Vacuum Tubes Versus Solid-State
The tube versus transistor debates that you hear most often occur in the pages of consumer and music magazines, with descriptive, but imprecise language like “warm”, “liquid”, “smooth”, and “dynamic”.
But what do the engineers who actually design the equipment think about tubes versus transistors in terms of objective science and measurements?
The two professional societies that have the most to say on this subject are the IEEE (Institute of Electrical and Electronics Engineers) and the AES (Audio Engineering Society). Both of these professional societies publish peer-reviewed journals, with articles written by engineers and scientists who work in the professional and consumer audio industry, as well as in cutting-edge academic research. If you are seeking a balanced view on this debate, direct yourself to either or both of these societies.
Below we point you to some readily available IEEE and AES publications that will help you better understand the differences between solid-state and vacuum tube electronics, their performance, and ultimately their sound.
IEEE - The Cool Sound of Tubes
The IEEE published “The Cool Sound of Tubes” in their August 1998 issue of IEEE Spectrum. In the same article, there is also a useful sidebar on tube versus transistor distortion. Finally, there is a useful table summarizing the advantages and disadvantages of tubes and transistors from both sonic and design points of view. Since the table is only available as a graphic image, we transcribe the text from the summary table below while highlighting some of the key points that directly impact sound quality:
Vacuum tubes – Advantages
- Highly linear without negative feedback, specially some small-signal types
- Clipping is smooth, which is widely considered more musical than transistors
- Tolerant of overloads and voltage spikes
- Characteristics highly independent of temperature, greatly simplifies biasing
- Wider dynamic range than typical transistor circuits, thanks to higher operating voltages
- Device capacitances vary only slightly with signal voltages
- Capacitive coupling can be done with low-value, high-quality film capacitors
- Circuit designs tend to be simpler than semiconductor equivalents
- Operation is usually in Class A or AB, which minimizes crossover distortion
- Output transformer in power amp protects speaker from tube failure
- Maintenance tends to be easier because user can replace tubes
Vacuum tubes – Disadvantages
- Bulky, hence less suitable for portable products
- High operating voltages required
- High power consumption, needs heater supply
- Generate lots of waste heat
- Lower power efficiency than transistors in small-signal circuits
- Low-cost glass tubes are physically fragile
- More prone to microphonics than semiconductors, especially in low-level stages
- Cathode electron-emitting materials are used up in operation, resulting in shorter lifetimes (typically 1-5 years for power tubes)
- High-impedance devices that usually need a matching transformer for low impedance loads, like speakers
- Usually higher cost than equivalent transistors
Transistors – Advantages
- Usually lower cost than tubes, especially in small-signal circuits
- Smaller than equivalent tubes
- Can be combined in one die to make integrated circuit
- Lower power consumption than equivalent tubes, especially in small-signal circuits
- Less waste heat than equivalent tubes
- Can operate on low-voltage supplies, greater safety, lower component costs, smaller clearances
- Matching transformers not required for low-impedance loads
- Usually more physical ruggedness than tubes (depends on chassis construction)
Transistors – Disadvantages
- Tendency toward higher distortion than equivalent tubes
- Complex circuits and considerable negative feedback required for low distortion
- Sharp clipping, in a manner widely considered non-musical, due to considerable negative feedback commonly used
- Device capacitances tend to vary with applied voltages
- Large unit-to-unit variations in key parameters, such as gain and threshold voltage
- Stored-charge effects add signal delay, which complicates high-frequency and feedback amplifier design
- Device parameters vary considerably with temperature, complicating biasing and raising the possibility of thermal runaway
- Cooling is less efficient than with tubes, because lower operating temperature is required for reliability
- Power MOSFETs have high input capacitances that very with voltage
- Class B totem-pole circuits are common, which can result in crossover distortion
- Less tolerant of overloads and voltage spikes than tubes
- Nearly all transistor power amplifiers have directly-coupled outputs and can damage speakers, even with active protection
- Capacitive coupling usually requires high-value electrolytic capacitors, which give inferior performance at audio-frequency extremes
- Greater tendency to pick up radio frequency interference, due to rectification by low-voltage diode junctions or slew-rate effects
- Maintenance more difficult; devices are not easily replaced by user
- Older transistors and ICs often unavailable after 20 years, making replacement difficult or impossible
AES - Tubes versus Transistors: Is There An Audible Difference
The AES (Audio Engineering Society) published the a journal article in May 1973 titled "Tubes versus Transistors: Is There An Audible Difference" that focuses primarily on the distortion aspects of tubes versus transistors.
One of the more interesting quotes from the AES article:
“Our extensive checking has indicated only two areas where vacuum-tube circuitry makes a definite audible difference in the sound quality: microphone preamplifiers and power amplifiers driving speakers or disc cutters. Both are applications where there is a mechanical-electrical interface.”
