I decided I wanted a little bit more information about op-amps. Just soldering in the audio amp into the SW40 kit board and moving on with only vague recollections from 20+ years ago about what these chips do wasn’t satisfying to me. So, to the internet, and my old college textbooks, I went.
The top photo is of a generic op-amp circuit configured as a “subtractor”. If you make all the resistors the same, you realize a difference function between the two inputs, va and vb. The derivation for how this works isn’t all that hard and was found in the cited textbook in the caption. You make shortcut assumptions which makes most of the algebra drop out and leaves you with simple expressions. The main assumptions are things like input current into the op-amp can be neglected (it’s essentially zero), and the gain “Ad” is super large, and can be approximated as infinity, especially when on the denominator of an expression you want to go away. Other important facts about op-amps are a very high input impedance (nice so as not to load down your inputs) and a low output impedance.
The all-important gain expression that I was struggling to find (to no avail) through random internet searches was the ratio RF/RS. If all resistors are the same, the circuit will just give you the difference in input signals without any amplification. But by manipulating this ratio you can boost the difference signal up to the point where you start clipping it (which happens pretty quick I found).
A couple of significant gotchas are lurking with hooking up this chip. First, I went with an LM741 because I’d just purchased a couple of bags of these cheap items from the Radio Shack liquidation a few months back, and I figured if I blew them up it was no great loss. But looking at the spec sheet and the little package pin description shown above, I didn’t realize fully what the terms meant. It took some digging online but I learned that the offset null could mostly be ignored and is used to kind of “zero-out” internal chip discrepancies that might, say, cause two equal inputs to seem as though they were not equal. And I thought I knew what V- and V+ were for — namely, ground and +12V connections. Oops!
Although I found some possibly questionable websites showing the chip connected in this manner, I discovered that I could not get anything but nonsense results doing things this way. I spent a couple of hours and tried different chips and resistor values but to no avail, the circuit was NOT finding difference voltages (I’d get things like a DC +8V signal even with AC inputs).
So, more Googling turned up some college physics lab notes on op-amps and lo and behold some used the 741 chip. They clearly stated you needed to apply +12V to the V+ and a ** -12V ** to the V-! I made the change (although clumsily, using a battery pack to deliver the V-), and it worked! Wow! Difference signals very clearly and crisply seen on the scope. So if anyone out there in internet land has had similar struggles, I hope this helps (let me know if it did).
What was a little off though was I was still getting gain even though all the resistors were the same. I can think of a few reasons for this. With such high gain, the op-amp may magnify subtle differences between resistor values. We know these cheap 1/4 W resistors are not precision, generally. Another factor may just be that whole “offset null” thing (just speculating on that). But, whatever the cause, I discovered quickly that if I made the feedback resistor lower, around 10 ohms while keeping the others at 100 ohms, I made the gain unity and the circuit worked just like the textbook equations predicted.
The next step was to see, could I make an audio amp out of this with 20 dB gain, just like the one in the SW40 kit? Well, why not? I already was showing gain when using all 100 ohm resistors. I found that if I made that resistor around 70 ohms (with the others being 100), I got the desired gain. With no load (ie, speaker) attached and just watching on the scope, I could inject a 1000 Hz audio tone into the input and see the 10x voltage gain on the output, clean as a whistle. Note, this is actually better performance than I got with the SW-40 audio amp, which for some reason picked up a lot of 60 cycle AC noise. Not with this simple 741 configuration.
The final piece of the puzzle was to hook up a load. I tried a small speaker, those little 2″ type things you see on kit rigs. Sure enough, the op-amp could not deliver to that load very well and distortions took over with very little input signal. I fared much better when using iPod headphones. I could tell the volume would be adequate and distortions really only got bad when cranking it so loud you’re hurting your ears. Still it seemed reluctant to deliver much volume and so I can see that this is probably a limitation in using op-amps for audio amps. I feel like I’ve heard or read that this is the case. The trusty 386 chip seems much more willing to deliver audio oomph.
Still, I found the ability to scale gain up and down dramatically and getting clean outputs very satisfying and cool. And it was also neat to finally play with these circuits, which had intrigued me for the past 25 years but which I’d never had the time or inclination to actually study. It was high time to bust out the breadboard and fire these things up and play.
I’m finding I have this 20+ years of pent-up need to explore circuits in me! These kit rigs really help serve as a guide down that path of exploration I think. Each stage is something that opens a door into a wider understanding. And once learned you can use that knowledge going forward into your own future projects.
Next I will probably look at the op-amp chip used in the kit again and see now if I can understand it’s configuration a little better.