9VAC Power Supply for X0xb0x




Introduction: 9VAC Power Supply for X0xb0x

About: Electronic boards for cnc machines, 3d printers, software PLCs and knight rider replica cars
Converting a spare 12VDC power supply into a 9VAC power supply for x0xb0x, the popular replica of the legendary TB-303 analog bass synthetizer by Roland.

You need less than 5 minutes to get the entire job done.
There's no need to buy an expensive and hard-to-find 9VAC power supply.
Only thing you need is a 12VDC power supply and a soldering iron.

Find info about x0xb0x project by ladyada here:

This is ideegeniali talking, please visit my cyberexistence homequarters:

Step 1: What You Need

You really need only a 12VDC spare supply to convert to 9VDC

- A spare 12VDC power supply
- Soldering iron
- Solder
- 2 short pieces of jumper wire

In the pic the supply I had laying around. 12VDC and 800mA.

Step 2: Theory of Operation

The power supply should be of the transformerful type. I.e. should consist of a transformer, 4 diodes (or a compact 4-diode-bridge) and electrolityc capacitor. This is the most common type: transformer + rectifier. What will do is remove all parts, and connect the transformer directly to the output. There exists switching power supplies, but they're really uncommon in the wall-cube fashion.

You'll have to identify two of the four diodes wich need to be replaced with a jumper wire to connect input to output directly. Failing to choose the right couple of diodes may short out the output and transformer will start to heat up quickly, with possible injury. So: be careful!

One good question is: how came from a 12VDC power supply we're getting only 9VAC after mod?
The answer is simple if you reverse the order by wich you think at the process: in the original power supply, you have a 9VAC voltage from transformer. 9V is the root mean square of the sinusoidal wave wich is 9 * 1,414 = 12,71V half-amplitude, and 25,4V from peak to peak. Once it gets rectified, it becomes a 12,7V-peak pulsing voltage if diodes wouldn't drop another 0,7V, leaving just a 12V pulsing voltage. Wich becomes a 12VDC with the electrolityc capacitor smoother.

This to say that if you strip out everything, but the transformer, you'll have the 9VAC you need for your x0xb0x!

Step 3: Open Thing Out

Open original power supply and reveal interiors
My power supply had screws exposed and easily found and opened with a screwdriver.
Sometimes they're hidden under a label, wich you'll have to remove.
Or just go through the label with your screwdriver, without removing the label at all.
Some other times there are no screws at all and the wallcube was hard glued in manifacture.
Try to open it with a hammer (I'm not joking - it works!) or be patient and open it with a screwdriver used as lever, or a blade (be careful!).

Arrangement may vary, but you'll see:
- the mains connection (WORK WHILE DISCONNECTED!)
- a transformer
- a small PCB with circuit

PCB has two sides: component side and track side. I showed both on these pics.
You see parts are 4 diodes and an electrolitic capacitor.
Sometimes you find also an led and resistor. Some others a fuse as well.
Some other times a selector switch. It doesn't matter, since we're removing everything!

Step 4: Remove All Parts

Use your solder to heat solder joints, and remove everything.
The only thing you won't remove, are the soldering points to the transformer and to the output cable.
You'll just leave these two connection in place, and remove everthing else.

I'm not telling you how to use a soldering iron. There are already tons of tutorial for this around.
Basically, you'll like to put fresh soldering on the joints, so that the flux contained in it will help transfering heat and easing out the process.

Step 5: Identify the Correct Pads

Identify where to put jumpers
You need to identify two of the four diodes with need to be replaced by jumper wire.
With the correct choice, you'll connect straight transformer to output.
With an incorrect choice, you may end up shorting out transformer, wich will heat up a lot and possibly cause damage/injury. So BE CAREFUL! Read back theory on step 2, what PCB track side and component side many times until you find the right spots.

Remove solder with a desoldering pump from the designated pads.

Insert two small pieces of jumper wire
(quicktip: reofors of spare resistors will do).

Bend their legs so that they won't get out once you flip PCB.

Solder them on place

Cut excess leg of jumpers

Step 6: Close and Test - It Works!

Carefully close the power supply, screw it up again.
Check you don't mess up with wires, or whatever.

We're about to test for correct operation.
Mains voltage can KILL YOU! So BE CAREFUL!

Connect to mains, wait 1 second or 2. Disconnect. Make sure nothing is heating up fast.
Connect again to mains, and check output voltage with a multimeter.

Put multimeter to VAC position. You should get 11VAC reading in open circuit or so.
Connect a load (i.e. 4,7Ohm/10W resistor, for example), and VAC should drop to 8,5VAC or so.
Such variations from a transformer with no-load full-load conditions are correct, and show circuit is working well.

Final check: be sure you match your plug on x0xb0x with the plug of your transformer, or replace either of them until they match up. Original x0xb0x mounts a 5,5mm x 2,1mm plug so use this kind of plugs to be compatible with the rest of the world!

If things don't work, disconnect mains, reopen, and double check everything.
If things work, put a 9VAC label on transformer to remind you it's been hacked and it's not a 12VDC supply anymore.

Place the transformer back to the original box for future use, or just plug it in your x0xb0x and start playing some tunes!

This is ideegeniali talking, please visit my cyberexistence homequarters:

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    Question 1 year ago on Step 6

    I get 12.6VAC without a load. Is this Ok or too high?

    Jan ArloV
    Jan ArloV

    Question 2 years ago on Step 6

    How to make it isolated and noise free


    12 years ago on Introduction

    The x0x will run off 9vDC as well, no need to bust anything up for AC!


    Reply 6 years ago

    i don't believe this to be true.


    Reply 12 years ago on Introduction

    I never tried running on 9VDC, but looking at schematic it looks like you really need AC for x0xb0x to work. How can you get +12V at C61 with a 9VDC power supply at D41/D42? But, hey, electronics behave in weird manners. Your suggestion is worth a try...


    7 years ago

    gday, what if i wanted 9v AC at 1500 to 2000 ma? i need an adapter for a digitech rp12 and i have heaps of old wall warts. the closest i can find is 10v at 1000ma, i also have a9v at 700ma. it requires at least 1300ma, but since the standard wall warts overheat and die for this particular unit. i thought that if i upped the amperage and added a heatsink i could solve these problems. cheers.


    8 years ago on Step 5

    Would this be possible with a switching power supply as well?


    12 years ago on Introduction

    How many amps does the converted adapter put out? the x0xb0x needs min 200mA, max 500mA... Going on some forum posts over at adafruit, putting anymore than 500mA in and you're going to blow capacitors! i live in Australia and i can't find a 9V AC 200mA to 500mA adaptor anywhere! so its either a $100 or so 240V -> 110V stepdown transformer (so i can use the supplied 110v AC adapter), or grab one off ebay and have to pay for shipping... hmm..


    Reply 12 years ago on Introduction

    C3 and C5 are 2200uF/16V. And 9VAC after rectifing give around 12VDC, plenty of safeness to 16V (cap damaged). Trasformers output a higher voltage than nominal when are bigger than required. This one in facts output 11VAC instead of 9VAC at no-load condition. Still this doesn't damage C3 or C5, since 11VAC after rectifing are around 14V, still below 16V (cap damaged). But there is no universal rule, you have to test yours out. I'd say more than 1A is not safe, 500mA is safe. Test in between 800mA and 1A.