Can you plug 12v ac into a 9v pedal?

[Dr Huge Longjohns]

My son had just bought a digitech ex 7 multi fx pedal which he wants to power from his power block rather than the supplied wall wart. The pedal takes 9v AC ie not the usual DC. His power block's only AC option is 12v. I know it's OK to run 12v dc into a 9v pedal but, just for safety's sake, is it ok for AC too? Cheers

[. . . Delete This User . . .]

no

[damoore]

Not a good idea. You will be making the power regulator in the unit to dissipate a lot more heat, and it may not be capable of doing that.

[Dr Huge Longjohns]

no

Idris, I yield to no-one in my admiration of brevity and succinctness, but in this instance perhaps I could persuade you to elaborate, just a little? If it's ok for dc why not ac (as you can see I am a long way from being an electrician).

[ef37a]

no

Idris, I yield to no-one in my admiration of brevity and succinctness, but in this instance perhaps I could persuade you to elaborate, just a little? If it's ok for dc why not ac (as you can see I am a long way from being an electrician).

AC is normally specified as the "rms" voltage (you don't need to know why!) but when rectified the off load voltage developed will be the "peak" voltage. In the case of 9V ac that is ~12.7V dc but for 12V ac it would be about 17volts.

Now it is unlikely that with modern devices that an extra five volts or so would do any harm*, but not a good idea to risk it especially when you cannot account for other factors such as the regulation of the 12V supply and the variations in mains voltage.

NB: "Vintage" kit using Germanium transistors or "bucket brigade" delays are VERY voltage sensitive.

*There is in fact a "sect" that circuit "bends" and put 18V+ on pedal circuits, More money than sense IMHO!
Dave.

[The Red Bladder]

There is in fact a "sect" that circuit "bends" and put 18V+ on pedal circuits, More money than sense IMHO!
Dave.

Which reminds me of a story - I once learnt a valuable lesson about circuit bending when just a humble gear repair person.

I must point out at this stage, that I usually had to learn all my lessons the hard way, electrocuting myself, setting fire to myself, causing thousands of Pounds of damage and even, on one occasion, driving over myself. But this time, I learnt all I needed to know about circuit bending something you do not understand from a young man with a Siamese cat on a lead.

I say, on a lead, but this was a cat that knew nothing about walking to heel, so it was a cat that was being dragged along the ground on the end of a length of string.

The guy had long black hair and a trench coat and was carrying a valve amp under one arm. He used the other arm to pull the cat, who protested loudly, particularly about the steps leading up to the workshop. "She's still in training!' he explained.

My boss (who BTW, was the same guy who had the affair with the chinless girl in the mop-and-bucket room at the piano salon - you know the one where the pianist turned to see me standing on the table, looking at him through a pair of binoculars - yes, that's the one, the one who got the clap and asked 'Why does it hurt when I pee?') turned to the guy and asked "Yes?"

"I've like circuit bent this amp man!" said our new friend.

"And does it work?" asked my boss.

"I've no idea. I thought I'd come here to have you guys, like, check it out and stuff!"

My boss was beginning to see the comic potential of the situation and asked him what exactly he had done.

"I've like connected wires inside to see what would happen."

"And what did happen?" asked my boss. "I mean, have you tried it out?"

"I thought I'd let you guys do that." he said and put the amp down on the workbench.

"OK!" said my boss and, instead of driving the thing up slowly, whilst watching the current draw, he plugged it in and (smiling broadly) just turned it on.

There was a bit of a flash and a loud bang and volumes of brown smoke poured out of the amp. The cat went nuts and did a tour of the workshop rather like one of those 'Walls of Death' motorbike riders at fairgrounds and then went to earth in a Leslie cabinet.

I was given the job of explaining to the owner of that Leslie cabinet, that it must have come to us smelling like that in the first place. "I mean, it's not as if we have cats in the workshop!" I had to tell the man, with a perfectly straight face.

[Dr Huge Longjohns]

extra five volts

Well the pedal is rated at 9 so going up to seventeen or so would actually be an extra 8. Brilliantly explained, by the way. I'll tell him to plug it in the mains plug bank then!
Cheers

[Hugh Robjohns]

extra five volts

Well the pedal is rated at 9 so going up to seventeen or so would actually be an extra 8.

