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Ground. (What it isn't)

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

  1. elpico

    elpico Tele-Holic

    674
    Sep 14, 2011
    Vancouver BC
    I've noticed that questions about "ground" come up in many threads, sometimes sidetracking them a little from the intended topic. I wonder if a thread dedicated to discussing ground might be useful?

    I'm going to start with a blunt opinion:

    It would probably be best if you forgot you ever heard the word ground.

    The ground. The thing we are standing on, the earth beneath out feet... it plays almost no role in the function of these gadgets we like to play with. Consider the basic common cathode gain stage:

    [​IMG]

    You may be accustomed to seeing a ground symbol attached to the bottom line of this drawing, but there's nothing about the operation of this circuit that requires you to connect that point to the earth. In fact you don't have to connect ANY point on the circuit to the ground. As drawn above there is no connection to earth at all (imagine V2 is a battery if that seems confusing) yet the circuit is happy to operate normally floating this way.

    No current is being "drawn from ground", no signal or electrons are "returning to ground". It's not connected to the ground in any way yet the tube will settle at the same bias current it always does and the input signal being applied to the tube's grid will be amplified and appear in a much larger version at the output the same as it always has. In truth you can directly connect ANY point in this circuit to the earth:

    [​IMG]

    and it won't make a bit of difference. I think that last one in particular may be unintuitive to some people:

    "If you connect the input signal to ground doesn't the signal disappear into the ground?"

    No. Nothing is going into or coming out of the ground. The ground is not involved in the operation of this circuit at all. That's the point I'm trying to get across here.

    So why are you used to seeing a ground symbol connected to the bottom of this circuit then? If nothing goes into or comes out of the ground connection, if earth plays no role in the operation of the circuit whatsoever, then what's the point of connecting it to ground at all? The answer is more simple than you might be thinking:

    Safety.

    Guitar amps traditionally make a direct metal to metal connection between the guitar and the first tube. That means whatever voltage appears on the grid of that tube appears on your strings as well. Expensive gear might have an isolation transformer on the input to prevent this from happening (and eliminate the possibility of ground loops in the studio) but guitar players are underpaid and guitar amps have rarely gone to the expense of an isolated input. Luckily, as we've already seen you can connect any point of the circuit you wish to the ground without altering how it works. So we can simply connect the point that we're touching to the earth we're standing on to avoid any potential shocks.

    That's probably enough for an opener. There's a lot more to say about the topic, but for step one I think trying to forget that you ever heard of anything going to or coming from ground is a good start.
     
    Last edited: Jun 5, 2018

  2. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    Well put !
    It has been mentioned before, but nothing like as often as the many varied false claims contradicting the basic factual truth in what you state here.
    Thanks for starting this (sadly) necessary thread, and bring us all back to earth....or do I mean ground....:twisted:
     
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  3. LudwigvonBirk

    LudwigvonBirk Tele-Holic

    Age:
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    Aug 26, 2017
    Madison
    Great post elpico.

    Haiku-summary:


    amps had 2-prong plugs

    they made a nice sound back then

    but they could shock you
     

  4. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    Great post



    what ground is not
    It is not negative or positive. It is a reference level.

    what ground could be


    It might be an arbitrary reference level, or in the case of earth ground or chassis ground it physically is the best choice for the ground reference, and provides a convenient 'common ground' to connect many components together.

    even in cars, the chassis makes a logical and convenient ground reference, but it is arbitrary whether it is referenced to the positive or negative terminal of the battery.

    if ground is referenced to something relatively massive and conductive, then it is both a source of endless electrons and a place that will receive endless electrons.






    there are many interpretations of ground, and they all have their place, but I agree with the OP that ground is often misunderstood.
     
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  5. Recce

    Recce Tele-Holic Silver Supporter

    970
    May 3, 2016
    Northern Alabama
    Somewhere in the middle of the first poster I started skimming. I don't understand enough to be involved. I will stick to newer gear where this is worked out for me and with a three pronged plug.
     
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  6. LudwigvonBirk

    LudwigvonBirk Tele-Holic

    Age:
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    Aug 26, 2017
    Madison
    You did such a fine job in your first post elpico, I would like (at some point, when you have time) to request you add some additional commentary about the need-for/use-of resistors and capacitors between a component and ground.

    Here's a randomly-grabbed schematic with several such scenarios. If I snipped every connection to ground in the picture would everything in the circuit still work and just be unsafe?

    upload_2018-6-5_15-42-4.png
     

  7. Bill Moore

    Bill Moore Tele-Holic

    When you use a battery, there is no necessary direct connection to chassis/ground. However, when we use voltage from the wall outlet, not just the (green), ground connection is tied to "earth", but also the neutral at the service panel! So whether or not you draw the connection "earth" symbol in your circuit, an A/C circuit will connect at some point. You may lift all chassis connections, but the neutral feeding the transformer is "grounded".
     

