Click here to purchase the entire book in PDF format. Introduction If we take enough transistors and build a circuit out of them we can construct a magic device with three very useful characteristics. In no particular order, these are infinite gain infinite input impedance zero output impedance At the outset, these don't appear to be very useful, however, the device, called an operational amplifier or op amp, is used in almost every audio component built today. It has two inputs (one labeled ``positive'' or ``non-inverting'' and the other ``negative'' or ``inverting'') and one output. The op amp measures the difference between the voltages applied to the two input ``legs'' (the positive minus the negative), multiples this difference by a gain of infinity, and generates this voltage level at its output. Of course, this would mean that, if there was any difference at all between the two input legs, then the output would swing to either infinity volts (if the level of the non-inverting input was greater than that of the inverting input) or negative infinity volts (if the reverse were true). Since we obviously can't produce a level of either infinity or negative infinity volts, the op amp tries to do it, but hits a maximum value determined by the power supply rails that feed it. This could be either a battery or an AC to DC power supply such as the ones we looked at in Chapter 2.11. We're not going to delve into how an op amp works or why - for the purposes of this book, our time is far better spent simply diving in and looking at how it's used. The simplest way to start looking at audio circuits which employ op amps is to consider a couple of possible configurations, each of which are, in fact, small circuits in and of themselves that can be combined like Legos to create a larger circuit.