Paul's Posts — 14 July 2012
By Paul McGowan
Let’s start today’s post by showing you how easy it is to design an amplifier circuit with an op amp.
When an op amp is used in a circuit as an amplifier, all you need is two resistors, the op amp and a couple of batteries. You can have the op amp amplify as either an inverting or non-inverting amplifier – meaning that whatever signal you put in can have its polarity or phase intact or flipped over – the same way you can reverse the +/- of your speaker terminals.
So let’s roll our sleeves up and design a preamplifier together with an op amp. We will want to have 20dB of gain in our op amp, meaning whatever we put into it will come out ten times louder.
So here is what an op amp looks like in a schematic: it’s a simple triangle with two audio inputs, one output and the two power supply inputs.
The two audio inputs are marked + (non-inverting if we want to preserve the phase) and – (inverting – if we want to flip the phase over) and the output is, well, the output. Simple eh? You have no idea just how simple this thing is. Let’s keep moving.
Here is a drawing of an IC op amp package – a diagram of where each of these terminals are located on the actual device itself. You’ll see some other stuff marked on this one – like offset null – don’t worry, we won’t need to use that or have anything to do with it.
The -V and the +V are the battery inputs and between the two batteries we might use (like a couple of 9 volts) we have what’s known as ground – or the center between the two power supplies. Here’s what the two batteries tied together might look like and ground is in the middle between the two where it is written “0V”.
The picture I grabbed shows 6 volts – but imagine 9 instead – which BTW was exactly the circuit I first used to power the prototype of the PS Audio phono preamplifier – yep, two 9 volt batteries and a couple of op amps in a Roi Tan cigar box for a chassis.
OK, now let’s make our preamplifier. I think let’s start out making an inverting version, then we’ll go tackle a non-inverting one. As I mentioned all we need is two resistors and because the gain of an op amp is set by the ratio of the two resistors, then all I need to do is have one resistor 10 times bigger than the other. So I could use a 1K (1000 Ohms) and a 10K (10,000 Ohms). These are just little bits I could buy at Radio Shack if I wanted – maybe a whole $0.10 each. Here’s what our circuit would look like.
We would put the 1K on the in and the 10K between the out and the in. The little triangle symbol you see on the + input is ground (the meeting point between the two batteries). That’s it. There’s not much more. Just connect an RCA input to the input side and another RCA connector to the output and you’re done. Whatever you plug into the input will come out 10 times bigger on the output RCA without any distortion or problem.
If I used quality parts, a PC board to hold everything, a power supply instead of a battery and a fancy chassis I have a commercial preamp. Want a volume control? Just stick a potentiometer on the input and you have it. Want it “high-end”? Use a good op amp and a better power supply and fancy connectors.
But maybe we want to have a non inverting amplifier instead. OK, all we have to do is change where we connect ground up and where the input is. Here, take a look.
See? R1 is our 1K resistor and R2 is our 10K. Simple, no?
But this is a single ended preamplifier – where we can only put in the output of an RCA connector. Let’s get super fancy and make it a balanced input amplifier – where you would put in the output of a balanced cable with an XLR on its output – balanced out. We’re all familiar with that.
Here’s what that would look like – please ignore the values because I just grabbed this off of the internet.
The little symbol that looks like a rake is ground. Cold is the one side of the balanced signal and hot the other.
OK, this is getting lengthy and the purpose isn’t to teach you how to design an op amp but just how danged simple it is – and I am not simplifying anything here.
Tomorrow let’s see what’s inside of these little buggers that makes everything so easy and so widely used.
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Tom Devey
Title should probably be more accurate as “HOW TO DESIGN WITH AN OP AMP” as stated in the first paragraph.
Soundminded
I think it should be called “High End Audio Electronic Circuit Design for Dummies” or How to successfully design and manufacture high end audio equipment without having to go to engineering school. No need for load line calculations, Thevenins theorem, Kirchoff’s Laws, just a few basics from a cook book, some spare space in your garage or basement, a soldering iron and a few small hand tools. It’s the same principle as the five minute university.
Paul McGowan
If that’s what it’s appearing to be like then I’ve apparently done a pretty piss poor job of this series. My intention is to try and help people understand how all this works, what the differences are between IC amps and discrete amps and why one would choose one over the other. It has never been to teach people how to design products in their basements.
It’s been my feeling for many years that if you have an interest and are armed with even the slightest amount of knowledge that you can make informed choices much better. In the same manner that you might read an in depth story about how a financial instrument works and why it’s affecting the economy, it doesn’t make you a financial expert but at least you have a clue what’s going on.
If you have any suggestions how I might better accomplish my goal I am all ears.
