As you begin to tackle more complex and interesting synthesizer components you may begin to encounter schematics for circuits which require both a positive and a negative power input to operate. As an example Eurorack synth modules use plus and minus 12V power supplies. As a result many DIY synth enthusiasts will also use +\- 12V so that they can interface with their Eurorack Modular set-ups. This is referred to as a bipolar power supply and is necessary for circuits which include operational amplifiers.
Unfortunately this can cause something of a tripping point for electronics or synthesizer beginners. When I first came across this it took me far longer than I like to admit to wrap my head around it. How could a voltage be negative? Do I need special equipment to power these circuits? This confusion was primarily caused by two misconceptions I held at the time:
Voltage is a measurement of force not quantity:
When we think about voltage we tend to think of it as a quantity. We assume that the voltage of a battery is the number of volts which that battery contains. This is perpetuated by the way we refer to voltage ; “This battery is 9V.” This is however not correct. The voltage we refer to with batteries, power supplies and circuits is actually the voltage difference between the positive and negative poles. If you are familiar with the water analogy for describing electricity you may have heard that voltage is the pressure which pushes the electricity through the circuit. If you had a pipe where you applied equal pressure to both ends the water would not move through it. If however you had a pipe where you applied a greater pressure to the water at one end of the pipe, the water would begin moving. Further the force with which the water moves through the pipe would be equivalent to the difference in pressure between the two ends of the pipe. Similarly with a 9V battery the voltage at the positive pole is 9V higher than the voltage at the negative pole which pushes the electricity from the positive pole, through your circuit, to the negative pole.
Consider then if you turned the circuit upside-down. This would mean the same 9V of force was still moving through the circuit. However now it is moving through the circuit in the opposite direction. The 9V of force would now be pulling electricity from the ground connection and pushing it to the power bus. This is what would be referred to as a negative voltage.
Ground is a reference point:
When I started working with electronics I did not have a firm grasp on what exactly ground was. I got used to using the negative pole of a battery as ground and began assuming that it was the lowest pole of the battery or power supply. This understanding served me fine with basic circuits but became a problem as I began working with op-amps and more complex circuitry. The truth of the matter is though that the ground is not an intrinsic point on the power supply and has more to do with the circuit itself than your power supply (That being said some power supplies include circuitry to anchor or shield their ground to make it more stable). The ground ultimately serves as a reference point from which the voltage of the circuit is measured. With some basic components you could set up a ground anywhere between the maximum voltage of your power source and 0V.
Consider the circuit above. The most intuitive way to approach this would be to say that ground is point C. In this case we would measure the voltage difference between B and C to determine that the voltage at point B is 9V. Similarly by measuring the voltage difference between A and C you can determine that the voltage at point A is 18V.
However if you approach the circuit differently you will see very different results. Lets say that we assign point B as ground in the circuit. In this case by measuring the voltage A and B to find that the voltage at point A is 9V. Next we would measure the voltage between C and B and find that the voltage at point C is negative 9V. This means the voltage at point C is 9V less than the voltage at ground (point B). The schematic shown above is the most basic bipolar power supply you can create and is perfect for developing familiarity with these types of circuits.
To make my life easier I soldered this small bipolar power supply together on a scrap of perf board I had on hand. I’ve added two large capacitors (330 uf electrolytic) to provide some decoupling for simple circuits. Additionally I placed leads on the positive, ground and negative traces so I could easily connect this supply to my breadboard.
If you are looking to free yourself from batteries I would strongly suggest looking into MFOs Wall Wart Bipolar Power Supply as an option for moving to a more permanent voltage source (along with the wonderful documentation provided with all of MFOs projects). Alternately if you have a traditional bench power supply there are many projects available to help you create a bipolar supply using the monopolar output these provide.
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