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HI / LO input jack grounding query

Discussion in 'Amp Tech Center' started by owlexifry, Mar 9, 2021.

  1. owlexifry

    owlexifry TDPRI Member

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    currently building one of the designs from robrobinette.com

    it has a HI and LO input jack.

    I understand the grounding design uses the HI input jack as the primary grounding point (directly to chassis w/ star washer).

    looking at layout and specifics, i’m trying to understand why the HI input jack has a star washer indicated, but the LO input doesn’t.

    why not use star washers for BOTH input jacks?

    for the LO input jack, is the sleeve grounding just relying on contact with the chassis (without a star washer)? is that gonna be reliable?
    sleeve tab isn’t wired to anything.
    does it matter? i’m not sure.

    View attachment 830096
     
  2. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    You can use star washers for both. Good idea. Jacks are grounded by their body to the chassis. The main point is to make *sure* the jack with the ground bus on it *stays* 100% grounded. Oh, and welcome to the forum.
     
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  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    *Ideally* there should only be one connection to ground. Any input jack can be the *one* location used as the ground point. More than one will make a ground loop. Most of us have experienced a loose jack so relying on a single jack connected to ground may not be a good idea. Isolating the jacks from the chassis and using a dedicated bolt near the input jacks for the ground point would be best. Using a jack is a second best alternative.

    On occasion, I have used both input jacks for the ground, reasoning the likelihood of both jacks becoming loose is less and a small ground loop at this place in the ground scheme would probably create no noise. I know this does not meet the *ideal* but it has worked.

    To answer your questions more specifically... Use star washers on both. Does it matter? I doubt you will experience a difference wiring the two ground terminals together vs the chassis as a ground plane between the two. Reliable? Normal maintenance of checking for loose jacks should be satisfactory. YMMV.
     
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  4. 2L man

    2L man Tele-Meister

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    Chassis and ground are not synonyms!

    On every amp they should meet only once at input jack or very near it.

    :)
     
  5. 2L man

    2L man Tele-Meister

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    When you connect ground bus to chassis only at input jacks I think you can mount all steel jacks without insulation because there does not flow any current to chassis and when they are close each others not much ground loop noise forms although technically there comes ground loop(s) but they come active only when channels are connected parallel and obviously they don't cause any adverse effect.

    It is good practise to use star washers so that jacks do not come loose. If you use insulated input jacks install soldering lug using strong bolt and scrape aluminium just before installing. I don't put star washer between chassis and soldering lug but I put one to bolt side and one under the nut.

    If you have steel chassis you can solder to it if your soldering iron is strong enough but I just read somewhere that it is not recommended or was it even forbidden. Using screw mounting on steel chassis is more reliable than on aluminum.
     
    Last edited: Mar 9, 2021
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  6. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    With respect, in the case of the question here about grounding the input jack, is this relevant?

    More generally, although a single ground may be ideal in theory, many expert builds and very quiet small amps here and elsewhere are built with a simple split ground and a few other ground variants, so the idea that there is only *one* good method may confuse new builders.
     
    Last edited: Mar 9, 2021
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  7. owlexifry

    owlexifry TDPRI Member

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    many thanks all, i appreciate the responses.
    this has been most helpful.

    i guess the main part of my query is:
    - for an input jack, and speaker jack, where contact with the chassis is the only ground connection, how reliable can this ground connection be without a star washer?

    [robrob wiring layout indicates speaker jacks also ground to chassis via direct contact..
    ->OT secondary com lead (black) is connected to main speaker jack sleeve tab - and grounds to chassis there, but no star washers indicated...]

    i'm just confused why star washers would be indicated for:
    - HI input jack
    - presence pot
    - bias pot

    but not indicated for:
    - LO input jack
    - both speaker output jacks

    it’s not like they’re expensive... i just don’t understand why you wouldn’t indicate star washers for anything that grounds direct to chassis...

    am i mistaken?
     
  8. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    No worries. It’s not like they’re saying *not* to use 'em. Millions of amps out there have been built without 'em, but the contact will be even more secure with.
     
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  9. David C

    David C Tele-Meister

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    I agree. The key is what level of risk do you want to put on the tightness of an input jack without a star washer compared to one with a star washer. Both will work just fine, the amp won't blow up if the non-washer jack comes loose. You may have to get into the amp to retighten it every now and then.
     
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