Converting 9vDC or 12vDC to 9vAC.

[bob_adolphus]

Howdy!
Please excuse the newbiness. I'm not even an expert in building stuff.
I've recently got myself a new pedalboard power supply which offers 8 outputs in pairs consisting of a 9v and 12v with a total of 400mA each pair.
My issue is that I want to run my Digitech XP300 pedal which runs on 9vAC but would rather not have to mount a wall wart underneath my board if I don't have to.
Does anyone know of a little device I could run between my power supply and pedal to change one of the outputs to 9vAC?
I've read somewhere that the XP series pedals might actually change the AC power to DC inside anyway. Maybe that's another question in itself.
Cheers. :-)

[GibsonGM]

Hi Bob, and welcome!  The forum is FOR newbies - we're ALL newbies in some area, ha ha!

What you want to do is something many have struggled with for a long time.   It's easy to rectify AC and get to a DC output.   Sadly, it's not so easy to take DC and create a GOOD AC power supply with it, though!

It CAN be done (remember, they have inverters you can use in the car to run AC-powered equipment...) but it tends to be complicated.  If the sine wave that outputs from such an inverter isn't clean, it can cause problems/harm your equipment.  Not something one does without a good amount of thought.

And hacking into the Digitech - well, you COULD, but you also might destroy it in the process.   I bet it DOES rectify inside somewhere, but whether or not it would be viable to go in and bypass all that ....  maybe not worth it....

I recommend just dealing with it and running a cord to the Digitech for AC.   Someone may come along with a better handle on "the new technology" for generating sine waves that's more reliable/simpler, but I think this is your best option!

[Govmnt_Lacky]

The XP pedal DOES rectify the incoming 9VAC into 9VDC. This said.... DO NOT connect DC power to the XP300.... it WILL damage it!

Since those pedals have an absurd resale value AND they are becoming as rare as crystalized unicorn tears.... I recommend you run the seperate 9VAC power supply.

Why take chances? 

[R.G.]

Hi, Bob. As noted, we were all beginners, and still are in one way or another.

GGM is right on all counts. It is possible to modify the Digitech to DC, but complex, and error prone for a non-expert solderer. The Digitech does use DC inside, and does use a rectifier to do this. Making AC from the DC on your power supply will be complicated and expensive.

My day job involves tech support for a pedalboard power supply, and we have had people use this 9V dc adapter to drive "9Vac" pedals. DC will find its way through one side of a full wave rectifier too.

But there are subtle issues. If you use a DC adapter to drive the Digitech, it must both have enough current to do this, and then NOT be used for other pedals. The Digitech pedals use a lot of current. I don't remember off hand whether 400ma is enough DC to run them. The "NOT be used for other pedals" is based on grounding issues. Full wave rectifiers tend to use diodes to control the direction of current flow on both + and - sides. The - side is used by most pedals as ground, and so the Digitech signal ground will be one diode drop, about 0.7V, higher than the DC coming it. If you then connect other pedals to the Digitech with cables, the ground on those other pedals is held at the same voltage as the Digitech signal ground. If these same pedals share the power supply with the Digitech, then the other pedals and the Digitech are trying to make the power supply - side be at two voltages at the same time. This is one of those things that Mother Nature won't let you do successfully.

If one of your isolated 9Vdc supplies will run the Digitech, you're in! If not, you need another power supply for the Digitech.

And here's one final gotcha. If you use the Digitech's 9Vac power supply on your pedalboard, you have introduced a trap for yourself. That 9Vac plug is the same size and shape as the plugs/sockets on other 9Vdc power supplies. But 9Vac inserted into other pedals' 9Vdc power supply jack will kill most pedals dead. The polarity protection in most pedals can't live with an AC power supply inserted, so they overheat and die. So if you ever plug the 9Vac into a 9Vdc pedal, or worse, a daisy chain for multiple pedals, they will potentially all die. If you put a 9Vac adapter into your pedalboard, it makes a great deal of sense to somehow mark the plug so it's easy to recognize. Some kind of paint or cable tie or something is good.

[Govmnt_Lacky]

Digitech XPs use 2.5mm ID power plugs (as opposed to the BOSS-typical 2.1 ID that most other pedals use). Still.... there is the potential, as RG mentions, to damage other pedals on your board.

Use a seperate 9VAC power supply for the XP and label it accordingly 

[R.G.]

We get a tiny but persistent stream of pedal users that claim that our power supply killed their pedals. Not big, every month or two out of many thousands of users, but it does happen. I get to investigate these on the chance that our power supply really is killing their pedals. So far, that has not been the case.

