Sewer vs signal ground question

[eddieddie]

Hello! I've done as much searching and reading as I can and was wondering if somebody could give me a quick sanity check re: grounding for stompboxes. Does the following logic make sense?

Ground on a voltage divider is signal ground because signal oscillates around it, but the filter caps on the actual divider go to sewer ground (because we don't want all the noise to bleed into signal).

The cap and resistor in a LPF/HPF respectively go to sewer ground, because we don't want the signal to bleed into signal grounds.

[PRR]

> go to sewer ground

Poop? Dirt? What kind of pedal is this?

I think these "grounds" need specific descriptions to know what we are talking about.

Also, everything is some kind of voltage divider, and it may matter when we want to know if it is poopy or dirty.

[eddieddie]

> go to sewer ground

Poop? Dirt? What kind of pedal is this?

I think these "grounds" need specific descriptions to know what we are talking about.

Also, everything is some kind of voltage divider, and it may matter when we want to know if it is poopy or dirty.

It'll be a dirt pedal eventually, but right now it's a pile o poo. So both, I guess.

I'm trying to figure out a reasonable grounding scheme that minimizes unwanted noise/bleed/etc. I realize I may be overthinking things, but I enjoy learning and the process!

The voltage divider is a divider providing ref voltage for an op amp gain stage. The LPF/HPF are just passive RC filters.

[ElectricDruid]

Welcome Eddieddie!

I agree with PRR that we might need a bit more information before we get what you're talking about. "Signal" and "Sewer" grounds is not terminology I've heard before.

I *suspect* that your "Sewer ground" is what I'd call "Ground", and your "Signal ground" is what I'd call "Vref" or "Vbias", the midpoint level that things are referenced/biased to inside the pedal. But I'm guessing.

The only trouble with this is that when it comes in, the "signal" has the input jack with the ground connection to your "sewer ground" not your "signal ground", which makes the naming a bit confusing. But that's what input capacitors are for, I guess

I can think of situations in which LPF or HPF might go to either ground. So I don't think it's clear cut, or at least, there isn't one rule for everything.

HTH

[eddieddie]

Welcome Eddieddie!

I agree with PRR that we might need a bit more information before we get what you're talking about. "Signal" and "Sewer" grounds is not terminology I've heard before.

I *suspect* that your "Sewer ground" is what I'd call "Ground", and your "Signal ground" is what I'd call "Vref" or "Vbias", the midpoint level that things are referenced/biased to inside the pedal. But I'm guessing.

The only trouble with this is that when it comes in, the "signal" has the input jack with the ground connection to your "sewer ground" not your "signal ground", which makes the naming a bit confusing. But that's what input capacitors are for, I guess

I can think of situations in which LPF or HPF might go to either ground. So I don't think it's clear cut, or at least, there isn't one rule for everything.

HTH

I've been here a long while, just quietly learning, much from both of you (Tom and Paul). The terminology I picked up from one of RG's many explanations. I hope he doesn't mind me copy and pasting below from a previous thread here: https://www.diystompboxes.com/smfforum/index.php?topic=124462.40

Here's the basic grounding scheme I'm thinking: Signal grounds/ input jack/ output jack connected from board to power jack, 'sewer' ground (op amp ground, other noisy grounds, LED) on their own path meeting again at the power jack. Now the task is to figure out which path to connect each point of the circuit that is looking for 'ground'.

From RG:

I classify "ground" into several categories, depending on the currents they carry. I think of them this way:
1. Shield ground: this is a ground that intercepts radiated fields and dissipates them, or moves the currents to where the implicit electrical loop can be satisfied. The more a shield ground totally encloses a circuit, the more effective it is. Shields dissipate RF waves and AC magnetic fields by converting the RF energy to heat in current loops in the shield conductor. They prevent electrostatic field interference by shunting the field voltage to the One True Ground, where it can't get back out. Shields carry any random voltage that comes in, and therefore there should be one and only one wire connecting the shield and the One True Ground in any box. There are some asterisks and caveats to do with high frequency RF, but for pedals, this is how it works. There is some subtlety involved about grounding input jacks, output jacks and so on.
2. Power grounds: Electricity runs in loops (... circuits...) and so power out of the power supply must return to the power supply. If you have one wire from the power source to the whole mess of stages in a circuit, all of the return current is mixed on that one wire, creating what I call a "sewer ground", a wire that carries all of the "used electricity" back to the power supply.
3. Rectifier grounds: This is a special case. The pulses of current from rectifiers into/out of a first power supply filter cap generate a 2x power line frequency voltage in all the two wires leading to/from that cap. If the rectifier return wire goes anywhere else than the negative terminal of that first cap, you WILL get power line ripple/hum that you can't get rid of by anything other than moving that wire.
4. Signal grounds: these carry the signal return currents. If you let any other currents (like power return currents) onto these wires, you will have interfering signals introduced into your signal. A test of practicality applies to this. If the currents are small enough, the introduced noise will be small, maybe small enough to not be audible. Pedals often qualify for the "small enough" exemption.
This could go on. You see the point: don't mix current sources on signal ground wires, because if you do you will introduce the currents times the ground wire impedance into your signal.

[idy]

Usually your "Vref" has a big ol' cap across it to.. the sewer. So for signal (AC) it is ground. For DC it isn't. (This is fresh from a recent back and forth.)

With digital circuits and things with LFOs is where you got to engineer such issues.

[ElectricDruid]

Ah, ok, thanks! That does clarify things!

Yeah, RG is talking about "different" grounds, but they're all the same thing. I was talking about two totally different levels. No wonder there's room for confusion!