  8. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Not mentioned is the concept of “circuit common”, which is not always earth ground,but can be. Usually shown on schematics as a triangular symbol. Very convenient to use because it reduces clutter on a schematic, as opposed to drawing lines everywhere. Some schematics use the ground/earth symbol for common.
     

  9. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    No, it wouldn't work, you wouldn't have circuits anymore, just many open-ended strands of components.
    However, if you then joined all the snipped ends together, it would work.......but only as long as all the conections in the whole amp that where connected to ground are snipped and rejoined together, (which is just the same thing as snipping off earth wire at the power cable)....or at least, if all the power and signal connections that are needed to keep the thing going as you'd like it to - if you left off certain side-chains (such as lifting a tone-stack ground reference) then you could leave some disconnected, but they would not be working.

    and YES..IT WOULD BE UNSAFE
     
    Last edited: Jun 5, 2018
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  10. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    generally,

    AC grounding is done thru a capacitor, or a resistor and a capacitor.

    DC grounding is either directly connected to ground or referenced to ground thru a resistor
     

  11. dsutton24

    dsutton24 Poster Extraordinaire Gold Supporter

    Dec 29, 2010
    Illinois
    For the most part, it's a technically accurate rant, but that's all it amounts to. The terminology is pretty well entrenched, it is not going to change to suit you. Unless you're willing to refer to 'tubes' as vacuum tubes, or (gulp!) valves, 'amps' as amplifiers, and 'grids' by their specific function, trying to get everyone else to refer to circuit commons as anything other than ground is just selective curmudgeonry on your part.
     
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  12. schmee

    schmee Friend of Leo's

    Jun 2, 2003
    northwest
    Well.... your ac plug is likely 3 prong and one actually does go to the Terra Firma Earth! There's a copper rod pounded into the ground that it goes to.
     
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  13. Snfoilhat

    Snfoilhat Tele-Holic

    Age:
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    Apr 8, 2016
    Oakland, CA
    IMG_2059.JPG
    As an amp-building hobbyist I only ever post the link to Valvewizard's freely downloadable chapter on grounds. http://www.valvewizard.co.uk/Grounding.html

    As a teacher (my profession, trained as a researcher but teach for a living), man my hat is off to y'all. This is a very challenging format for teaching. Questions will arise that sometimes have false premises which need to be identified and addressed before tackling the question itself, well-meaning folks will chime in w/ answers that are potentially incorrect or misleading, and it all has to be managed faster than misinformation or thread-dissatisfaction overwhelms the whole thing and it sinks...

    Safety ground is an unbroken connection from the metal chassis to the actual earth, with no current flowing through it (other than in some fault condition I suppose), maintaining the same potential in both (voltage = difference in potential). Has nothing to do with the circuit, but with the circuit's packaging*, as other's have said.

    But there are 100 other ways in which common or whatever you choose to call it is crucial to the well functioning and understanding of the circuits we're interested in.

    ***Edit to say the 'circuit's packaging' is pretty darn important, since guitar players are always touching it! The strings and bridge and pickup covers and control plates of our guitars, by tradition, are directly electrically connected to it.
     
    Last edited: Jun 5, 2018
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  14. elpico

    elpico Tele-Holic

    674
    Sep 14, 2011
    Vancouver BC
    dsutton, It's only an opening post not the end of the story. I'm trying to discuss the behaviour of the circuit and clarify what ground is and isn't doing in it which is extremely relevant to how to physically build the thing, not just what to call it's parts. I didn't even get to begin ranting about what to call the parts. Never said a word about that! (that post can come later :lol:) I'm going to keep going into how the thing works though and will certainly use the symbols for chassis and common so you're entirely welcome to tune out now if you want and find a thread that's more to your liking.

    Bill Moore & schmee, one end of the primary winding of the power transformer is connected to ground via the neutral wire at the house panel, but there's no connection between the primary and the secondary windings of the transformer. That means the secondary (and the circuit we build across it) is free to be entirely floating, connected to none of the three wires coming from the wall. The only thing "connecting" the circuit to the wall outlet is a magnetic field. Unless we choose otherwise.
     
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  15. LudwigvonBirk

    LudwigvonBirk Tele-Holic

    Age:
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    Aug 26, 2017
    Madison
    Can those who understand why please summarize the reasoning for each? (or provide a trustworthy link?)

    thx!
     