These instances fall into two major buckets: (1) it wasn't really our power supply they were using, and (2) they had both our power supply and a 9Vac supply with a plug that would allow it to mate with stock pedals on their setup.

I deplore the "difference" between the 2.1 and 2.5mm plugs. A 2.1 plug won't go into most 2.5mm jacks. However, all 2.5mm plugs will go into a 2.1mm jack, and every one I've tried (and I've tried a bunch of them) will make contact fine. The 0.4mm difference is 0.016", and that's apparently within the range of the spring contacts. But my deploring won't fix the industry. 

I spent a lot of time trying to kill pedals with our power supply. Never could. This includes shorting, soft shorting, which is worse, reversing,  intermittent switches, overloading, everything I could think of. The internal protections always shut things down before damage was done. I also rigged up a "typical" pedal power plug with "typical" reverse polarity protection and tried that with a 9Vac source. This generally fried the protection diode within a few minutes. I can just see someone rearranging their pedalboard and re-plugging power plugs, then trying it out and no sound coming out. Hmm... what's not connected? Trace, debug, trace. Hey! Do you smell something burning?

So I've come to the conclusion that the difference between 2.5mm pin and 2.1mm pin plugs is good for keeping 2.1mm plugs from powering 2.5mm jack pedals sometimes (many will go into a 2.5mm jack, though) but the 2.5mm plug will usually make contact in a 2.1mm jack just fine. ACK!!

[Processaurus]

The XP pedal DOES rectify the incoming 9VAC into 9VDC. This said.... DO NOT connect DC power to the XP300.... it WILL damage it!

It's a little different, if you look on page 3 of the schematic at Colin's website:

http://experimentalistsanonymous.com/diy/Schematics/Vibrato%20and%20Pitch%20Shift/Digitech%20XP100.pdf

The power supply is typical for older digital pedals that run off an AC wallwart, it rectifies and regulates the AC power into positive and negative 5v rails.  It also has a separate regulator for the digital +5 and the analog +5, I'm guessing most of the current is used by the digital supply.  It might actually be possible for a dedicated person to rework the internal power supply with a charge pump for the negative supply for the analog stuff, but RG is the one who guessed the real issue:  The current requirements of this digitech pedal are massive.  I measured mine at one point, I believe it was around 750mA!  Much more, I'm guessing, than a tap on the original poster's 8 output pedal power.

In a more abstract discussion, one could make AC from DC, but it would basically mean making a 60hz (or 50hz if you're in Europe) sine wave and using a power amplifier to play the sine wave into the power input of whatever you're trying to power.  I've seen people do that to make 220v, 50hz power here in the US.  That's what inverters for the car do, to take 12v DC and turn it into 120v AC.  Long story short, you can do it, but it's way easier just to start with AC, or convert DC to DC than to try to manufacture AC after its been turned into DC.

[Govmnt_Lacky]

Sorry... I misspoke. The XP does do an internal conversion from AC to DC (albeit NOT 9V to 9V)

The current rating for each of the XP pedals is 9VAC at 500mA. Don't know if the current draw would be greater for a DC supply though... 

[GibsonGM]

Yeah, these seem to be quite the current hogs!  My RP350 power brick is labeled 9V 1.3A, and also 3V 100mA.  So we may have a COUPLE of things going on here.

Seems like the consensus is - use the supplied transformer, and maybe paint it/mark it up to identify it.   That includes the jack end! 

[PRR]

> look on page 3 of the schematic at Colin's website:



There's ways to hack this, but a simple 9V_AC_ wart is *perhaps* the best way to handle this oddball (by 2014 conventions).

[Dave W]

bite the bullet. use a 9 VAC wart. clearly write XP/ 9VAC on it and color code the jack to avoid any confusion. a piece of colored heat shrink works great for this.

[R.G.]

Good idea - colored heat shrink. I have used the yellow and read plastic dip stuff from Home Depot to paint color on, but it's messy.

[bob_adolphus]

Thanks for your input guys, and thanks for letting me know it's ok to be a newb - I actually failed electronics back in college so i'm running on very limited knowledge here! hahaha
Looks like I'll have to stick to running a separate 9VAC wall wart.
There are rumours that Digitech are working on a reissue of the XP300 Space Station but there's been no official confirmation of that yet. If they did release one it would make sense for them to change it to 9VDC like they did with the Whammy V pedal. I suppose I could go for the wall wart until this comes along.
More stuff to have to try and fit underneath my board! Oh well!
The prices on the XP series pedals are ridiculous - especially the 100 and 300. The other two seem to go cheaper. I was hoping to switch the chip of my 300 into a 200 because my expression pedal is beyond repair but apparently the 200 and 400 have less RAM than the 300. Bummer! Maybe I could somehow wire in an external one??? I think that might be a bit beyond me though. hahaha

[Govmnt_Lacky]

@bob

I suggest you start a NEW thread with some pictures! I am interested in seeing what you mean by your "expression pedal is beyond repair." 