In addition to speakers, disc cutters and microphones we can include phono cartridges and musical instrument pick-ups (ie. guitars) in the world of mechanical-electrical interfaces where tubes have an advantage.
For impatient readers, skip to the section titled DISTORTION CHARACTERISTICS OF PREAMPLIFIERS, as this section covers the more relevant aspects of tube versus transistor sound.
This AES article dispels the myth that tubes overload more gently than transistors. This conclusion is reached by comparing variations in the slopes of the distortion characteristics (THD) for silicon transistors, pentodes and triodes. The finding is that there is little variation in how the transistors and vacuum tubes overload. However, there is a difference when they overload. Specifically:
“Overloading an operational amplifier produces such steeply rising edge harmonics that they become objectionable within a 5-dB range. Transistors extend this overload range to about 10 dB and tubes widen it to 20 dB or more.”
In the tests conducted for the AES journal article:
“Further listening revealed that it was only in the range of early overload where the amplifiers differed appreciably in sound quality. Once the amplifiers were well into the distortion region, they all sounded alike -- distorted. In their normal non-overload range all three amplifiers [transistor, hybrid op-amp, and vacuum-tube triode] sounded very clean.”
“Engineering studios show that any amplifier adds distortion as soon as the overload point is reached. The tests show that all amplifiers could be overloaded to a certain degree without this distortion becoming noticeable. It may be concluded that these inaudible harmonics in the early overload condition might very well be causing the difference in sound coloration between tubes and transistors.”
The article then digs deeper into the perceived “sound” of the relative distortion harmonics of tubes versus transistors. It was found that:
“There is a close parallel here between electronic distortion and musical tone coloration that is the real key to why tubes and transistors sound different.”
Further reading explains in detail the effects that harmonics have on sound coloration:
" The primary color characteristic of an instrument is determined by the strength of the first few harmonics. … The odd harmonics (third and fifth) produce a "stopped" or "covered" sound. The even harmonics (second, fourth, and sixth) produce "choral" or "singing" sounds. The second and third harmonics are the most important from the viewpoint of the electronic distortion graphs in the previous section. Musically the second is an octave above the fundamental and is almost inaudible; yet it adds body to the sound, making it fuller. The third is termed quint or musical twelfth. It produces a sound many musicians refer to as "blanketed." Instead of making the tone fuller, a strong third actually gives the sound a metallic quality that gets annoying in character as its amplitude increases. A strong second with a strong third tends to open the "covered" effect. Adding the fourth and fifth to this changes the sound to an "open horn" like character. "
" The higher harmonics, above the seventh, give the tone "edge" or "bite." Provided the edge is balanced to the basic musical tone, it tends to reinforce the fundamental, giving the sound a sharp attack quality. Many of the edge harmonics are musically unrelated pitches such as the seventh, ninth, and eleventh. Therefore, too much edge can produce a raspy dissonant quality. Since the ear seems very sensitive to the edge harmonics, controlling their amplitude is of paramount importance."
The last section, RELATIONSHIP OF FACTORS AND FINDINGS, ties everything together. The final paragraph sums it up best:
“Vacuum-tube amplifiers differ from transistor and operational amplifiers because they can be operated in the overload region without adding objectionable distortion. The combination of the slow rising edge and the open harmonic structure of the overload characteristics form an almost ideal sound- recording compressor. Within the 15-20 dB "safe" overload range, the electrical output of the tube amplifier increases by only 2-4 dB, acting like a limiter. However, since the edge is increasing within this range, the subjective loudness remains uncompressed to the ear. This effect causes tube-amplified signals to have a high apparent level, which is not indicated on a volume indicator (VU meter). Tubes sound louder and have a better signal-to-noise ratio because of this extra subjective headroom that transistor amplifiers do not have. Tubes get punch from their naturally brassy overload characteristics. Since the loud signals can be recorded at higher levels, the softer signals are also louder, so they are not lost in tape hiss and they effectively give the tube sound greater clarity. The feeling of more bass response is directly related to the strong second and third harmonic components, which reinforce the "natural" bass with "synthetic" bass . In the context of a limited dynamic range system like the phonograph, recordings made with vacuum-tube preamplifiers will have more apparent level and a greater signal to system noise ratio than recordings made with transistors or operational amplifiers.”
When playing back early 78 RPM (shellac) disc recordings through a tube phono stage like the Wavestream Kinetics Archival Phono Stage, you will notice a different dynamic character because of the above tube response and dynamics. Subtle artist intonations are clearer and more pronounced, fostering a greater sense of realism and emotional connection to the recording.