Steampunk Technology: The Inner Workings of Vacuum Tube Buffers
by Phil Taylor
This article explores the deep inner workings of tube buffers, comparing and contrasting them with their solid-state transistorised opamp (operational amplifier) equivalents, and suggests why tube circuitry sounds so euphonic and musical in comparison. Note: You can zoom in and enlarge many of the photographs in this text by clicking on them.
Just what is a ‘Buffer’?
Buffer circuitry is found in all kinds of electronic audio hardware from budget transistorised guitar stompboxes to high-end vintage tube studio compressors. Many Effectrode effect pedals are built on a simple tube buffer shown in fig. 1. This circuit originally appeared in the RCA Receiving Tube Manual back in the 1940s – the same text Leo Fender referred to when designing his first guitar amplifiers. The men in white coats refer to this circuit as a cathode follower stage but it’s also commonly known as a ‘buffer’. This is because the circuit has high input impedance and low output impedance meaning it can effectively isolate or buffer one part of an audio circuit from another to prevent loading. A practical application of this is the use of a buffer to isolate a high impedance signal source, such as guitar pickup from a long length of cable to prevent high frequency loss.
Next to an opamp a tube is an expensive, fragile, bulky and power hungry device that takes a while to warm-up and gets pretty hot during operation, yet despite these technical shortcomings there’s a handful of nutty, artisan audio designers who are prepared to go to great trouble (and expense) to understand and work with this archaic and virtually obsolete technology. The reason for this is that buffers do much more than buffering – the buffering example I’ve just cited is just their typical, classic textbook use. The physics of a tube buffer also imparts something more to the sound of a musical instrument – beyond merely buffering – that works very much to the advantage of guitarists, musicians and recording engineers [see ‘The Cool Sound of Tubes’ article written by Eric Barbour]. They can fatten-up weak, thin solo guitar parts, add sparkle or life to chord work, impart additional depth and richness to ‘flat’, one-dimensional sounds, add ‘shine’ to a guitar track or help it sit better in the mix and even bring a lack-lustre solid-state transistor guitar amp to life. Let’s take a closer look at the construction, materials and physics of this tube buffer and see what it reveals about this vintage circuit’s nature as a tone enhancer.
Tubes and Opamps Look Different
Although vacuum tubes and opamps both perform the task of electronic signal amplification, they are physically very different devices, so different, that tubes seem like steampunk technology from a bygone era, and they are! A vacuum tube is constructed from metal electrodes and wires made up of several different metals, including nickel, tungsten, molybdenum, copper, strontium and barium oxides supported within a glass tube using insulating mica washers. Although much of their assembly was automated using machines some of it could only be completed by hand and this coupled with some of the rare and exotic materials utilised in their construction makes them considerably more expensive to manufacture than an opamp. A tube is also a relatively large device and requires larger quantities of raw materials, further elevating fabrication costs relative to an opamp which contains a tiny integrated circuit (IC) inside a plastic housing. The IC itself is a tiny silicon wafer (or die) and the circuit is created using hi-tech etching and deposition processes to create transistors, capacitors, resistors and connections on its surface. Fig 3. below shows a 741 opamp with the outer casing removed. The photo is greatly magnified and the die containing all the circuitry is only 1.3 mm square. The transistors are made of silicon (a semiconductor) containing impurities of boron or phosphorus, the capacitors from insulation materials and the internal connections from aluminium.
Tubes and Opamps Operate on Different Principles
The fundamental physics of how tubes and transistors operate is also completely different. In a vacuum tube electrons are emitted from a heated ‘cathode’ (the electrode that glows orange inside the tube) and are attracted to another electrode, the ‘anode’ which is held at a high voltage of several hundred volts. Signal amplification is achieved by control of the electron stream (a current) by varying a voltage on an electrode called the ‘grid’. The electrons travel at high velocity in a vacuum, whereas in a transistor they make their way more slowly through a crystalline silicon lattice. Now please bear with me over these next few sentences as the operation of a transistor isn’t as straight forward as a tube and things get a bit technical. Here we go… In an opamp the tiny transistors are made of a sandwich of silicon with boron and phosphorus impurities deliberately introduced by a ‘doping’ process. Doping silicon with phosphorus introduces extra electrons into the crystal lattice, whereas doping with boron causes a lack of electrons. The outer phosphorus-doped layers of the silicon lattice are called the ‘collector’ (analogous to the anode in a tube) and ‘emitter’ (analogous to the cathode) and the thin inner boron-doped layer is the ‘base’. The base controls the flow of electrons from the emitter to the collector which is connected to the power supply rail which is held at a voltage of several volts. Small currents on the base affect the conductive properties at the boundary between boron-doped and phosphorus-doped silicon. The physics of transistors is much more abstract than tubes, however that’s essentially how they operate.
