PS Audio: More on op amps

Paul McGowan writes:  This subject is actually very interesting because this class of amplification device is at the core of almost everything in audio today and while simple on the surface, there’s lots to know.  Therefore I will break this up into a bunch of little posts to help you assimilate the info one day at a time.  First let’s start with a bit of history.

Op Amp stands for Operational Amplifier and despite the fact it permeates nearly every audio design in solid state electronics it was never designed to be an audio amplification device.  In fact, operational amplifiers were designed to work as the core building blocks of analog computers.

“What’s an analog computer?  I thought that computers were all digital?”  Just like everything we think of as new digital wonders they all started with analog versions and, of course, there are many types but a slide rule is a good example of one.  The electronic equivalent or a slide rule is what op amps were originally charged with doing and they did it well.

Here’s an example of how we might make a calculation using an op amp.  Because op amps are essentially perfect voltage amplifiers you can easily set their gain or multiplication factor with a simple set of resistors.  Let’s say I want to multiply a number by 10.  I simply set the gain of my op amp to a gain of 10.  Now when I input a voltage, let’s say 1 volt, the output of the op amp is 10 volts.  Volia!  We just did a math function and all I need to do is connect a meter to the output to know the answer.  This sounds too simple to be valuable because you can do the math in your head but now let’s try a harder math problem.  Multiply Pi 3.414 times 8.215 and the op amp merrily (and instantly) spits out 28.04601 volts.  You can divide, add, subtract and perform complex math of any kind.  Limiting you say?  Consider that when we sent men to the moon the calculations needed were performed on slide rules (mechanical analog computers) – and I’ll bet setting foot on the moon took some heavy math!

So op amps using vacuum tubes and then transistors were running computers years ago but then something happened: Bob Widlar (from yesterday’s post) integrated this circuit onto a single piece of silicon, then packaged this perfect voltage amplifier into a small and, soon to be, affordable package with 8 pins for a single op amp and 14 pins for a dual op amp.  This changed everything for every engineer in the world including, you guessed it, audio designers.

The popularity of the packaged IC op amp was immediate and overwhelming.  The original Fairchild 701 and later their 709 had a two year waiting period for advance orders so swamped were they with orders – and they were $100 each!  By the late 1960′s and early 1970′s when most audio designers starting using these little gems, the price had dropped to around $1 each.  So imagine purchasing two perfect voltage amplifiers in one package for $0.50 each.  This was the Garden of Eden and we all took a bite out of that apple.

Why?  Well obviously the low cost but perhaps most important is the ability to use a building block rather than having to build the block itself.  There are only 5 pins to connect on an op amp and two of those connect up to the battery or power supply.  This meant that you could design a preamplifier for an audio product by connecting a few resistors up to three pins – two inputs and one output and you had a product.  Design time for the schematic?  Less than 1 minute and you were guaranteed it world work.

Enticing?  You bet.  Tomorrow let’s look a little deeper.


Leave a Reply