Question on power jack wiring

[dfro]

When adding external power to an effects pedal, why is the power jack wired with the positive terminal on the outer sleeve?

Most of the wall warts that I am familiar with have the positive terminal on the inside sleeve.

If the negative terminal was connected to the outside sleeve of the power jack, it would not need to be isolated from the chassis of the effects pedal.  Ground would be connected through the jack to the chassis, which would provide shielding.  I thought ground being connected to the chassis was a good thing.  I thought shielding was a good thing.  Am I missing something?

Thanks.

[LyleCaldwell]

The shielding is provided by the audio ground, because the audio signals are AC.  A DC negative won't drain a shield (someone correct me if I'm wrong).  But aside from that I don't know why tip is negative.

[dfro]

A DC negative won't drain a shield.

Lyle, I don't know what you mean by that.  I am fairly new at electronics. 

From what I have learned so far, I thought all parts of the circuit share a common ground point.  Sometimes there can be two ground points if you are superimposing an ac wave on a dc current (i.e. 0v and 4.5v).  You can have +12v and +5v and -9v in a circuit and they will all share a common ground point.  Right?

It seems that the power jack, by the way it is made, is asking to be grounded to the cassis through the outer sleeve.

The first stomp box I made was a tube screamer.  I wired the power jack with the ground on the outer sleeve.  It worked great.  I then read somewhere that this was backwards and I changed the wiring, without isolating the power jack from the chassis.  I now hear weird fluttering noises.  I am surprised it even works.  I am using a battery.  If I plugged in an external power plug, I am sure I would short out the box.

Any more thoughts?  The black art of grounding and shielding is a big hurtle for newcomers to electronics, imho.

[Paul Perry (Frostwave)]

I hoped someone else would add to this.
My feeling is that the - centre, is probably because Boss does it, and most follow.
And the original reason is, with the first effects (all Ge) the ground connects to the + of the battery, because most Germanium transistors are PNP.
A DC negative has no trouble shielding, but of course if you have two effects, one with + ground and the other with - ground, they had better have completely separate power supplies (or batteries) because, you will short the - and + via the cable shield when you connect them together!!

[Mark Hammer]

My understanding is that it works like this...

Any power jack that involves disabling an internal battery supply, in order to replace it with an external AC-based supply, has to physically dislodge the contact which brings battery power to the circuit board.  By virtue of how they are normally installed, there is no other choice but to make the tip of a phone plug carry V+, and use the tip to move the jack connector away from the leaf contact that would normally allow battery voltage to reach the circuit board.

By virtue of *their* construction, barrel-type plugs have to use the outside shaft to do the physical dislodging.  If you look deep inside the barrel jack, you will see that the shaft of the plug pushes a little spring type contact when inserted.  Normally, that contact touches another contact to bring the battery voltage to the board.  Insert the barrel plug in and the spring contact the plug shaft bumps against is moved out of reach of the one carrying the battery voltage, so that now the only positive connectiuon to the board is coming from the shaft of the wallwart.

But of course, that doesn't explain why there are inner-pos and inner-neg adaptors.  Or maybe it does...

If there is no battery switching to be concerned with, since the barrel jack is not grounded to the chassis, the wallwart manufacturer has the choice to make the outer shaft V+ *OR* ground.  From a safety perspective, any hapless wallwart user that just happens to be holding the bare plug in their hands with the wallwart plugged into the AC outlet,
is probably safer if that outer shaft is ground rather than V+.  So, when the wallwart is not presumed to be making a physical switch between internal battery and external powering, the manufacturer tends to go with safety and make the inner pin connection V+ and the outer shaft connection ground.  Remember than many wallwarts are AC not DC, so it's not like there can be any universal standard that DEMANDS the outer shaft to be ground, because plenty of wallwarts/plugs are going to be "hot" whether intended or not.  When a DC wallwart has a "second job" of disabling the internal battery, however, the outside connection HAS to be V+ because there is no other physical choice.

So, what you will see on the market is a mixture of pin-pos and pin-neg adaptors, because there are reasons for manufacturing each.  I don't have proof in the form of a corporate memo, but my sense is that it is this very variability in commercially available wallwarts that leads all the commercial pedal manufacturers to make a big deal about using THEIR adaptors.  They don't want you to blow up the pedal.  They don't want you to mistake hum from a poorly regulated wallwart for a poorly designed pedal.  They don't want you to fry yourself.  They don't want you to accidentally stick an unrectified AC wallwart and plug it into a DC-needing pedal.  And maybe most importantly, they don't want to have to teach you basic electronics so that you know what to look for in an adaptor.  The easiest thing is just to sell one and tell you to use.  It's simple and it saves them headaches.  Maybe it makes them money, but maybe not.  I suspect saving headaches is more important when you consider the prospect of making a buck off the wallwart vs the risk of having someone return a $200 pedal and tell their friends "it's crap".

In sum, your confusion is well-justified.  Between the things that are optional for wallwart makers, and compulsory for pedal manufacturers, you will run into a bunch of contradictions.

[dfro]

Thanks guys for the responses.

It seems to me that either the positive or the negative side of a battery can be disconnected to remove it from an electric circuit.  The circuit can be broken anywhere.  Am I correct in my thinking?

When I first wired up my tube screamer pedal (which sounds awesome , by the way), I just made it so that the negative side of the battery was disconnected when the power plug was inserted.  I also wired in an on/off switch, which would disconnect battery or external power, so that I don't have to unplug the pedal in order to turn it off.

Mark, based on your very good explanation of how a barrel-type plug operates, and the fact that (at least the metal ones) are designed to have the outer barrel conduct to the chassis,  I still think outer barrel negative, inner pin positive, is the logical choice.  I have seen where guys overdrill the hole and then fill in with bondo to isolate the plug from the chassis, or they use a rubber grommet.  Why not just wire it differently?  I am belaboring my point.