It’s been the aim of this article to examine the physical and functional differences between tube and op-amp buffer circuits to highlight the subtleties of their operation. It’s common engineering practice to evaluate audio circuitry solely using easily measurable parameters such as frequency response, harmonic distortion, etc, and on paper op-amp parameters often look very impressive, much better than tube specs. But what is ‘better’. Behind the specs there’s real physics going on and the unique physics of tubes, i.e. controlling electrons in a vacuum with high voltages, can sound sublime when the designer knows what he’s doing. All these graphs and numbers can sometimes blind us to what’s important—does it sound good? For an engineer these technical specs are vitally important tools in circuit design, however, they’re really just guidelines and should just be a means unto an end, not a means unto themselves. For a guitarist the tone of their gear, how it reacts or feels and if it inspires them are what counts. These are the specs that really matter and this is where tube buffers excel.
On a final note, as an engineer I prefer working with the idiosyncrasies of tubes, there’s something about their simplicity and their limitations that appeals to me. For example, a 12AX7 contains only two discrete amplifier sections meaning that it’s only practical to utilise a few tube stages in a given design otherwise the circuitry becomes unwieldy and expensive. Op-amps are much smaller and much, much cheaper meaning you can toss as many of these into a circuit design as you like with far less concern for costs or complexity. You’ve got to be a minimalist to work with tubes and designing circuits with them is as much an art as a science. Their simplicity and limitations focus the engineer’s mind on the challenge of attempting do as much as they can with very little. Simple systems have simple shortcomings and to my ears the shortcomings of tubes sound wonderful.
Special thanks to Thomas Mayer for kindly granting permission to use his beautifully detailed close-up picture of the 6J5 tube—I’ve not seen anyone else achieve such detailed and well illuminated close-up photographs of tubes before. If you’re interested in seeing more examples of his superb work then you might want to check out his blog VinylSavor. Additionally, I would like to thank Gary Lecomte of Chemlec Consulting and Design for his superb close-up of the inside of a 741 opamp. You can find out how he managed to take this amazing picture on his website.