Star grounding, why the insulated output jack?

[Max999]

Allright, I have been reading a lot of posts about star grounding in a pedal and this is what I found:

Best practice is to ground the input jack ( so Maxwells law sends the rfi riding along the cable to the chassis). Then we connect the sleeve of the input jack to a point on the pcb where the different traces carrying grounds meet ( the star). This seperates the ground currents so they all will be connected directly to a clean ground and they will all be at the same potential, eliminating hum and other ground related artifacts.

The part that I don't understand is why it is also neccesary to use an isolated output jack and run a wire from the sleeve to the star ground.

Why can't this be an unisolated jack without the wire? Isn't the chassis also providing a single connection to ground using an unisolated jack?

[GibsonGM]

Nope; it could create a ground loop (in theory).  Whether it WILL do so (in any way you can hear) is open to interpretation, though.   
The main reason for not grounding the jack to the enclosure and using the wire is, however...because over time it will work loose, and you'll get a god-awful BUZZZ when that happens! 

....usually, what we end up with is some sort of 'modified star', in practice...at least in MY practice, LOL...The current levels we're dealing with aren't high enough to worry as much as one would with an amp, for example.

[GGBB]

Why can't this be an unisolated jack without the wire? Isn't the chassis also providing a single connection to ground using an unisolated jack?

Because then it isn't a star. The output jack ground would be connected to the input jack ground which then goes to the common star ground.

[Marcos - Munky]

It's because the star point is not in the chassis, and you need to directly connect every ground to the star point, including the output jack ground. If you use an unisolated output jack, the output jack ground will be indirectly connected to the star ground by the enclosure.

Btw, usually in pedals a star grounding doesn't really make a huge difference. A proper ground planning is required when building high gain preamps and/or circuits that use noise sensitive parts (J201, LM386, CD4049, to name a few), but in most of the cases, just avoid ground loops and you'll be fine. Ground loops are bad for any pedal.

[EBK]

Ideally, you use the chassis as a shield, but not a ground current conductor.  This means only a single point on the chassis gets connected to ground, and you aren't using the chassis to provide a ground for anything else.

(I prefer to ignore the fact that we easily get away with ignoring these types of "best practices" and the fact that following them does not make any audible difference. )

[Max999]

Thanks everybody.

I see now that in my example the output jack ground is daisy chained with the input ground to the star point.

But what if I make the input jack ground the star point ... still wrong?

[PRR]

In audio gear as small as a pedal, "star grounding" is pointless.

You do want to be sure high-current paths don't share the wire with low-current paths. Otherwise none of the wires are long enough that "star" or "bus" or "mish-mosh" really matters.

[Max999]

I like high gain PRR. What I noticed is that humfree circuit followed by a clean amp/pedal will suddenly have worse perceived noise specs because of the compression of the high gain. Offcourse high gain will always have a high noise floor, but if I can change 5% of it I will try to. (Emphasis on "try") 

[Paul Marossy]

Honestly, in my experience, the only place I've found star grounding to be useful is in a tube amp.

[EBK]

I use the DC jack as a star ground in my pedals, despite the fact that they would sound exactly the same if I didn't.  For me, it is part of the artistry/craftsmanship* and makes me feel more proud of my work.


*A personal aesthetic choice -- I don't feel that my stuff is better that anyone else's just because I choose to do it my particular way.

[Paul Marossy]

Ideally, you use the chassis as a shield, but not a ground current conductor.  This means only a single point on the chassis gets connected to ground, and you aren't using the chassis to provide a ground for anything else.

True, but that was not common practice for a long time. Almost all of the older effects used the chassis as a common ground. But back then tape hiss and pops on records were just accepted, as well as some hum from amp/pedal/guitar.

[vigilante397]

I completely ignore star grounding convention in pedals and just ground everything everywhere. 

It tends to work.

[EBK]

I imagine if you purposely tried to create an infinite number of ground loops, you'd end up with effectively zero ground loops.... 

[vigilante397]

I imagine if you purposely tried to create an infinite number of ground loops, you'd end up with effectively zero ground loops.... 

That's exactly my strategy when building pedals. In amps I absolutely go for star grounding, but in pedals I find the Infinite Ground Loop Cancellation convention to be extremely useful.

[MrStab]

forgetting all concerns about ground loops, IMO star grounding is way more reliable for enclosure-shielding generally. use a coupla tooth washers along with your crimp, nut and bolt and that thing will never come loose.

RG's system of using a PNP to power up charge pump circuits via. a stereo input jack often makes me wonder if there are other scenarios in which it's bad to pass circuit grounds so close to the high-impedance input. That's another reason i always avoid grounding everything to the input jack.

[reddesert]

IMO, star grounding is a well-tested tube amp practice that has made its way into pedals with no particularly good engineering reason.

[MrStab]

i contend that as the input jack is a mechanical component which may be used fairly frequently and has some risk of loosening, its role should be purely that of carrying the input signal and its specific ground, and shouldn't be relied on for the entire ground network to reach the enclosure. So while the intention may often be wrong, i think star grounding is superior for that reason.

[GibsonGM]

Nope; it could create a ground loop (in theory).  Whether it WILL do so (in any way you can hear) is open to interpretation, though.   
The main reason for not grounding the jack to the enclosure and using the wire is, however...because over time it will work loose, and you'll get a god-awful BUZZZ when that happens! 

....usually, what we end up with is some sort of 'modified star', in practice...at least in MY practice, LOL...The current levels we're dealing with aren't high enough to worry as much as one would with an amp, for example.

[MrStab]

i should have re-read other posts before opening my beak! anyone got a cracker?

[blackieNYC]

Star grounding can really be fruitful in building a studio. In fact, you'd be crazy not to. There usually is a couple "sub-stars" going on, but you can't stray too far.
I believe the size of the loops is one of the factors- in the sense of accidentally fabricating an antenna, and ground runs that have a little resistance, thus potential difference, between ends, are the kinds of factors that lead to problems in the high- fidelity atmosphere of a studio. It's hard to believe these problems can happen in a little aluminum box.