  16. Snfoilhat

    Snfoilhat Tele-Holic

    Age:
    37
    621
    Apr 8, 2016
    Oakland, CA
    peteb's use of the word 'grounding' is exactly in conflict with the entire point of elpico's post, the entire point of the thread. This is what I mean by false premises. The safety ground is a direct connection through good conductors. No resistance, no capacitance.

    Furthermore, if we accept that peteb is using ground to mean circuit common like so many folks do (so let's cut him some slack), then all he is really saying is that resistors and capacitors are some components often found in a circuit. And that common or the negative side of a circuit or the electron source or whatever is also a part of a circuit. There's no there there. It's an observation without a principle behind it, but superficially appears to be some kind of electronics rule or piece of useful knowledge. Sure, let's remember the basic info about caps, how they pass AC and block DC, and let's remember the definition of a resistor, but it's not really conceptually connected to grounding. Meaning no offense to anyone. But pedagogically this is the mess.
     
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  17. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY

  18. elpico

    elpico Tele-Holic

    674
    Sep 14, 2011
    Vancouver BC
    Okay so I left off by saying that our basic amplifier circuit up there operates just fine when it has no connection to ground and is left floating. Also that you can connect any part of the circuit to the earth that you want and it makes no difference to the way the circuit works because nothing comes into or out of the circuit through that ground connection. We choose to connect one point of the circuit to ground purely as a failsafe to protect the player in the event that something should fail or come loose in the amp that ends up connecting the plate side of the power supply to ground. (with both sides then connected to ground the power supply would be shorted, produce no voltage, and blow the fuse)

    The second bit bears repeating - nothing is flowing through the connection to ground.

    One of the common confusions I see on here is a mental picture of the amp that imagines the power supply as just a red wire running around the amp and the ground connection as an endless sink that electrons are being "sourced from" or "dumped to". But I've just said that nothing at all is flowing through the ground connection, so where are the electrons flowing through that red power supply wire actually coming from or going to?

    The answer is also ...the power supply.

    A power supply consists of two leads, not one. All electrons are being sourced and sunk into and out of the terminals of the power supply, not the connection we made to ground. Here's a basic power supply we're all very familiar with:

    [​IMG]

    The two leads are equivalent. The red one is not a "noisy, nasty power supply" and the other a "quiet, endless sink of ground potential". They're both power supply wires. Two equivalent wires that have a DC voltage between them. Or at least we hope a purely DC voltage is the only difference between them. Any AC difference between them can cause unwanted noise in our circuits.

    And electrically speaking, the world is a noisy place. Your average household now has nearly a dozen radio transmitters putting out noisy fields of electromagnetic energy at all times (your cell phone, your wife's phone, her ipad, your pc, the router, your tv etc) and all your neighbours have another set of the same gadgets. The world is doubly noisy inside your amp where you've got all that noise plus rectifier noise, filament wires etc in close proximity to our sensitive, high impedance tubes.

    The thing about this noise is it's not even across physical space. The type, frequency and amplitude of the noise is different in one physical spot than another, even on very short distances. This is where the difference between the two mental pictures can start to produce important results for how you build your amp. If your mental model is "the power supply is one red wire and ground is ground, an endless sink of electrons" then you have no tools for avoiding this noise. When you recognize that ground plays no part in this and the circuit is actually a loop between two power supply wires then you do have a tool. You can't make the noise stop existing, but by treating the two power supply wires as equally as possible, making them run the same path through the noise field, twisting them tightly together wherever you can, you ensure that equal noise will be induced in both of them, which leaves the difference between them a purely DC voltage like we hoped.

    That's probably enough for a second post. I'll come back tomorrow and see what you guys have to say so far.
     
    Last edited: Jun 5, 2018

  19. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia

    It's hard to find good descriptions of all the types of grounding. AC grounding is the same or similar to signal grounding and you will find more info on signal grounding. About the only mention I found on AC grounding is that AC grounding is done thru a capacitor. That's it.

    AC grounding is done thru capacitors as they block DC. That's the basics.
     

  20. SSL9000J

    SSL9000J Tele-Meister

    Age:
    46
    102
    Dec 18, 2016
    North of Atlanta, GA
    More than once I've heard the concept of "ground" spoken of as something everyone with a soldering iron must struggle with at some point. Not least of which because, as Elpico has pointed out, it means different things to different people.
    Indeed. Perhaps a good starting point, for those of us with misconceptions who wish to un-learn them, would be to think of the "ground" in our amplifiers as a conductor; nothing more, nothing less?
    This is the part that really piques my interest. I hate noise. Hate it. But I've read that proper grounding technique, (or "grounding philosophy" if you prefer,) can banish detestable noise from our signal path. For this I shall stay tuned; pun only slightly intended. Thank you, Elpico, for taking the time to explain these elusive concepts.
     

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