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Ground. (What it isn't)

Discussion in 'Amp Tech Center' started by elpico, Jun 5, 2018.

  1. LudwigvonBirk

    LudwigvonBirk Tele-Meister

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    Thank you for volunteering to deal with us so firmly, politely, patiently, and decently elpico. Canadians are the best in the world at being like that at present, kudos to all, everybody should aspire to be so IMO.

    I'm going to now re-ask and self-hijack what I was thinking about in my post #6. I wrote it that way but my mind wasn't really asking about circuit/continuity.

    In the post#6 schematic we see various things connected to "ground" (using the same ground symbol- perhaps there's some confusion caused by the "coarse" symbol shape which is only one of several it could be.?). Some are direct to ground, some have a resistor as the last thing connected to ground, some have a capacitor+resistor as the last thing connected to ground. (Fwiw, I have built to this schematic, works wonderfully. Hoffman PR circuit if you want to see the whole thing).

    If on my 3 prong plug, I snip off the ground lug, this amp will still work. Maybe I'll get shocked (Safety feature eliminated, but the amp still works, fine, got it.) I have scabbed in dozens and dozens of light switches, ac plugs, cieling fans etc into houses and never once flopped inspection or got shocked, perhaps amazingly.

    So, amp-centric questions:

    1) If the amp still works after I snipped off the ground plug on the power cord, the things in the schematic that are shown symbolically "connected to ground" are going to ground.... via the neutral AC power wire (which has a connection to ground in the house power box)?

    Perhaps something about how neutral/white and ground/green relate but aren't the same would be good to state here.

    2) Resistor or Cap or Resistor+Cap sitting between "ground" and the upstream components as shown in post #6. If there's a resistor and/or cap sitting between xyz upstream component, that's not done for purely safety reasons.

    Perhaps something about noise, potential, etc is the response here? My ears are open...

    Thanks TDPRI!

    Ps: I run an engineering dept of 40+, a few of whom are EE (pretty much everybody is CS). The only on-team person I could ask the above 2 questions to is 50+ and a Russian chess-god and awesome EE. I haven't dredged him into this but could. Poor fellow...
     

  2. LudwigvonBirk

    LudwigvonBirk Tele-Meister

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  3. Speedy454

    Speedy454 Tele-Meister

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    Oct 1, 2013
    Highland, IL
    I have a BSEET, and have been working in the medical electronics field for 35 years. I install, maintain and repair Cat Scans, MRI systems, X ray systems and portable X ray machines.
    We regularly troubleshoot to the component level, reading and following detailed schematics.

    Never have I ever seen a capacitor in series with a ground.

    Since I began in 1983, all of the systems I have worked on, GE, Siemens, Picker, Philips and Toshiba have 2 well defined ground circuits.

    The DC ground circuit and the AC ground circuit.

    They connect together at one single location, near the system main inputs.
    Upon completion of an installation, one of the final tests is to open that connection, measure the resistance between the AC and DC ground. If it isn't in the megohms, we go on an easter egg hunt and fix it.

    They must connect at one single point for noise suppression.

    This is NOT ground and neutral. It is the dedicated DC and signal ground vs the AC power ground.

    I get the feeling that is what the OP is trying to get across?

    The difference between the small signal return path/ground and the input power ground. (Which should really be called a neutral.)

    The green is truly a ground. Mostly a safety ground. So if something in the circuit fails and sends current/voltage to the chassis, the safety ground will conduct the current to ground, not through you.

    The white wire is a current carrying neutral. This is the return path for the AC input for the power supply.
    It should never connect to the chassis. That is a shock waiting to happen.

    The only place that the neutral and ground are allowed to connect is at the fuse panel or at a "separately derived source" such as a full isolation transformer or a full UPS.

    Time for a sandwich.
     
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  4. LudwigvonBirk

    LudwigvonBirk Tele-Meister

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    This^!

    What is the tube-guitar-amp-land, holy-grail link, that explains this?
     

  5. Nickfl

    Nickfl Tele-Holic

    933
    May 24, 2016
    Florida
    In your example, nothing connects to household ground or neutral, they have nothing to do with the function of the amp, the power transformer isolates them from the circuit. Read post #18, it answers your question. The 9v battery analogy is key to understanding it, the transformer center tap is like the negative terminal of the battery and that is the common/chassis "ground" everything must return to for the circuit to work. I'd try to further explain it, but I'd just be getting in the way of elpico's elegant summary.
     
    Last edited: Jun 6, 2018
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  6. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    The common denominator between all uses of the word ground is that it always implies zero volts.




    The two leads of the 9 volt battery are not equivalent. One has an electrical potential 9 volts higher than the other.


    Which ever terminal you decide to be ground is ground, and the common probe on the DMM will contact that spot if a voltage measurement is taken.



    It is common practice to designate the lower potential as ground, as the name 'ground' naturally implies the lowest level.
     
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  7. SSL9000J

    SSL9000J TDPRI Member

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    My life changed forever when I began to understand that what we call "voltage" is a measurement of the difference in electrical energy potential between two points.
     
