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idiots guide to amp building

Discussion in 'd.i.y.' started by say it as it is, Feb 26, 2021.

  1. say it as it is

    say it as it is pfm Member

    Ok it's a 500va transformer I thought a softstart was needed? When i eventually build monos I won't use soft start as smaller transformers.

    I have the cheapo UPC 1237 but no 12Vac supply.
    Do I need another transformer to feed the protection board?
    If so what transformer or just anything presume no real load so as small as can get?

    Do I need the softstart as well as speaker protection?

    Finally will this detract from the sound? ie do I need to spend more on a better protection board?
  2. Mike Hanson

    Mike Hanson Lovely!

    See my post on the other thread.

    Regarding the UPC1237 supply, it's going to draw very little power, so pretty much any little 12VAC transformer will do. For example, this is plenty big.

    As for the quality of the UPC1237 boards, Les @ Avondale told me thinks they're fine, except that he replaces all the capacitors. I'll be doing that going forwards, especially if it's for an amp that's going somewhere other than my own house.
  3. say it as it is

    say it as it is pfm Member

  4. muchtoofast

    muchtoofast pfm Member

    Guess you'd need to know the current draw of your UPC1237, thats 3W 12V.

    I made the K4700 kit in the end - only needed one - £17 inc. VAT. Les W. detailed the 0V separation to use one unit on both channels in an earlier thread. (I can find it again if necessary). Mine's been working for a couple of weeks now. Just another option, maybe not preferrable to you as you have the UPC1237 already.

    I use an Avondale SSM2 soft start to feed the 820VA tranformer and 23000uf x2 per channel. Even with it, I still managed to blow a 5A fuse in the test plug when switching it on second time around, so now I'm on a 13A fuse. It should in theory also protect switch contacts by restricting initial contact load... so I didn't bother with that idea either lol... see my earlier picture of the short from the input switch across the thermal cut off contacts.

    As for whether you need it or not... if you are not worried about initial high load through the inlet circuitry, a growl from your transformer as the core fills up, and it doesnt trip your RCD/RCBO etc. ... then no worries. I however, being new at this, am wary by what I've read and have put it in regardless. Slow blow fuses come into this equation also. 300VA apparently you are ok, 500VA you may be able to get away with just a slow blow fuse of the right rating??

    Maybe, someone with fab experience can cut in here and put us all right?
  5. Mike Hanson

    Mike Hanson Lovely!

    I just did a quick search, and it looks like the max current draw should be 160mA, and that transformer should be able to do 300mA, so it's probably fine.
    muchtoofast likes this.
  6. say it as it is

    say it as it is pfm Member

    Cheers guys ok am clueless with caps the protection board I have has 5 caps .
    All rated at 25v

    Do they all need replaced if so with? Brand or type,?

  7. Mike Hanson

    Mike Hanson Lovely!

    Any reliable brand with those specs will be fine. Apparently the cheap caps on those boards can't be trusted.
  8. say it as it is

    say it as it is pfm Member

  9. S-Man

    S-Man Kinkless Tetrode Admirer

    Looks OK to me, but you should test it:
    Power it up, wait for the relays to click-in. Connect a variable bench supply to one pair of terminals (eg. LIN and GND) and slowly increase the voltage. The relays should click off at about 1.6V.
    You should find that there is a shorter time delay at turn off if the "fault" voltage is higher. At >10V it should be perceptibly instant. Repeat with the other channel.

    If you don't have a bench supply, one AA cell (well used) should not trip it, but 2 in series should.
  10. say it as it is

    say it as it is pfm Member

    will do not got a 12v supply yet, will test when I do.

    Got me thinking about the caps now if they should be changed?
  11. S-Man

    S-Man Kinkless Tetrode Admirer

    Oh, and check it works with both polarities of fault!

    Cap swapping?
    If the failure mechansim of the caps is a short circuit then they should be swapped.
    If the failure mechanism is high ESR, reduced value or open circuit, then it will make the protection circuit more sensitive or it could fail to close the relays. I.e. fail safe.

    In general I am not aware of electolytics (except tants) failing short.
  12. say it as it is

    say it as it is pfm Member

    yes they look like they will fail to safe, just the pain t strip them out if they fail quickly I know not all no name are poor quality some are ok just luck
  13. grahamdocman

    grahamdocman pfm Member

    I too am researching speaker protection options (though mono in my case) - Mike has given me some useful ideas (via PMs).
    However, I'm still undecided.
    Recapping with e.g Panasonic radials works out only a few £ per board - so still not expensive.

    My questions:
    1. specifically for 'say it as it is' : can you check the value of the lowest value cap is 19uF as I can only find 18uF or 20uF 25V caps as replacements from Panasonic ?
    2. if only 18uF or 20uF are available, does it make a difference and which one is the most appropriate replacement
    3. is there a built in 'power switch on' delay on this board ?
  14. say it as it is

    say it as it is pfm Member

    hi no delay that I can see I have a soft start from Avondale will connect via that.

    having the same issue finding 19uF

    am tempted to leave them and try it think on it this way these cheaper caps are used on lots of budget kit that lasts a reasonable time
  15. say it as it is

    say it as it is pfm Member

    my other issues is finding a UK based supplier or on that doesn't add £15 to a £3 order
  16. muchtoofast

    muchtoofast pfm Member

    Looks like 10uf in your link
  17. chiily

    chiily PFM Special Builder

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