A real effects pedal power-supply

BatteriesWhy?

Over my playing years (more than 30 now) I’ve owned many effects pedals, spent a ton on 9V batteries and even tried many different power supply devices. I had one of those VisualSound “1-Spot” daisy chain things, which worked quite well. And recently I was given a Furman SPB-8 which appears to be the mother of all pedal boards. If you’ve been to this site before you know I’ve built an amp, and a whole slew of different pedals. All of these things I mention have one thing in common: there’s a lot of attention paid to converting alternating current from the wall outlet into direct current to power the circuit.

How?

After looking at many different power supply circuits, I realised they looked very simple, but I did not know the first thing about why each component was there and what they did.

My cousin Mike was in need of a power supply for his pedal board that could handle anything that he threw at it (i.e. multiple voltages and a useful current handling capability). Naturally there are products one can buy, such as the Voodoo PedalPower or the MXR Brick, but these can be quite expensive. And lets face it, I wanted to build one, so I could find out how they work.

What?

So, what does this thing need to do? Basically the following is desirable:

  • Provide a steady 9 volts.
  • Do not introduce noise into the circuit.
  • Handle at least ten pedals’ loads, which might approach 1 Amp.
  • Be small enough to fit under Mike’s pedal board.

Transform, rectify, smooth and load

From my work with tube amps and reading everything I could find in print or online, it became clear that, at a fundamental level, there are only a few discrete steps to accomplish:

  1. Transform the outlet supply from 115V to something closer to the desired voltage.
  2. Use a rectifier to convert the AC supply to a DC supply.
  3. Use filters to smooth out the now rippling DC.
  4. Load the circuit with whatever you wish to actually provide power. Whatever that is, it require voltage, will draw current and therefore has resistance.

A transformer is easy to get hold of. Even Radio Shack sells a 9V AC ‘wall wart’ that will handle 1 amp. A rectifier is easy too; just a bunch of diodes arranged in a certain way.

The filtering though was a mystery. How do you know what values to use? After gathering quite a few different circuits it seemed everyone was using different values, so who had it right?

The idea is this: use the characteristics of a capacitor to ‘fill in the gaps’ of a rippling DC supply. Remember, we rectified AC, which gave us rippling DC. That ripple will be at a frequency that can be heard by us humans, so we need to reduce that ripple to be inaudible.

Here’s a graphic showing a number of things. Firstly, this is a time-series showing voltage over time for the circuit at the top of the bitmap. Its a simple circuit showing an AC source going through a rectifier. Then the AC signal flows across C1, a capacitor. This graphic is to explain how to calculate the value of C1, and the influence it will have on the green waveform, which is the rippling DC source (post-rectification).

Ripple smoothing calculation

Ripple smoothing calculation

So imagine it like this:

1) The rippling DC source voltage rises to reach about 32 volts. During this time C1 is charging.

2) as the DC source starts to come back down, C1 starts to discharge. This serves to “fill the gap” between the ripples.

The chart is showing, with the different coloured lines, the influence of different values of C1 on the rippling DC.

Here’s a great article on this subject.

A more modern approach.

It seems to me that it is rather than painstaking to install a network of capacitors to perfectly smooth out the rippling DC. Also, transformers are far from perfect, so under load they sag, in terms of voltage. Even if you got all your calculations right, under load it will behave differently.

It is far easier, but not necessarily more efficient (in terms of trees and polar bears), to use a voltage regulator. The 78XX series works very well. You give it more voltage than you require, say 12V, and it does the work to provide you with an almost entirely ripple-free steady 9V. Any excess voltage is ‘given up’ as heat. This is why you might need a heat-sink (and why dolphins and rainbows are destroyed). Here’s a typical circuit using regulators:

AMZ power supply

A typical regulated power supply for guitar pedals

The end result

I came up with a simple circuit that used part of the above circuit to provide 9V and 18V for about 10 pedals. My initial circuit was flawed in that I’d ordered the wrong voltage regulators. They could only handle 100mA. Once we’d connected up all of Mike’s pedals, it would literally cut out. I even connected each one individually to my bench supply so I could measure each ones current draw.