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  8. elpico

    elpico Tele-Holic

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    Sep 14, 2011
    Vancouver BC
    LudwigvonBirk, yeah Nickfl is absolutely right there. If you snip the ground prong off your plug then you'd have exactly the situation I've been describing: thanks to the voltage isolation provided by the power transformer the amplifying circuit would now be floating with no connection to earth or the wall or anything else. Even the chassis is floating in this situation, since the green earth wire you've connected to it doesn't go anywhere. So where do all the earth symbols actually go to if they don't go to earth? They go to the power supply's negative terminal. That's the black wire in our 9v battery example, or the power transformer center tap/negative lead of the filter cap in a typical amp power supply.

    And yeah that's probably a good time to address what some members have already given us a hint about, there's more than one "ground" symbol available to us. Unfortunately the terms and symbols used to communicate about electronics have always been slightly different on either side of the Atlantic. I'm North American so naturally I inhabit that tradition and it's the only one I can really speak to. (Maybe one of the UK members can explain the different way ground symbols are used in the British/IEC standards)

    Let's talk about the NA tradition of using the symbols D'tar posted on page 1:

    [​IMG]

    The first one may be the most familiar, but possibly the least appropriate choice for most guitar amp drawings. It represents a wire that you connect to the earth. As in, hammer a long metal spike into the earth beneath your feet and connect a wire to it. That absolutely is something you do when you're making an off grid device, like a solar panel at the cabin or a portable generator at an outdoor show. Not so much inside a guitar amp though.

    The second one means this wire connects to the metal chassis the device lives in. That definitely is something we do in a guitar amp.

    The last one only means that this wire connects to a point we've decided to call "zero volts". It's our choice what that point is, it's definition is completely arbitrary. It may or may not be connected to the earth, the chassis, or anything else.

    So getting back to the topic of LudwigvonBirk's question, if your drawing had used the third symbol for most of the connections it would, in my opinion, be a lot more accurate to how the circuit is actually built. I think there'd be less confusion about what happens if you snip the earth connection since earth wouldn't even appear in the drawing. Now I'm not picky about what people call that symbol. It's still quite normal to call the symbol "ground", although it can also be referred to as "common" to clarify that it does not necessarily have any connection to the ground you stand on. It doesn't matter what you call it though, as long as you remember that very important second part. (that it doesn't imply any connection to earth ground)

    It was suggested earlier that electronics practice is pretty entrenched, that nothing ever changes. Well all I can say is that does not match my personal experience. Your own experience may be different, the world is always full of variety. I'm a programmer by trade, but I spent a little over a decade working in an electrical engineering shop working directly with EEs, soldering flea sized components under a microscope, testing and repairing circuits etc. The number of times I saw either of those first two symbols in use? Zero. That can be largely explained by the fact that we were in the business of designing battery powered RF transmitters, but I think it's safe to say things have changed in the larger electronics world as well. For example, like many people I use LTSpice to draw and simulate my circuits. When I push 'g' on my keyboard to draw a ground in spice that third symbol is what I get. I don't even have the option of drawing the other ones. To create the drawings on the first page I had to draw the earth symbol by hand myself.

    So is the larger electronics world really locked into the ideas of the 1950s? Or is that something that could be more true of the people in the tube guitar amp hobby? :twisted:

    Interestingly Fender's own usage of these symbols in their schematics has flip flopped over time. The earliest tweed drawings use the earth ground symbol throughout. By 1960 they seemingly had modernized and began using the third symbol, which is what you'll find in all brown, blonde, black, and silver era schematics. Now in 2018 their schematics seem to randomly flip flop between the two symbols, some models use the common symbol, others the old earth ground. Beats me.
     
    Last edited: Jun 6, 2018
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  9. elpico

    elpico Tele-Holic

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    Sep 14, 2011
    Vancouver BC
    Now that sounds really interesting. You're going to have to tell us a lot more about that!
     

  10. elpico

    elpico Tele-Holic

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    Sep 14, 2011
    Vancouver BC
    That's not necessarily true. Although I've said a power supply consists of two leads, I should've said it consists of at least two leads, because one of the most common power supply configurations in the world is this:

    [​IMG]

    Which has three leads. This is basically what you'll find in every solid state amp, a computer, anything modern like that. The thing to note here is that the point we've chosen to define as zero volts (and marked with the common symbol) is actually in the middle of the voltages present in the device.
     
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  11. Bill Moore

    Bill Moore Tele-Holic

    In the circuit below, the green "ground" is completing the circuit in several places, and since the green, and white wires tie together at the entrance panel, the "ground" is necessary to make this circuit work.
    I have one of these chassis on my shelf now, and can attest that the "green", or in my case "green/yellow" wire is also soldered to the chassis. I don't intend to imply that if all signal returns were isolated from the chassis, (but connected together), that the device wouldn't work, but rather in real life we find circuits as the UL Twin wired as the schematic.[​IMG]
     

  12. LudwigvonBirk

    LudwigvonBirk Tele-Meister

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    I agree! Thanks much for your helpful posts.
     