[SISKO]

I converted my XP100 to run on DC. Made a 555 inverter (not the CMOS version, as it is beyond its power capability) and put it inside. I was happy for a while but, strange as it may seems, yesterday i was thinking about getting rid of it and going back to normal.

As always, there are prices to pay. I was happy because i can use just one power supply to power everything on my pedalboard but the XP100 consume lots of current (did not measure it, i should...) and the 7809 on my PS was getting really hot as it provided current for the XP plus all my other pedals.
In order to make my rig realiable  i got to build another voltage regulator just for the XP100.
All of this, keeping in mind that someday ill hack my XP100 to convert it to XP300 and that pedal has 3 drams chips more (wich means, MORE current, and maybe an inverter re-design)
Also theres some heterodying as i can hear the multiplexed leds. At low volumes its not a problem buts its noticeable at high levels. Its only there in certain settings and if you are not playing, but its definetely annoyng.


So bassicaly, yes, you can do it. If you want you can make the inverter more powerfull, aisolate all the (heterodying) noise, keep the regulators cooled,etc. But for me, it just dont worth one more transformer on my pedalboard. Besides, the resale price is something that interest me, so ill leave it original.

[lietuvis]

Hi all. By looking at schematic it seems that -5V is for few OPAMPS negative rail and they are fed from 79L05 which I think is 100mA max. Only positive side is draining faire amount of current where one 7805 is for digital part an the other is for audio part. This what I think can be adopted to feed the circuit from 9VDC is by using LT1054, if there is a room for it and few capacitors (C1, C2, C3, C5 and C6)

[bob_adolphus]

@ Govmnt Lacky

What I mean by "beyond repair" is that the metal arm that holds the LED attached to the expression pedal has been bent and it no longer lines up. Now, I've had it fixed a few times but it is now so flimsy that I doubt it will handle being straightened again. The issue started because the rubber 'feet' that stopped the expression pedal from hitting the chassis were 'chopped through' by the expression pedal and the arm attached kept hitting the bottom of he inside of the unit causing it to bend. Being a big fellow I tend to be a bit rough on my pedals. Hahaha
I really love the idea of being able to run an external expression pedal because the XP pedals are quite bulky and I'm slowly running out of real estate in the front row of my board. I'd love to put it at the back and put an expression at the front.

I've pretty much resigned myself to having to run a second power supply. Gotta get it working again first though.

[Cykeltur]

Interesting question!
I have similar question :
I have 12-24vdc and need 9v, is there any converters?

[R.G.]

Interesting question!
I have similar question :
I have 12-24vdc and need 9v, is there any converters?

I spent a good part of my engineering career designing power supplies and converters.

Yes, there is a converter. There is, in general, some converter that can be made to convert any power source to any other power source. The problems are whether it is (1) affordable and (2) practical. Well, also whether it is "polite" enough once it gets finished converting.

The easiest converter of 12-24Vdc to 9Vdc is a 7809 three terminal regulator. This will supply 9Vdc, regulated and relatively quiet, from zero current up to either 1A or the current where the product of voltage across the regulator and the current through it makes it overheat.

That last means that you have to know how much current you need, and maybe provide heat sinking.

There also exist down-converting switching converters that will make 9Vdc out of 12-24Vdc. They do not dissipate nearly as much heat as the linear regulator, but they are complicated and may broadcast RF noise, as well as putting a "whine" on the output voltage and ground.

So the answer is "yes", but the devil is in the details.

[J0K3RX]

What kind of 8 output power brick do you have? If it's like a voodoo labs you should be able to get the 9VAC directly from one of the 9VAC secondary taps before it gets rectified and with the voodoo labs those are .5A which sounds like exacty what you are needing...

Edit: Looks like the XP 300 uses a  PS750 power supply which ironically is also 750mA...   so, unless there is a tap for 9VAC that is 750mA or greater then you're stuck... Or, you could run 2 of the 9VAC secondary taps in parallel and that will double the current.. So, if you have 2 secondary 9VAC taps rated @ 400mA then if you run them in parallel you'll have roughly 9VAC - 800mA . But that will use up 2 of your 9V power outs.