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Guitar Effects Pedal Power Supply










Abstract: I built a power supply (R.G. Keen's Guitar Effects Pedal Power Supply) as a gift; it is one of my first projects using the Valkyrie-clone CNC machine for PCB fabrication.  The Power Supply is a bow power supply, and the source is at http://www.geofex.com/Article_Folders/Spyder/Spyder.htm
One board has 6x basic regulators, another has 2x "Deluxe Regulators".  The resistors for "deluxe" are specified 240, 1.1k, a 500ohm trimpot and a 100ohm trimpot.  The 100 ohm trimpot must be capable of 0.5 Watt dissipation.


The Xicon transformers (Mouser part #41PG006) are mounted seperately: the leads from the secondary coils go to the AC jumpers.  Each 4 DC outputs are connected to a DB-9 connector; the DB-9 connector attaches to four 2.1mmx5.5mm DC barrel jacks, negative pin, which connects to most effects pedals.

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Effects_Pedal_Trimpot_Evaluation.xls18 KB
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Hi John,

Hope you get this message. I also built a power supply myself for my guitar effects. I am still looking for a suitable enclosure though that fits under my pedalboard. What did you use, that looks like a nice compact box.



- Koen.


Hi Koen,

I used a homemade enclosure, so it was custom built in a wood shop.  If you have a table saw and a drill press you could probably build one the same way.  The base is 1/2" thick and the sides 1/4" thick wood, the top is acrylic.  I can get you detail on the type of wood, if you want to build one. 

The duct tape enclosure was cut with a box-cutter from a double-thickness cardboard box used to hold oranges.  It's really easy to make, and is pretty sturdy (just don't stand on it).  Unfortunately the little variable resistors inside could not be adjusted easily, since there was a lot of duct tape and cardboard in the way.


Hi John,

Thanks for your reply. I never considered building a enclosure from wood or even cardboard. Especially since it is a PS, I assumed that it should be metal because of grounding. But I guess that I could put conducting foil on the inside of the box for this purpose.

- Koen.

Hardware version 2 is completed and shipped via USPS Priority Mail on 1 April 2010.

The problems were

  1. The 9V regulator was under-specified and did not meet requirements for all pedals.  A typical "wall wart" for a guitar pedal gives about 200mA current, but the regulators were only specified to handle 100mA.  Also, some pedals (digital delay) need more than 200mA, and are recommended to run on 500mA power supplies.  These battery-vampire pedals can burn through batteries in less than an hour, and need a power supply even more than most low-current pedals.  The regulated 9V channels must be able to handle 500mA.
  2. The adjustable voltage circuit was under-specified -- it contained a trimpot of unknown wattage rating.  A 100ohm trimpot was connected in series with the +9V output, which means that all current passes through the current limiting trimpot.  The trimpot emulates "internal resistance" of the battery.  The trimpot should be rated to 0.5 Watts to handle 200mA of current.  The trimpots I used might be rated to 0.5 Watt, or 0.2 Watt, or even 0.1 Watt, for all I know.
    A 200mA pedal would draw up to 0.45 watts when the trimpot is used to limit the current.  The default setting is about 80 ohms, so 0.4 watts would be dissipated.

The fixes were

  1. The 9V 0.1A regulator was swapped in the schematic for a 9V 1.0A ST Microelectronics regulator, and a new set of capacitors, bridge rectifiers, and headers was ordered from Mouser to populate a new board.  A set of new boards were fabricated (~5 hours on the CNC, while I sipped coffee and occasionally changed the bit).  Saturday morning I populated the 9V regulated power channels.  Swapping the revision 2 boards for the revision 1 boards should be easy.
  2. After taking inventory of my components, I ordered some trimpots that I know are 0.5 Watt trimpots.  Since I know the trimpots are 0.5 Watt, I can specify the allowable pedals one can use on the adjustable channel (up to 200mA according to my spreadsheet).

The result:  The power supply is rated to 200mA on the adjustable channel, and up to 1 Amp on the regulated 9V channels.  The AC current goes through a 125mA/240V SB fuse, so there should be 30 watts among the 8 channels to play with.  A pedal board setup with six 200mA pedals and two 500mA pedals drawing their full current would total 20 Amps.  I've included a ~5 extra fuses just in case they occasionally blow out.

I sent it out today, April 1st.  No fooling!

nice work!

 but i have a question: How much W is the trimpoti R4 capable off or how much power do you need out of the supply? When i see right all the output power is going through this. 


And another suggestion: When drawing schematics pleas use those point when two crossing wires should have contact (normaly you use them also for three wires) It makes your schematics much better readable ;-)





Thank you for the advice on the schematics!  I'm learning as I go, so the advice helps.  The trimpot is probably rated for <0.5W (based on similar parts from Tayda Electronics, which wasn't where I got the ones used for this circuit).  I did not worry about the power running through the trimpot for this particular circuit since it will be powering a pedal that normally gets a lot of use out of a 9V battery. That said, I probably should have roughly calculated how much power the circuit might be expected to use:

The internet says a 9V alkaline battery has about 500mAh capacity (5 Watt-hours), and the internet says a (worst-case) digital delay pedal can drain it in an hour.  That implies that the pedal impedance is R = V²/P ~ 9²/5 ~ 16 ohms.  The impedance of the power supply ranges from 0 to 100 ohms, so now I need my graphing calculator to figure out how many amps are going through the circuit and how many watts are disapated by the resistor over the 0-100 ohm range.  The amps I calculate by I = V/R, and the power through the resistor I calculate by P = I²*R4 (R4 is the resistor).


 The power disapation through the trimpot R4 will be 0.6 to 1.2 Watts for the worst-case pedal.

Thank you for the warning!  It looks like my circuit is not ready for production quite yet until that trimpot is fixed.

In addition to the adjustable "battery resistance" problem, there is a problem with the voltage regulators I chose.  The regulators have a 0.1A limit, which is less than the BOSS pedals' 9V 200mA wall wart, and less than the 500mA needed for a digital delay pedal.  I've tested the supply, but I'd like it to be robust enough to handle any pedal, not just some of them.

It's time to build another set of boards and order another set of components.  There are many lessons to learn before I can emulate an EE.



the above link is a  model of the expected voltage dissipation for a pedal that lasts a certain number of hours under use.  It looks like a pedal whose battery lasts 3 hours or more would be safe.