Why it's good to indicate adaptor polarity

[Mark Hammer]

I bought an old Visual Sound VV1 volume pedal at a swap meet/ garage sale a couple years ago, just like this one.

An odd little beast, it uses a pair of slider pots and a cam action in the treadle.  But more importantly (and the very reason for the "Visual" in Visual Sound), it ties one of the two channels to an LM3914 bar-graph chip to provide 10 volume "steps", indicated by LEDs, so that one can easily replicate, and monitor, the volume level.

The one I had bought for a song worked fine as a volume pedal, but the LED indicators didn't work at all.  Okay, I thought, must be a blown 3914, that needs replacing.  So, last night I took it apart, in preparation to do so.  Then something about the installation of the adaptor jack (it requires external power) caught my eye, so I did some probing.  Lo and behold, the thing actually works like new.  Problem was that the unmarked jack is centre-pos/outside-gnd, and I had been testing it in past with the usual outside-pos supply.

If you make pedals that somebody else might someday use, PLEASE indicate the jack polarity clearly in some way, and save someone a bit of agita.

[theehman]

Anything I build gets a polarity/voltage sticker.  If it's a repair or a jack replacement, it also gets a sticker.  You'd think that would be a no-brainer to have it labeled.

[italianguy63]

I was hard wiring my radar detector into my car this weekend.  Didn't check polarity on the plug first.  I let the magic smoke out.   

Expensive mistake...
MC

[Govmnt_Lacky]

Paging Mr. Keen..... Mr. Keen!   

[Mark Hammer]

Paging Mr. Keen..... Mr. Keen!   

In fairness, I believe this was before his tenure there.

[R.G.]

Yep, not on my watch - although I may have confused DC plug polarity a *few* times myself.   

The company owner was off making pedals to start his own company back before the internet blossomed. In fact, I first ran into him before there were web browsers, and the only "forum" was usenet. Like many people here, he is entirely self taught - and is now very good at understanding electronics. Back in those days, it was not clear at all that Boss would come to dominate the pedal power supply world with a center-negative DC power connector. It wasn't a standard at the time.

But Mark is absolutely correct - the biggest mistake there is that it's missing a label noting what polarity it expects. Labels are our friends.

[Mark Hammer]

I'm guessing that the reason for me being able to buy it for $10, or something like that, was likely because someone else (possibly the owner of the local vintage gear business who had bankrolled this event in the first place, and from whom I bought the pedal) had probably assumed it was Boss-standard (because it lacked a label), plugged in a supply, muttered "frigging thing doesn't work", and tossed it on the get-rid-of-cheap pile.

[bcalla]

This seems like an opportunity for someone like me - whose knowledge of electronics is more hole than doughnut - to ask a question.  Could we simplify our lives by opening up a pedal like this and swapping the leads on the DC jack, thus making it center negative? 

[Luke51411]

This seems like an opportunity for someone like me - whose knowledge of electronics is more hole than doughnut - to ask a question.  Could we simplify our lives by opening up a pedal like this and swapping the leads on the DC jack, thus making it center negative? 

That would be easy to do if the jack is wired, if it is board mounted it would probably not be so easy.

[Mark Hammer]

This seems like an opportunity for someone like me - whose knowledge of electronics is more hole than doughnut - to ask a question.  Could we simplify our lives by opening up a pedal like this and swapping the leads on the DC jack, thus making it center negative? 

That would be easy to do if the jack is wired, if it is board mounted it would probably not be so easy.

In fact, that is precisely what I pondered last night, thinking "Hmmm, I could cut/dremel that jack lug separate from the ground plane, that other lug separate from the regulator input lug, and then just add a couple of wires to connect them to where they need to be..." (with all lugs remaining soldered to their respective pads).  But it was late, and I figured it was just easier to take a paint pen or some of my white rub-on lettering, and indicate the polarity on the chassis.  It's not like there is a shortage of outside-neg adaptors in the world, or that difficult to add a distinctively coloured power jack to a daisy-chain cable with the polarity reversed.

[R.G.]

The other issue lurking behind the idea of swapping the leads to the DC socket is that DC sockets that have metal bodies are nearly always made so the outer, barrel contact is attached to the mounting bushing. This forces whatever is connected to the barrel to be shorted to the enclosure, and often this is electrically the same as the negative lead over on the circuit board. So a metal-bushing DC socket will cause a short between the incoming DC (+) and DC (-) in this kind of setup if you simply swap the leads on a metal-bushing socket.

On a PCB, they're generally plastic-bodied, so swapping isn't going to short + to chassis, but as Luke notes, you have to have the skills to cut and paste traces, and that can be difficult to do.

***simulpost***

[Bill Mountain]

I was gifted a "broken" Mesa Boogie Bottle Rocket.  I took one look at the power supply he had with it and it said 12V DC!

The guy bought it used and it never worked so he just wanted it gone!  I told him he simply needed an AC adapter but he still gave it to me anyway.