Soundminded
No offense intended Paul. So far you’ve explained in general terms what’s in an IC op amp and the most rudimentary basics of how they work (no real detail of what’s going in inside them though, nothing about how the circuit actually works.) But I haven’t seen any explanation of why an IC op a is different from the same amplifier circuit built with discrete parts. So far packaging would seem to be the only difference between an IC op amp and the same op amp circuit breadboarded. There has also been no discussion about the reasons for the difference in performance between one manufacturer’s version of an IC op amp and another’s of the same model number type with a different circuit topology let alone what makes one design better or worse than an equivalent different type by the same manufacturer. What do measurements show is the telling difference in peformance? Often manufacturers tell audiphiles the differences between their products and the inferior products their competitors offer can’t be measured. I don’t buy that.
There are several things that should be pointed out. Virtiually all of these parts are very inexpensive. There isn’t one of them that can’t be bought retail for just a few dollars and in quantity at a fraction of their retail price. High end audio manufacturers have been saved from even faster decline by the fact that the “can-do” DIY spirit that was so prevalent in America in the 1950s and 1960s when many teenagers were building shortwave transmitters and receivers and audio equipment from schematics and kits and others who tore down engines in old cars with the help of friends who liked to work on cars and had access to machine shops is all but gone now. Most high end audio equipment or similar designs could be reverse engineered and built by hobbyists at a small fraction of their retail price. For example, a few years ago I saw one manufacturer’s high end tube preamp that was designed around three 6AT7s that retailed for $10,000 yet this “box” was even less complex in its audio circuit than the all American five table radio that sold for a few dollars all over the country in the 1950s and that preamp circuit could even have been built then (except for the mylar film capacitors.) Except for power and output transformers what element of an analog electronic audio circuit is more expensive than a few dollars? What I hear from many high end manufacturers to justify their high prices is the “fantastic research” audio equipment designers of end user products brag about. Most strike me as little more than tinkerers experimenting by trial and error until they find something they like better than what they’d built before. I say that as someone who has been surrounded by real scientists and engineers all of my life and worked in many real research facilities.
Can you name even one other category of consumer electronics product where the long term trend hasn’t been to get more and more performance at less and less cost? Compare the models of current computers, TV sets, cell phones, electronc cameras with those offered just five or ten years ago. Then compare high end audio equipment. I’d like to point out that even five or ten years ago, many children were able to buy power supplies, motherboards, cases, a few more cards and hardware and software and assemble a desktop PC themselves in an hour or two and still can if they cared to and it would perform identically with a store bought computer of the same type. With the availability of entire amplifier circuit boards, especially class D amplifiers now, I see no reason why they couldn’t do exactly the same. What will the differences in price be though? I don’t know any other area where engineers don’t worry about price and value for price and this includes professional audio equipment too. You can be certain Bryston takes a very sharp pencil when they have to bid against Crown for a sound system in a sports arena. Those prices they sell similar designs to audiophile for are out the window or they’d never win a bid to get a contract.
Soundminded
At the general interest level you may be doing all you can. But to gain a real understanding of what’s going on takes much more. You can explain the general concepts of how a vacuum tube amplifier works in a few minutes. But if you were going to explain in detail how it works that takes a course in electrical field theory. And then there’s the complexity of screen and surpressor grids when you get to tetrodes and pentodes and why those elements are used. To just explain how a transistor really works puts us in a whole different world of quantum physics (and all that horrible math) let alone how circuits using them are designed (again a lot more math.) This is what a 4 year electrical engineering couse is about and why it’s so hard. Perhaps I’m just being too critical, too judgmental and unfair expecting far too much. If that’s the case then I apologize.
Paul McGowan
No need to apologize, we love you just the way you are. All this dialog is quite helpful keeping things in perspective. I just got a wonderfully nice note from a reader by the name of Tim which I will post here. For me, this is what’s important and why I do this:
“I would imagine the silent majority of audio hobbyists are not EE’s, but like me are busy with their work, family, etc. and have not had someone thoughtfully walk us through many of these building block concepts. I have heard and read in Stereophile and TAS (and listened to) tubes and op amps for years, but never Really had even a basic understanding until your series. I know my fellow Lexington KY audiophile friends (who have also been in the hooby for years) feel the same and have also subscribed after I forwarded them some posts. And some of your ideas we have emailed back and forth and discussed live.
Thanks again – and keep it up!
- Tim”
And there in a nutshell is why I spend my free time writing and contributing. I have no reward in mind other than to once in a while stimulate some thought or open a few doors of understanding. For me, that’s plenty.
Thanks to all of you who take the time to read and comment.