Power jack grounding - Problem

[niggez]

I finished building my tremulus lune today, it works wonderfully. I had some trouble getting it started, the problem was my power jack.
The new power jacks i bought are made of metal, so my ground flows through the jack into my effect chassis. The other effects i've built so far, i always used the plastic housed jacks, so no connection was made to the chassis.
As soon as i insulated the chassis around the hole of the power jack, the problem was solved.
At the moment my effect chassis is not grounded at all - should it be? I have the connection of ground from input to output through a wire - could i switch this to a connection through chassis and remove the insulation from the power jack?

[punkin]

I donno...maybe I've been lucky. I'm new to this stuff but I've made two boxes so far.

Both are metal enclosures and both are using metal jacks and metal instrument jacks that are connected to ground/chassis. I've purposefully connected wire between the insturment jack ground and the DC power jack ground. I haven't had any problems yet.

There must be something to the insulated power jacks. I see a lot of people using them. I suppose it's something to do with ground loops and possible noise issues. Ultimately, I'm thinking that unless the instrument jacks are insulated as well, by virtue of the cables, eventually the box(es) will get ground through the amplifier at the end of the chain.

Sorry I didn't add anything to the post but I too am currious...consider this a gratuitous bump 

[R.G.]

Metal housed DC power jacks invariably connect one side or the other of the plug to the chassis with their metal bushing. This would not be all that tragic if the side that was connected was always the one that went to signal ground, but it isn't. In particular, the effects industry standard was set by Boss - sleeve (outside) positive. So it only works OK for positive ground pedals. On the more standard negative ground pedals, the jack effectively shorts the power supply.

[punkin]

I see, so is it more typical to isolate the power and the audio jacks from the housing? I suppose I'm ok as long as I'm building all my pedals the same and don't mix them with "other brand" devices? Sorry, I'm a noob here. I have done some searching and have noticed the differences in power jacks but haven't paid any attention to weather or not the chassis' are grounded or floating.

[R.G.]

Enclosures should be metal, or made conductive with metal foil or conductive paint. This helps keep radio pickup from happening.

The conductive enclosure should be connected to signal ground. It is ideal if the enclosure is grounded only one place, that being the input jack. However, most people just let the bushings for both input and output jacks touch their metal enclosures and get away with it.

Only one side of the power supply can be connected to ground. For most pedals, this is the negative side, so these effects are called "negative ground" pedals. A few mostly older fuzz face pedals and their bazillion clones, imitations, and me-toos will have the positive side of the power supply connected to ground. This "positive ground" requires that such pedals have a separate power supply from the negative ground pedals, otherwise the ground contacts both positive and negative, and you have no power supply at all.

In the case where you just stick the jacks into holes in the metal box, the metal bushing connects the chassis to whatever is connected to the jack. In your case, you've been lucky enough that these were all at the same voltage.

[ianmgull]

RG, I've often wondered about what you said about grounding the enclosure at only one place. Would there be a significant improvement in regard to noise by not allowing both jacks to ground the enclosure? If so you're option would either be insulate say, the output jack from the enclosure (probably more practical) or let the output jack ground itself only from the enclosure (running the risk of loosing ground if it comes loose). Or probably still more likely it's not that big of a deal.

[punkin]

Thanks RG...that was helpful. I drew this out on paper last night...(I'm a visual thinker)...I see what you mean...most of the power supplies are transformer outputs with floating ground.  I've been building my boxes with negative ground with center pin hot (positive). I've been connecting ground on my DC jack to my signal jacks chassis/ground connection as well...I now understand that the signal jacks connect between the various pedals and that's where I really need to know how my "pro-built" boxes are set up. Thanks!

[R.G.]

RG, I've often wondered about what you said about grounding the enclosure at only one place. Would there be a significant improvement in regard to noise by not allowing both jacks to ground the enclosure? If so you're option would either be insulate say, the output jack from the enclosure (probably more practical) or let the output jack ground itself only from the enclosure (running the risk of loosing ground if it comes loose). Or probably still more likely it's not that big of a deal.

It's close to moot for most pedals. That's why most people get away with it. Special conditions can make it matter.

Here's what happens. If you connect one jack's bushing to the enclosure, the temptation is big to not run a wire to the other one's bushing, and just let the enclosure "ground" it. That works mostly, because because the enclosure is often thick drilled aluminum, with a good, fresh, raw machined cut that the bushing rests in. However, the return current from the enclosure-grounded jack does have to return through the enclosure. If the gap between the bushing and the enclosure gets dirty, the aluminum grows an oxide skin, dirt gets in the way, etc. then the return resistance builds up. A voltage then has to build up across the enclosure, and that wiggles the circuit board ground up and down because the input ground is usually where the DC power comes in. That voltage is added to the input signal voltage.

In most cases, the input impedance is low enough, the gain of the pedal is low enough, and/or the output current is low enough that the voltage induced at the input is too low to cause oscillation. But with FET input devices, high gain distortion, and super-mondo outputs of the full power supply, you'll get enough feedback to make it sing all on its own.

It's kinda like almost all days you can get away with not brushing your teeth. But if you do this all the time, chances are it'll bite you one day.