As the nice man said; the input 9VAC supply is rectified internally to produce around 12VDC.

"In the case of 9V ac that is ~12.7V dc but for 12V ac (input) it would be about 17volts (DC)."

When I was at school 17VDC - 12VDC = 5 ... but maybe things are different now!

We all like to nitpick Dave's posts from time to time, but on this occasion I really don't think he's left us much ground for manoeuvre!

Hugh

[Dr Huge Longjohns]

Wasn't trying to nitpick at all--just not understanding the rectifying business! But, in a nutshell, it sounds like it's probably a bad idea yes? Or no?

[. . . Delete This User . . .]

yes, or , witness my earlier comment to the original question.

No..

also , beware the tendency to take the optimist view of the word "probably" in phrases like "probabkly be alright"

more often than not it means.... something like...

will be dead very shortly....
or will work okay for now, but come the big gig on the 28th, will die spectacularly in mid solo.

or will be dead the first time you try it.

I'd also in general advise against routinely using 12Vdc on 9v equipmentl, for much the same reason.

what MAY work.... is not necessarily a GOOD idea..

[Dr Huge Longjohns]

OK, cheers. On balance, I think we'll stick with the No option! Thanks for all the input. I've genuinely learnt something today, too.

[ef37a]

OK, cheers. On balance, I think we'll stick with the No option! Thanks for all the input. I've genuinely learnt something today, too.

wise decision HL!
To add further weight to it I have not mentioned "regulation". This is a quality that transformers have and it is expressed as a percentage. We need not get too technical about it, suffice to say it means that transformer specified as say 9volts at 0.5Amps will in fact measure more than 9V when not fully loaded to 0.5A and very small "rat" traffs are appalingly bad in this respect, that 9V/0.5A job could easily be 12V or more off load, add in mains up 10% and things can get nasty!

On the bright side, such transformers will soon be outlawed...Ungreen don't yer know!

Dave.

[grab]

It's not really recommended for DC either. It'll work, but it's not best for your kit.

One major way things can go wrong is simply exceeding a voltage rating somewhere and making the weakest link say goodbye in an explosive kind of way. Electrolytic capacitors (the can-shaped ones used in power supply circuits) are the major issue here. Common voltage ratings for these beasties are 10V, 16V and 25V in this kind of ballpark. 10V is out because 9Vrms AC peaks at 12.7V, but 16V would usually be fine. Higher-voltage capacitors are physically larger, and sometimes slightly more expensive, so generally people will use the smallest voltage rating they can reasonably get away with, which here would normally be 16V. But 12Vrms AC peaks at nearly 17V, which is going to play hob with the electrolytics in your regulator circuit if they're 16V-rated ones.

If you're lucky, the maker has seen this coming, they've picked higher-rated caps, and you're OK. If you're not lucky, you'll be picking bits of exploded capacitor out of the inside of the case. And once the cap's gone, the voltage is a lot less steady and various other things could be cooking off too.

For DC incidentally, in theory people might get away with 10V caps. In practise though it'd be stupid - quite apart from the possibility of plugging in a 12V DC supply, there's also the vagaries of whether it's a regulated supply or not, and simple tolerance problems too, so best not to. And if they run 16V caps, then their kit is safe for 9V DC and 12V DC.

Backtracking, another way things can go wrong is simple heat. The more voltage things need to dissipate, the hotter they get. And generally, the hotter things get, the more likely they are to fail early. This is a subtle and stealthy sort of failure - heat stress on joints and things like that - which basically means you should expect a shorter lifespan. This holds true for DC as well as AC, which is why you're best not running a stomp box of 12V DC, even if it seems to work perfectly.

[narcoman]

I'd also in general advise against routinely using 12Vdc on 9v equipmentl, for much the same reason.
.

Absolutely. The best you can hope for is it'll not work right.

[dubbmann]

I must point out at this stage, that I usually had to learn all my lessons the hard way, electrocuting myself, setting fire to myself, causing thousands of Pounds of damage and even, on one occasion, driving over myself.

i just have to say that i LOVE this sentence.

on a metaphorical level it pretty much sums up my experience about life, the universe, and everything.

i can see this as a laurel and hardy or 'two ronnies' skit. glad you survived to share the story

cheers,

d

[Studio Support Gnome]

i wonder if Mr Bladder ever trained Brian Harvey???