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Testing a +ve 24v ALWSR using a SNAPS scheme

Discussion in 'd.i.y.' started by chrisn, Sep 6, 2019.

  1. chrisn

    chrisn pfm Member

    I am about to test my newly built regulator and need some advice. I am following the SNAPS build here:-

    My power supply delivers 31v:-
    https://www.dropbox.com/s/cudvgnajhvhtu5u/alwsr pos d.JPG?dl=0

    I have connected the regulator using the prereg and remote sense but I am a bit reluctant to swith on for a couple of reasons. The BOM has 100uf 16v sepc capacitors (the two with the aluminium cases) will these be OK?
    When I was testing the prereg it was fine upto about 20v reading 2.5v but above this the output rose to 3.3v this could be corrected by adding a load (meaning 100R power resistor I usually use for discharging capacitors) to the output.
    So I am not sure what to do with the final test, should I connect the load before switch on?
    Any help would be appreciated.
  2. chrisn

    chrisn pfm Member

    Well I did the test after a lot of thought and checking.
    First with a TPR4 30v which was fine and then with the cap bank shown, x2 positive ALWSRS and Nait 22-0-22 transformer.
    This was also fine producing a steady 25v-0-25v.
    Great credit to the designer and the documentation!

    So time for a glass of wine and a listen with the Prefix.
  3. Andrew L Weekes

    Andrew L Weekes Reverse Engineer

    Excellent stuff!

    A bench PSU is a very useful tool for anyone that does DIY, get one with a current limit, you can then set the output voltage to a suitable voltage, then set the current to zero.

    You then use the current limit to slowly power up the circuit, observing the current draw - if it seems to be rising too rapidly or over what one expects, that's indicative of a fault, so you can power down and check, if things look good it saves letting the smoke out and is safer than going straight to mains power.

    They are often relatively inexpensive these days and a useful and worthwile investment in my view. My cheap 'chinese' twin output one came from Maplin many years ago, and still works a treat.

    As an example, this RS pro single output one is £69 (OOS at present though) https://uk.rs-online.com/web/p/bench-power-supplies/1757368/

    Similar unit here from Farnell for £58 https://uk.farnell.com/tenma/72-2685/bench-power-supply-1-ch-30v-3a/dp/2563981
  4. chrisn

    chrisn pfm Member

    Thanks, yes I did look at the one suggested in the manual. A bit of a steep learning curve for me.
    Sounds really good with the Prefix phono - better than the NAC 52 powered input I think.
    Runs cool too. Wiring needs to be tidied up, so no pictures yet.

    I am wondering about the maximum input voltage which is 36v DC stated in the manual.
    I am currently using a 22-0-22v AC Nait transformer to deliver a conservative 30v DC.
    25-0-25v AC transformers would seem to be the limit. This would rule out a drop in for a Hicap.
    I have a spare 28-0-28v AC (about 40v DC), would this be too much?
    Also optimum VA for a ALWSR pair? Not sure what the Nait is delivering on the 22v winding?
  5. Andrew L Weekes

    Andrew L Weekes Reverse Engineer

    The maximum input voltage is only quoted in the context of testing the pre-regulator in isolation: http://alw-audio.co.uk/Manuals/ALWSR/#t=Testing.htm&rhsearch=36V&rhhlterm=36V&rhsyns=

    The pre-regulator by default maintains a 2.5V potential difference between the output of the super regulator and the input (i.e. the super reg input voltage = super reg output voltage + 2.5V).

    The great advantage of the LM317 style adjustable regulators is that only the input to output voltage differential matters, because it has no pin tied directly to ground. You could regulate a 100V PSU theoretically with one, providing you stay within the acceptable operating limits under all conditions. Start-up and fault conditions are where things get challenging so it isn't recommended.

    When testing the pre-regulator alone, we are referencing the regulator to ground (via the TRSx pins) and the output voltage is set to 2.5V, so the maximum input voltage is 2.5V + 40V. The 36V was me being conservative and allowing for inaccurate measurement and poorly regulated inputs.

    The super-regulator portion is ground referenced and since the internal components (e.g. the op-amp) are powered from the regulator output and others are referenced to ground, the maximum operating voltage of those components is important to consider.

    In the case of a Naim application and a 24V output voltage, the output of the super reg will be 24V, the tracking pre-regulator output will therefore be 24V + 2.5V, which means the theoretical maximum input voltage for the regulator as a whole, is around 26.5V + 40V.

    You will run into severe power dissipation problems under these conditions though, so I wouldn't try it, and under fault conditions all bets are off, but operation in a HiCap is no issue at all and an application that has been tested repeatedly by others.

  6. chrisn

    chrisn pfm Member

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