So the gist of what RG is saying is to make sure that currents from one part of the circuit can't hit the ground in another part. The details of that are going to be very dependent on the specifics of what you're trying to do. Ultimately, everything is going to connect back to your power input ground point, one way or another. Perhaps some things you want to have their own route back there, so they're not mixed up with other stuff. So the bypass LED could have its own path for example, since it draws a decent current when it turns on (although many modern LEDs don't really, not like the 25mA basic red LEDs I remember from my youth). IDY's example of an LFO is a good one too, since many LFO design have an op-amp that crashes from rail to rail back and forth, and the current drain jumps about as it does it. You don't want that on the same ground wire as you use for sensitive stuff - the dreaded ticking!.
You've got plenty of gain, so you could use shielded wire for the input and output wires from the jacks to the switch and to/from the board, and those shields should all be grounded to one place and close to the original power supply ground, or given a low impedance path back to it (eg. a fat wire).

Note that I'm not saying that everything has to have its own individual wire back to the power supply ground, not at all. I think people *do* overthink this. As long as the path to ground is nice and easy for current to flow down, there'll be no problems. Resistance is your enemy in ground paths. The lower the resistance, the closer they all are to being exactly the same thing, and the less you have to worry about it. In many low current pedal circuits, that means you don't really need to worry about it at all. As gains go up, things get more sensitive, but still, you don't need to go crazy. If you start adding digital and analog elements in the same circuit, you throw in another complication ("mixed signal design", as it's known) since high frequency digital chips like processors often chuck a lot of noise onto their power supply rails, and you ned to keep that away from any analog circuits in the same box or on the same PCB. But a dirt pedal doesn't have that sort of stuff to worry about, so you're good.

HTH.

[Rob Strand]

Dogs know where sewer ground is.  Birds, maybe not.

[antonis]

I presume plumbers wouldn't have any objection..

[anotherjim]

I'm not sure about the sewer analogy. Before it gets back to the supply (reservoir) it will need to pass through a treatment plant!

[Kipper4]

Any more mucky jokes? 

[eddieddie]

Note that I'm not saying that everything has to have its own individual wire back to the power supply ground, not at all. I think people *do* overthink this. As long as the path to ground is nice and easy for current to flow down, there'll be no problems. Resistance is your enemy in ground paths. The lower the resistance, the closer they all are to being exactly the same thing, and the less you have to worry about it. In many low current pedal circuits, that means you don't really need to worry about it at all. As gains go up, things get more sensitive, but still, you don't need to go crazy. If you start adding digital and analog elements in the same circuit, you throw in another complication ("mixed signal design", as it's known) since high frequency digital chips like processors often chuck a lot of noise onto their power supply rails, and you ned to keep that away from any analog circuits in the same box or on the same PCB. But a dirt pedal doesn't have that sort of stuff to worry about, so you're good.

HTH.

Thank you for that Tom..! Definitely over-thinking a little. Maybe a better exercise is to figure out a better grounding scheme for breadboarded circuits. Mine's got all sorts of fun noises.

Dogs know where sewer ground is.  Birds, maybe not.

My dog treated the world as his sewer ground. No shame.

[anotherjim]

Any more mucky jokes? 

I'm not sure why "analogy" isn't the study of butt holes.

[POTL]

This is an interesting topic, I have seen pedals many times in which the ground is divided into parts. Separate ground for LFO, separate ground for digital, etc. Including, I saw how each land was made by a polygon, it was just that the board was divided into 2 parts or more (a thin strip of copper was removed). We know how to separate the voltage with a simple RC filter, but how to separate the ground? Can you show a diagram or draw a drawing?

[antonis]

We know how to separate the voltage with a simple RC filter

Do we..?? 

[garcho]

Ground is needed in order to make a circuit complete (sewer), but it's also needed as a reference point (signal).

[ashcat_lt]

I didn't see it addressed above, but generally the cap to ground in an LPF can really go to either ground or Vref because there's no DC getting through it anyway, and presumably the cap across the Vref divider will be big enough that it won't impact the cutoff frequency of the filter.  In an HPF, the R is on the bottom, and since DC can flow through, you have to at least consider the relative DC voltage on either end in order to avoid throwing your whole thing out of bias and/or put it back to the bias it had before the cap stripped it out.


Edit - maybe that's a long of way of saying that for AC signals the two places are essentially the same place (given the Vref bypass cap is big enough), but when there's DC involved, it very off matters very much which one you pick.

[R.G.]

Nota Bene: the somewhat strained analogy on grounds - including sewer grounds - was intended to illustrate the fundamentals of how the exact path from a circuit element back to power supply ground can matter. It's difficult for beginners and some pros to remember that (1) every wire is really a resistor, (2) currents flowing through those resistors create voltages and (3) you can remove the maybe-ugly mixing of ground voltages by separating the currents onto different "ground" wires.

Sewer ground is where you pour currents that you don't want stinking up the rest of your grounds.   

[anotherjim]

With shielding, I wonder about the effect of Eddy Currents.
Take your metal enclosure and put it near a strong AC magnetic field.
I will then suppose that Eddy currents may get induced into the metal and they will circulate.
Will the paths the current takes flow around the box, effectively making it a single turn short circuit coil? Then the current can be large as the material resistance is the only limit on it.
Can the currents in the enclosure in turn induce Eddy currents in the enclosed circuit?

[DrAlx]

The strong AC magnetic field will induce eddy currents in both the enclosure and the circuit within.  If you take the enclosure (and it's eddy currents) away then less energy will be removed from the external field (because the eddy currents in the enclosure used up some of the energy from the external field).  So no, the enclosure and it's eddy currents do not make things worse by being there.