Star grounding, why the insulated output jack?

[GibsonGM]

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

No problem, I just wanted to stick my nose back in!!   

Yes, for the 'trouble' of doing it, star grounding is a good idea.  Sometimes we end up doing the 'modified' approach, but anything you do to make a low-resistance groundpath that 'flows' correctly - will always yield less noise!  It becomes more and more important as your power levels increase.

[Max999]

Thanks everybody. I made a star ground with the star at the big filter capacitor. The casing however is only connected at the input jack for rfi protection, so they go together to the star. I hope this is not an issue.

[karbomusic]

, however...because over time it will work loose, and you'll get a god-awful BUZZZ when that happens! 

Just to drive this point home, if you have a ground loop issue, you'll likely know when you build the pedal, when the output comes loose you probably won't know until the most embarrassing and inconvenient time. IMHO - ask me how I learned this valuable lesson.

[R.G.]

Star grounding is a means to an end, not an end in itself. It is not the only way to minimize cross-contamination of signal from different sources, nor the only way to cure hum issues, but it is the only way known that definitely will cure such issues before actually building and testing (or tediously modelling) a circuit.

True star grounding is probably never done in practice, because there would simply be too many wires. Modified star grounding, which clumps together "related" circuit elements in a local "star" and then star grounds the clumps is much more practical.

I've typed in my polemic on the number and kinds of ground conductors and currents before several times, so I won't do it again. But the discussion in this thread has highlighted some areas that are confused, so I'll hit those a bit. DC and audio grounding are close to identical, as the frequencies where electromagnetic field effects happen are mostly above audio. This matters because hum is down in the audio range where you just need to have a low-ohms connection in for low-current circuits. For pedal circuits, unless you have some odd cases like high current paths, you can ignore it.

One counter example where you can't ignore this is high pulse current  sections in a pedal, and the best simple example of this is a power converter running at audio frequencies, like a 555 or a charge pump. The 555 in particular has the nasty habit of sucking an amp or two for a microsecond or so every time it changes state. So its ground wire - and power wire, for that matter - had better not be part of your signal ground path, or you'll hear the "ticks". Charge pumps do a similar thing as they charge and discharge their caps, but most charge pumps run above audio where it's much easier to filter the stream of ticks out and you can't hear that high in any case.

At yet higher frequencies, the electromagnetic fields force electrical currents to flow in ways that you don't expect until you've studied RF setups. When you get into radio bands, you need to think of the shielding type of ground. Shield grounds keep radiated RF out of circuits. That's one reason we put effects in metal boxes. We also live in a sea of power-line-frequency EM fields, so metal boxes prevent both the capacitive transfer of power-line hum out of circuits, and the RF-buzz of power line related spikes of RF as generated by fluorescent lights and such out of circuits.

Inputs are special. They are the circuit's most sensitive point and they're exposed to the world outside the box. They need special consideration. Input jack grounds must connect to the shielding ground to keep RF out, and much connect to the signal ground to get the signal to the circuit. For pedals, the shield ground of the box CAN act as a signal ground, as long as the jack is connected tightly to the shield/box ground. But as noted, it doesn't hurt to have a backup wire. Backup wires can break, but this happens less often that fixing nuts come loose.

Input ground wires are special too. They are the conductor that carries the tiny but real currents back to wherever they were generated. So mucking up an input ground wire with carrying power supply currents is a not-too-great idea. And many pedals do this by using the trick of using a stereo input jack to ground a battery or DC negative wire to carry DC power to the circuit board. It is true that the currents are small, and that the wire resistance is low, so nearly always you get away with it.

Nearly always.

I vastly prefer using a PNP transistor or a P-channel MOSFET inserted in the positive power line to switch power to a circuit, and connecting the base/gate to the stereo contact to turn the device on/off. It makes any current carried on the input ground be not only dramatically smaller, but also constant. Of course, you could do the same thing by just using the output jack for your power switching.

[Max999]

Thank you R.G. for your very complete answer. It seems that you are very much in line with PRR's answer, by stating that the currents are too low to really matter for hum, except as mentioned by you in oddball cases and directly at the input sleeve ( by using the power switch trick).

Regarding the input jack connecting to the shielding ground for RF protection, would it be a good safety measure to also connect a wire from the input sleeve to the chassis with a bolt?

Or would this destroy the RF protection and thus need the low value capacitor from sleeve to shield you mentioned in another grounding thread?

[R.G.]

In little circuits and boxes, the differences are small.

For the very, very noise conducted in, you'd insulate the input jack and carry the signal ground lead all the way to the input of the first stage. This is likely to be unnoticeable in most pedals. I could design one to show it up, but that's not the point of pedals. If you did an insulated input jack, you want a small ceramic cap to the chassis from the input jack's 'ground" contact. This bleeds RF off to the shielding shell instead of conducting it into the circuit. If you connect the input jack to the chassis by the bushing, a cap is useless. If you like belt-and-suspenders, you can do a ground wire from the circuit board to the bushing of the input jack, and be assured that if one fails open, the other will keep the big buzz from happening.

I would consider it overkill to make a wire connection from the jack to a bolt on the chassis. Sure, there are some situations where this would help if something else is failing, but those situations are way out on the edges of rarely happening. Not worth the worry. Shoot, if you're on stage and something like this happens, replug your effects to get rid of that pedal, and get back in and play. As Frank Zappa famously said, shut up and play your guitar. 

[Max999]

Thank you again R.G. but I am a bit confused about this sentence:  "If you like belt-and-suspenders, you can do a ground wire from the circuit board to the bushing of the input jack, and be assured that if one fails open, the other will keep the big buzz from happening".

We always need a wire from the input sleeve to the circuit board when the circuit board is not in contact with the shield, right?

[EBK]

I've had my Peterson strobe tuner embarass me on stage due to an input jack that worked its way loose, resulting in loud popping every time I retuned.  During a break, I tightened it back up with a socket wrench from my gig toolkit and stopped the popping. 

It does not matter how a pedal is built until it does. 

[GibsonGM]

I had that happen exactly ONE time.  Total loss of contact..."BZZZZZZZZZZZZZZZZZZZZZZZZZZ!" for like 20 seconds, at stage volumes and thru the PA.   

That will NEVER allow that to happen again 

[EBK]

This could be useful for pedal building:

I've used it to secure strap locks to straps.  Never thought to use it on jacks, but I might give it a go sometime.

[R.G.]

Thank you again R.G. but I am a bit confused about this sentence:  "If you like belt-and-suspenders, you can do a ground wire from the circuit board to the bushing of the input jack, and be assured that if one fails open, the other will keep the big buzz from happening".

We always need a wire from the input sleeve to the circuit board when the circuit board is not in contact with the shield, right?

Sorry - I was not very clear there. I was distinguishing in a muddy, indistinct fashion, between a wire to the jack's  bushing contact only, and two wires, one from the circuit board to the jack, and another from the jack's bushing to a dedicated bolt/nut on the chassis.

There are lots of ways to ground the metal enclsure, including wires from input and/or output jacks, dedicated wire from a ground pad on the PCB, wire from the power entry, etc.