trying to fathom why the passage of a few hours should make any difference... it eventually clicked when I remembered that I'd switched the central heating off and that temperature was the only parameter which had changed. I still couldn't believe that the change in ESR with temp change of "only" 15C or so could be responsible but any doubts were well killed off when I got the freezer spray and hairdryer out! IIRC I ended up with a combination of film caps with high ESR electrolytics acting as CR dampers which sorted it under all conditions.
     
    orangeart likes this.
  8. timH

    timH Senior Moment Member

    Would these work for C1? 100uF 35v 0.36Ohm ESR
     
  9. andrewsutton

    andrewsutton pfm Member

    Looks like Farnell are running their stock of LM329DZs down and out.
    Might be worth keeping an eye on them.......
    Cheers Andy.
     
  10. IDM

    IDM pfm Member

  11. john.luckins

    john.luckins pfm Member

    They look perfect. Well found.
     
  12. andrewsutton

    andrewsutton pfm Member

    Looking at the old BOM there are a few queries springing to mind.
    Are more modern 3 pin regulators a simple drop in, assuming they are better?
    Is the AD825 still the best bet or is a more modern chip better and a simple drop in? The AD825 is out of stock at Farnell anyway.
    Cheers Andy.
     
  13. Andrew L Weekes

    Andrew L Weekes Senior Reverse Engineer

    I noticed that.

    Seem to be available from Texas, but concerned it's going obsolete!

    There's a PCN (product change notification) from Texas, saying it's going obsolete, LTB was ironically April 2nd 2019! (https://www.mouser.com/PCN/Texas_Instruments_PCN20170721000_2018040212142643.pdf)

    They're suggesting LM4040 as a replacement, but it's not a buried zener device and is not comparable for noise (and there's no 6.9V option).

    RS have LT parts in packs of 2 (https://uk.rs-online.com/web/p/voltage-references/7798909/)

    ADi still list the part as current production, if we can find a distributor with a sensible MOQ (https://www.analog.com/en/products/lm329.html#product-discussions)

    Buy 'em while you can!
     
  14. Andrew L Weekes

    Andrew L Weekes Senior Reverse Engineer

    This was kindly sent to me by forum member geoturbo, I've been through the items in green, I've shared it so anyone can comment on it, maybe add comments in here and we'll review it, been a bit busy this week so haven't had as much time as I would have liked:

    https://docs.google.com/spreadsheets/d/13tO70sIFgCgXZmhLwoeHpELZPhMEoeDfRBIJbwYmDDg/edit?usp=sharing

    I was wondering about OSCON's for C2 / C4, the former is the noise filter for the reference and the latter is brute force AC shorting the level shift zener, so low ESR is useful here, particularly for the latter case. I have no experience of the SEPC types I've looked at though.

    Andy.
     
  15. martin clark

    martin clark pinko bodger

    The LM723, ancient thing it is, is still available, and cheap, and is a true buried-zener without the active circuitry wrapped around it (that makes the LM329 a bit more reactive than you might first think ...IME)



    (yes I know the LM723 is a >45yr old, 14-pin DIP voltage regulator kit-on-a-chip dinosaur, but it is worth buying a few if only to use that true 6.9v buried-zener reference for fun. It is genuinely quiet. but no drop-in for the LM329 - mentioned only for the curious)
     
    Andrew L Weekes likes this.
  16. Andrew L Weekes

    Andrew L Weekes Senior Reverse Engineer

    I've used LM317's and LT108x series as pre-reg's with good results.

    One thing to be wary of that I'm aware of as something that cropped up in my day job recently, is that TO220 devices are frequently available in packages with different thermal performance. In a cost-cutting exercise many are now in a 'single gauge' package, where the metal tab is much thinner and has higher thermal resistance. Double-gauge packages are like the old parts we used to buy, with a chunky tab. Probably not a big issue for those using them in low current applications, but something to be aware of.

    For an example, look at pages 14-17 of this data sheet, and compare dimension 'F' in the two packages http://www.farnell.com/datasheets/1815554.pdf
     
  17. Andrew L Weekes

    Andrew L Weekes Senior Reverse Engineer

    I remember those, amazed they still make them, it's a versatile chip! I have a 12V / 20A linear ham radio PSU with one of those in, complete with the noisiest, buzziest transformer in the world :). Will probably drop it's innards for a switcher when I get back into that (in my dotage I expect!).
     
  18. martin clark

    martin clark pinko bodger

    Drop the transformer in a bucket of varnish for a few days, then leave outside this summer... ;)
     
  19. Andrew L Weekes

    Andrew L Weekes Senior Reverse Engineer

    Worth a try!
     
  20. timH

    timH Senior Moment Member

    Quick question on op amp orientation. I’m building positive and negative versions. For the pos version I’ve fitted the ad825 with the identifying mark to top right (lm317 will be top left)
    My question is whether I should do the same for the negative version?
     

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