super velcroboy wrote:ground is ground. All the same. When you connect the patch cables between pedals, the patch grounds the pedals together.
Almost. And that's how it's supposed to be. The problem is that all we have to connect "ground" points is real wire with real resistances. So like all real resistances, the connecting wires show a voltage equal to the product of the wire resistances and the currents through the wire.
"But..." I hear you say, "but that's so tiny it couldn't possibly cause a problem!"
Not exactly. Some distortion boxes have HUGE gains before clipping. If you have a setup where the "ground" an input sees is referenced to one end of a copper wire that carries what I call "sewer ground current" ( i.e. the power supply ground) from another stage or another circuit, it will happily amplify that V=IR ground voltage, and you can easily hear that. The bigger the gain, the bigger the noise problem. This can cause noise and oscillation as well, and happens on the power supply rail too if the rail is not well bypassed. You can't bypass ground with a capacitor, though.
pandadandan wrote:Are there any rules as to getting them hooked up?
I'm happy doing single-box setups but recently I've had an urge to squeeze 3 distinct fuzzes into one enclosure. One of them will possibly be a positive ground circuit. Apart from a separate power supply for that particular circuit, does this affect anything in regards to grounding? Star ground perhaps?
The most fundamental, underlying rule is that you have to know what currents flow in what ground wires. Well, OK, you can just do it and trust to luck, too. A lot of times, luck gets you through and you don't happen to step in... er, don't happen to have a problem. You can just string them together anyhow and don't get something critical in the wrong place.
But you did ask about whether there are any rules. Star grounding is sufficient in all cases, but you may not need to work that hard. Inside one box, I would run a separate power supply ground wire for each board, and then connect the boards with a single signal wire. The power grounds are then starred and the signal voltage carries only the "sewer ground current" -induced voltage from the section that created it. That plus a 100uF + 0.1uF mono ceramic cap decoupler on each board will usually kill sewer current issues. Ground either the input jack or the output jack to the chassis, not both. Both can set you up for circulating currents, although mostly you get away with it in effects because of the small currents. This gets critical in things like car stereo and recording studios. You probably won't need shielded signal wire. All of this gets trickier the higher the gain you put in the box, and three distortions can have a whale of a lot of gain.