Investigation showed that the regulator was extremely hot, so it became clear that the thermal protection was being tripped. I replaced the 7809V regulators with higher current handling ones and all was well.

Here’s some photographs of that build:

The finished 9V/18V power supply

A very simple decal makes the enclosure look quite spiffy, I think.

Gut shot of 9v/18v power supply

The circuit mounted inside, with all the DC connectors

The final schematic

The final schematic

And the final schematic, with values underneath:

  • D1 – 1N5401
  • C1 – 470uF
  • C8 – 0.1uF
  • D2 – 1N4148
  • IC1 – L7809CP
  • C3 – 10uF
  • R1 – 100R 1Watt
  • C2 – 100uF

 

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15 thoughts on “A real effects pedal power-supply

  1. The Method says:

    this is pretty cool…i built something similar…but not as nice…
    would you be willing to share your schematic? where did you get the regulator?
    that one regulator takes the 18 and goes right to 9VDC no problem?

    • simonallaway says:

      I’ve updated the post with the schematic I arrived at in the end. And yes, the data sheet says that with a minimum of 11.5v and a maximum of 26v it will produce 9v (plus or minus 0.2v). I got the regulators from Mouser. They are the L7809CP, which will deliver up to 1A output current, which is more than enough for a pedalboard’s worth of devices.

    • Jon says:

      This post is over a year old, but I hope you get this message. This is the cleanest looking design I have found and want to try it, but my question is: the schematic calls for IC 1 and IC 2, but I only see one IC in the photo and on the schematic. I am sure I am missing something basic. Care to school me?

      • simonallaway says:

        Hi Jon!
        If you look at the first schematic you can see what amounts to four “sections” that each feed off the incoming power, right after C8:
        1) A 7809 offering a maximum of 1A
        2) A 78L09 offering 100mA
        3) An LM317 offering variable voltage
        4) A pass-through

        I simply omitted the ‘sections’ I didnt need, such as variable voltage, and I mnade sure I could handle high current demands. So I ended up with two identical sections using the 1A regulators, and the ‘pass-through’ of the 18v wall-wart power.

        The photo with the single IC was the first iteration, hence the disparity. What do you anticipate is your load? That’s what will decide your choices.

        Does that make any sense?

  2. Many thanks for writing valuable info about 9V batteries, There’s so much to learn from this article.

  3. Oliver T. says:

    Great simple design for an electronicly ignorant guy like me to comprehend. I would really like to build it, with help of someone more experienced in electronics, of course. My only question is: as far as i understand from the diagram, the output is center positive. Is there a simple way to make it center negative, say just flip the hot and ground leads, so that the connection with the DC jack/s would be the other way around compared to the schematic? Thanks

    • simonallaway says:

      Absolutely. You can wire up the connectors in anyway you see fit. But if you’re using it for ‘standard’ (defacto) guitar effects pedals then you’ll have problems.

      • Oliver T. says:

        Why so, if it`s not too much to ask? As far as i understood, the design is dedicated for guitar effects/pedalborad. Or im I missing something? Using it with pedals is exactly my goal.

      • simonallaway says:

        I do apologise. You are absolutely right. Definitely an error on my part as us guitarists generally want centre-negative. So yes, wire it exactly how you want🙂

  4. Oliver T. says:

    Thank You, Sir! Much appreciated!🙂

  5. Kevin C says:

    Hey there!
    I see this is a pretty old (but awesome) post, but I’m attempting something similar and have a couple of questions if you still check this..
    It seems you’ve come up with a solution for providing steady voltage for the higher current draw of multiple pedals, but in what way have you addressed the negative side effects of daisy chaining that most quality supplies combat with isolated outputs?
    Also, what type of decal do you prefer to use? I’ve been leaning toward waterslide since I have everything I’d need but screen and UV yield such nice results..
    Anyway if you get around to answering this thanks so much! Otherwise thanks for a great read!

    • simonallaway says:

      Hi Kevin,

      I’m glad you enjoyed the post, despite its age.

      What are the negative side effects of daisy chaining?

