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.
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.
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:
- Transform the outlet supply from 115V to something closer to the desired voltage.
- Use a rectifier to convert the AC supply to a DC supply.
- Use filters to smooth out the now rippling DC.
- 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).
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.
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:
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:
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