  13. LudwigvonBirk

    LudwigvonBirk Tele-Meister

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    This ^ was confusing me the most, wrt this overall topic.
     

  14. elpico

    elpico Tele-Holic

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    Sep 14, 2011
    Vancouver BC

    Maybe I should clarify that everything I'm saying so far is about what you could do to the simple amplifier drawing I posted at the start of the thread. By no means am I saying anything general about the way thousands of amplifiers have actually been wired. That's been done in every possible way you can imagine.

    For example, Fender's earliest amplifiers used random chassis grounds throughout:

    [​IMG]

    They just tacked the negative end of each component to any handy nearby spot on the chassis. Since I'm already saying some unpopular things I might as well push into full on blasphemy and say: Random chassis grounds are a sign that what you're looking at is the worst kind of cheap junk. (sorry Leo :D) When you wire an amp this way you save a few pennies on wire but you give up all control over the path of one side of the power supply, and when you can't control it you can't make it match the path the other power supply lead is taking.

    I've seen people online talk favourably about an amp with random chassis grounds because they've confused it with a "ground plane", a tool used in high frequency electronics, but that's not at all a thing at the frequencies we operate at. I might be straying off topic a bit here, so briefly a ground plane works for high frequency electronics because of a remarkable effect that only happens at high frequency - if you have a wide plate of metal and you run one power supply lead around above it then use the plate as a return, the path the electrons take through the plate will exactly follow the lead that's floating above it. No matter how you zig zag that lead around the plate. Basically the electrons are carving out a perfectly matching return wire for you without you having to make one yourself. But again, this is NOT something that can happen for a guitar amp. It only works with high frequency signals. Down in the audio band we need to use actual wires for both sides of the power supply if we want to control the path they take.
     
    Last edited: Jun 6, 2018
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  15. elpico

    elpico Tele-Holic

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    Vancouver BC
    No new questions. Was it my sacrilegious suggestion that anything less than perfection has ever rolled out the door of the Fender Electrical Instrument Company? If so I'm going to be talking to crickets by the end of this thing because there's necessarily going to be more of that :(

    In case it's still confusing you, that's purely a safety thing, not a functional thing. Three wires aren't used for any reason that has to do with the way the amplifier works. Technically two wires would be perfectly sufficient to run an amp, one "hot" wire (the black one) and one "earth" wire (the white one). Back when those two were all we had our amps functioned fine (although an unrelated problem with the physical design of the north american wall socket at the time could cause you to get shocked while the amp was functioning fine), but electronic devices are not always functioning fine. They fail.

    If you're going to build any kind of electronic circuit across the black and white wires you're necessarily connecting the hot wire to the earth wire in various ways, and that opens up the door to possible failure situations that could make the voltage of the black wire appear on the white wire. Not good if you're counting on the white wire to not shock you. The third green wire was added for this reason. Although both white and green are ultimately connected to ground somewhere the difference is that we put the white wire in harms way, so to speak, and keep the green safe. We make the white wire take the dirty job of connecting to the black wire in hundreds of ways throughout our home that could potentially fail, but we don't allow any connections to the green wire. The green is to be used purely as a protective shield, it's not to be made part of the electronic circuit in any way or have current running through it.
     
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  16. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia



    that's interesting.



    can you explain the need for 4.5, 0, and -4.5 volts out?
     

  17. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    It is usually refered to as a split-rail supply. Half the voltage either side of the central ground reference, one positive, one negative.
    One could apply the ground to the lower rail, and have two voltages both positive, the middle one half the voltage of the top one....or make two negative voltages with the ground on top.

    Why a negative and positive supply of equal voltage? It is a very good way to power an op-amp, you can make good symetrical outputs that way, to run straight to the mixing desk.
    Or you could use the negative supply to bias a preamp tube....or many other things...some bit that dont know or care if they are being powered with a negative supply, like the VACTROL'S in the Rivera-era fenders.

    Why split-rail?

    A rail is just a another term for the voltage supplys conductive line/wire/bus/plane/path/cable...etc.
     
    Last edited: Jun 7, 2018
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  18. elpico

    elpico Tele-Holic

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    Vancouver BC
    and the number 4.5 in the drawing I posted is just random. I knew there'd be 100s of images of split rail power supplies already drawn so I just googled and took the first one I saw. The exact voltages can obviously be higher or lower. +/-12 is very common.
     
    Last edited: Jun 7, 2018
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  19. SSL9000J

    SSL9000J TDPRI Member

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    I'd like to present a question, if I may: Could you please explain the correlation between ground and unwanted noise in an amplifier? Or perhaps more accurately, the correlation between grounding technique and unwanted noise.
     
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  20. LudwigvonBirk

    LudwigvonBirk Tele-Meister

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    I second, and propose that it is done in a cleanly titled brand-new thread.
     
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