      A power supply either has the ability to satisfy the demands of the problem presented or it doesn’t. This particular solution has the potential to distribute the load of as many pedals as you like over any number of 9V regulators. All you have to do is decide what you need and perhaps account for growth thus adding more regulators. Then make sure that the transformer you’re using to feed it can supply that much current. That’s what design specifications are for.

      An example being, my T.C. Electronic G-system; they added 4 x 100mA 9V power supply sockets to the back. These will serve 99% of normal pedals so they accommodate a large section of the guitarist population. One could complain and say “why aren’t there 8?” or “why aren’t they 250mA?”, and those would all be good questions. And all answerable with a different circuit. But someone somewhere (probably an accountant) decided 4 x 100mA.

      For decals I’ve always used water-slide, and it has worked well most of the time. Going for screen printing seems a lot of work for one offs. If I ever went bigger scale (which would break a major rule of mine) I’d end up doing that, I’m quite sure.

      Good luck!

      Simon

      • Kevin C says:

        Thanks for the response!
        Definitely get the idea of building for what you need, but when it comes to daisy chaining vs isolation, sharing DC and a common ground increases the likelihood for ground loops to occur which can lead to some pretty annoying hum with certain pedal combinations. Also, along with shared DC plugs, the pedals also share a common ground in the audio interconnect shielding. Each additional shared connection path increases the chances of noise leaking into the audio path, and two connection paths at once can create a feedback loop which amplifies the noise. DC isolation removes a path and the cause of the initial interference. Also, isolated supplies allow for center positive and negative to be run off the same brick, along with varying voltages and currents.
        For me, this applies because I run a few fuzz and overdrive pedals which are often create these issues.
        I’ll have to dig into this a little more to determine how to properly isolate, but the voltage regulator definitely seems like the way to go.
        Cool to know you’ve had success with water slide!
        Thanks again.

      • simonallaway says:

        Ah, now I understand what you mean. I had looked into isolation and indeed had considered the route taken by this circuit:

        http://www.geofex.com/article_folders/spyder/spyder.htm

        But when I considered my needs, which didn’t include any positive ground fuzzes (I’m just not a big fan of fuzz for some reason, don’t judge) I didn’t go ahead with it. The one fuzz that I do own was designed by my friend Shawn over at DIY Effects. Here’s my build of his board:

        https://hotbottles.wordpress.com/2012/09/04/ac128-germanium-fuzzface-with-fuller-mods-from-diy-effects/

        It deals with power supply polarity right on the board itself. Quite ingenious.

        I’ve since moved to using a T.C. Electronic G-System for all my time-based effects, and I use the 4 loops for various OD pedals. I don’t suffer from any ground-loop induced hum, but I can’t claim that’s through any solid approach on my part.

        Here’s a build I did of the Tremulous Lune pedal with my most pleasing water-slide results. I’ve also used it with great success on my guitar builds too:

        https://hotbottles.wordpress.com/2013/10/07/building-a-tremulus-lune-on-vero-board/

  6. simonallaway says:

    Ah, now I understand what you mean. I had looked into isolation and indeed had considered the route taken by this circuit:

    http://www.geofex.com/article_folders/spyder/spyder.htm

    But when I considered my needs, which didn’t include any positive ground fuzzes (I’m just not a big fan of fuzz for some reason, don’t judge) I didn’t go ahead with it. The one fuzz that I do own was designed by my friend Shawn over at DIY Effects. Here’s my build of his board:

    https://hotbottles.wordpress.com/2012/09/04/ac128-germanium-fuzzface-with-fuller-mods-from-diy-effects/

    It deals with power supply polarity right on the board itself. Quite ingenious.

    I’ve since moved to using a T.C. Electronic G-System for all my time-based effects, and I use the 4 loops for various OD pedals. I don’t suffer from any ground-loop induced hum, but I can’t claim that’s through any solid approach on my part.

    Here’s a build I did of the Tremulous Lune pedal with my most pleasing water-slide results. I’ve also used it with great success on my guitar builds too:

    https://hotbottles.wordpress.com/2013/10/07/building-a-tremulus-lune-on-vero-board/

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