Op-amps (operational amplifiers) come in an integrated circuit, or IC. The one pictured on the right is in a form called DIP-8, which is short for dual in-line package with 8 pins. “Dual in-line” refers to two lines of pins, in this case 4 on each side. Each pin has a special function and they are numbered from 1 to 8. Often, there is a circle on the top of the case to show the location of pin 1. Also the case is usually notched at the same end with half-circle cut-out. At least one of these two markings appear, but not necessarily both.
The manufacturer, its documentation, and the name of the chip are printed on the top. In this case, the leading symbol is the circular N logo of National Semiconductor and the name of the chip is LM741CN. The DOD Overdrive 250 and the MXR Distortion Plus are popular examples of stompboxes built around this chip.
The names of chips are often abbreviated to their numbers, like 741. Different manufacturers may use different prefixes and suffixes while keeping the same numbers. For example, JRC produces the NJM741D. Although generally they can be used as functional substitutes, chips with the same numbers but different names are not necessarily identical. One of the characteristics that is often flagged with the suffix is the package or case of the IC. For example, National Semiconductor also manufactures the LM741H which is the 741 in an 8-pin “metal can.”
The numbering of the pins is always the same. With the notch or pin 1 marking at the top, count sequentially from pin 1 down the left side and then up the right side of the DIP. In other words, count counter-clockwise (CCW).
Op-amps can be soldered directly to the circuit board but often op-amps are seated in sockets instead. The sockets are soldered to the board, avoiding the risks of destroying the op-amp with too much heat. Also, one can exchange op-amps easily to make a repair or to try an experiment (like on a breadboard). A DIP-8 machine-pin socket by Mill-Max and a DIP-8 dual-leaf socket by AMP are shown below.
These sockets also have notches like the op-amps so that even without the op-amp on the board its orientation is still clear.
Basic op-amps are denoted on schematics by a simple triangle that usually points to the right.
They have two inputs on the left-hand side, denoted by positive and negative signs, and a single output at the point on the right. The inputs are often shown in the opposite order.
Above and below are power supply connections, with the positive supply above and the negative supply below.
These directions are associated with typical audio schematic layouts, where the input is on the left, the output is on the right, the positive power connections go up, and negative or ground connections go down. Often, when they are implicitly clear, the power connections are not shown (as on the right).
On circuit board layouts, one sees the outline of the IC package. The characteristic notch appears at the end where pin 1 is located. On gaussmarkov.net layouts, the pad for pin 1 is also square while all of the other pads are round.
Op-amps do not have a single salient characteristic like resistors and capacitors do. Op-amps are designed with various special tasks in mind so that they are particularly good in specific ways. The design goals include gain, low noise, fast response, high input impedance, and low output impedance.
DIP-8 op-amp ICs may contain one or two op-amps. The 741 contains only one op-amp. The 4558 that is found in tubescreamers has two and is often called a dual op-amp. You will also run into quad op-amps that contain 4 op-amps in a DIP-14 package. The pin assignments (or pinout) for the 741 can be described by drawing the op-amp into a pin diagram
so that the two inputs are connected to pins 2 and 3, the output is connected to pin 6, the positive power supply to pin 7, and the negative power supply to pin 4. Pin 8 serves no purpose. We will discuss pins 1 and 5 in a later note.
Dual op-amps like the 4558 generally have the same pinouts so that their ICs can be interchanged in circuits that use them with sockets. This is a popular thing to try with stompboxes like the tubescreamer. Op-amp A has inputs on pins 2 and 3 and output on pin 1 on one side while op-amp B has inputs on pin 5 and 6 and output on pin 7 on the other side. The positive and negative power pins are 8 and 4, respectively. On a schematic these op-amps would be labeled IC1A and and IC1B and the power supply pins for one would serve both.