Just cause you label stuff doesn't mean people won't actively try and do it wrong anyway.

[Mark Hammer]

I was gifted a "broken" Mesa Boogie Bottle Rocket.  I took one look at the power supply he had with it and it said 12V DC!

The guy bought it used and it never worked so he just wanted it gone!  I told him he simply needed an AC adapter but he still gave it to me anyway.

Just cause you label stuff doesn't mean people won't actively try and do it wrong anyway.

True dat.  A very long line-up of obstinate horses at that particular water fountain.

[anotherjim]

I don't understand why they make the polarity label can be so hard to read when it is printed or embossed.
I do my own (with rub down lettering).

(-)+ means centre negative, positive outer and...
(+)- means... well you know.
Or sometimes just (-) or (+).

[Mark Hammer]

You have to wonder how many folks know that the wiggly lines mean AC while the flat solid and hashed line mean DC.

[Hatredman]

This seems like an opportunity for someone like me - whose knowledge of electronics is more hole than doughnut - to ask a question.  Could we simplify our lives by opening up a pedal like this and swapping the leads on the DC jack, thus making it center negative?

In my years of pedal making I used to fit a full wave rectifier ( 4x 1N4148 ) between the DC jack and the power lines of the circuit. Cheap, small and makes your pedal work with any power supply.

Downside: the 1.4V voltage drop, which could be a problem depending on your circuit. For distortions it never mattered much.

And this is a hard hard mod to do in Boss/TS9 pedals. I only put it in stuff I built.

But hey, it was in the 80's...

[armdnrdy]

I made up these standard labels for different voltages because...sometimes I can't even keep track of the correct voltage!

9 volts is definitely not the standard for the circuits that I build.

I always look at the label before I plug in the power....just to be sure! 

[R.G.]

In my years of pedal making I used to fit a full wave rectifier ( 4x 1N4148 ) between the DC jack and the power lines of the circuit. Cheap, small and makes your pedal work with any power supply.

Downside: the 1.4V voltage drop, which could be a problem depending on your circuit. For distortions it never mattered much.

And this is a hard hard mod to do in Boss/TS9 pedals. I only put it in stuff I built.

There's another subtlety lurking in here, too - ground offset. In addition to the loss of 1.4V, there's half of that which appears as an offset on the negative side, which is ground on most pedals. No biggie for many pedals as you know. But only if there is no other pedal connected to that same power supply and signal ground.

The signal ground on the shield of the interconnecting cables forces the negative side of the power supply in both pedals (for typical, ordinary, negative-ground pedals) to attempt to be at 0V and +0.7V at the same time. Mostly it fails. This can also lead to pops, clicks, and mysterious interactions between pedals.

I had a run at making this practical some years ago. I thought I could use a "bridge" of MOSFETs as what amounted to a "synchronous rectifier" and keep the low-side offset to be miniscule. I could get this down to about 20mV with some pedals, but it gets bigger with pedal current. I was never able to get a single one-size-fits-all solution, so I tanked it.  But it's better than simple silicon diodes or even germanium bridges.

[Bill Mountain]

I saw a power supply design once before that could take any AC or DC voltage and output a DC voltage.  I can't find the design but it seemed to rectify and filter whatever was plugged into but I seem to remember it also kept the output at a nominal voltage regardless of input.

I've always wondered why this was not more common.  Maybe it has limitations like the ones R.G. just discussed.

[R.G.]

Pretty much, you can do anything if you have enough power available.

I spent some back-and-forth time with aron back when I was doing this, and the rectifier bridge was only part of it. The overall idea was precisely that - just connect enough power and the power circuit makes it into whatever you need. And this is entirely -possible- if you want to work hard enough at it. The problems lie in making it happen for a reasonable price without ugly side effects and in a manner that isn't more complex than any pedal you plug it into.

First, we know it's possible. The DC adapters we make take in any ac voltage between about 90 and 260 and turn out a steady 9.x volts DC that is very well isolated from the input voltage. The first thing the AC coming in hits is a full wave bridge rectifier, then it's filtered to DC. After that, a switching control circuit makes a power transistor switch DC on and off into a transformer, after which the AC on the secondary side of the transformer is itself rectified to DC and filtered again. All switching power supplies work basically this way. The only difference between this and some hypothetical pedal power supply that runs from, say, 8V to 24V is the details of the primary side design of the transformer and power switch.

The tricks all lie in whether you can do it with a linear regulator (probably not - heat and input voltage ranges make this impractical) or get away without a transformer (mmmmmaaaaaybeeee) and whether you have to use fancy controls and switches or there is something elegant you can do.

I've actually attacked this a couple of times. I've developed a "fake battery", a device that plugs into a nominal 9V adapter and gives you 9V regulated and isolated from the input ground; I've also done one which is capacitively isolated from input to output, but it's tough to get the right caps for it. Specialized magnetics pretty much makes something like this not a DIY project.