Tube Circuit Minimalism
The circuit topologies of tube and opamp buffer circuits also differ. Again, this is partly down to the differences their physics, but also because of many years of development in the art of electronics and circuit design in the quest for the ideal amplifier. Tube circuits, being more primitive and less sophisticated, invariably utilise fewer components and are more straight forward in their operation. The component count in the RCA buffer circuit is minimal with just a handful of resistors, capacitors (passive components) and the vacuum tube (the active component). Contrast with the opamp buffer circuit in fig 4. Although its external physical appearance appears simple, it actually contains dozens of tiny transistors, capacitors and resistors. An opamp is intrinsically more complex than a tube, in fact it contains more components than my Fender Tweed guitar amp! Fig. 3 shows an excellent close-up view of the guts of a 741 opamp – a labyrinth of current sources, current mirrors, differential and push-pull amplifiers, temperature compensation and current limiting circuitry etched onto a tiny silicon wafer. All this circuitry is in the pursuit of technical perfection, the dream of an amplifier with wide, flat frequency response, vanishingly low distortion and low output impedance, and the opamp, to all intents and purposes realises this. Near perfect specifications realised through circuitry that harnesses ‘negative feedback’, an invention first conceived at Bell Labs during the 1920′s and early 30′s. More on negative feedback later.
Tubes are Electrifying!
Looking more closely at the opamp buffer circuit in fig 4. it can be seen there are 25Ω and 50Ω resistors in the push-pull output stage. This is an indication that higher currents can flow than in a tube buffer where the cathode resistor is several tens of kilo ohms. Additionally opamp devices operate at much lower voltages than tubes. Solid-state gear such as effects pedals the opamp power supply rail is typically derived from a 9 volt battery and some audiophile solid-state gear may even operate at higher voltages of 12VDC or 18VDC. Professional audio equipment usually has positive and negative supply rails of +/-15VDC allowing almost 30V total rail-to-rail signal swing which gives more headroom, however this is still low when compared with tube circuitry operating from 300VDC or 400VDC rails. It’s a huge potential difference and the tube cathode follower has a nice wide linear region of operation giving it excellent dynamic range. Not only that, if it is driven into the non-linear region then soft clipping occurs, generating a spectrum of harmonic components that sounds subjectively more pleasant than the abrupt hard clipping in opamps.
Nicer Negative Feedback
It’s worth highlighting how negative feedback is used in tube and opamp buffer circuits. Yes, that’s different too. In the opamp buffer feedback is externally applied from the output back to its inverting input with the well understood advantages of lowering output impedance and lowering harmonic distortion. This negative feedback control loop is non-localised, that is, it’s applied across the transistor stages within the guts of the opamp. The effect of such non-localised (or global) feedback on tone is the subject of heated debate amongst audio engineers and I must confess I’m not an enthusiast for it. I prefer the feedback arrangement in the cathode follower, which is beautiful in its simplicity. Here the negative feedback is intrinsic to the tube and much less contrived – the principle of operation is elegance itself. If the cathode is made more positive, then the grid becomes relatively more negative and less current flows, decreasing the output voltage. Conversely, if the cathode is made less positive, the grid becomes relatively more positive and more current flows causing an increase the output voltage. The feedback control in the tube buffer is down to the fundamental physics of vacuum tubes, whereas in the opamp buffer the feedback mechanism is imposed by design.
It’s been the aim of this article to examine the physical and functional differences between tube and opamp 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 opamp 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. Opamps 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.