DIY Vacuum Tube Amplifiers
Website of EJ Jurich
Class A Audio Reproduction
High-Fidelity
Vacuum Tube Amplifiers
At home in an average-sized room, using reasonably efficient speakers, 0.4‑watt (400‑mW) to 1‑watt of amplifier power provides a comfortable volume level while keeping speaker distortion at its lowest level (1). From the perspective of High-Fidelity audio reproduction, in the home environment, an audio system does not require a lot of amplifier power to achieve 80 to 85 dB sound pressure levels (SPL).
Consider how your ear reacts to sound pressure levels and how much power is required to double the apparent
loudness (volume). With efficient speakers in a typical 144 square foot (44 square meter) room,
0.1‑watt (100 mW) of amplifier power provides a background level that allows normal
conversation; point A of the graph. As a protective reflex, sound pressure increases will cause the ear to
compress (limit) the apparent loudness.
To double the apparent loudness to point B of the graph, it will require about four times the amplifier power
or 400 mW. From this point up, it will take ten times the power to double the apparent loudness to
point C, or in this case, 4 watts. Any more doubling will take you past the point of ear-damaging SPL.
The graph is a generalization. Real-world amplifier figures would depend on distance from the speakers and
speaker sensitivity. The graph is used to illustrate the ear's reaction to gradually increasing SPL levels
in a normally quiet room.
The graph starts at 0.1 watts, comparable to a small pocket radio playing background music in a room. As volume is turned up, the ear starts to compress the apparent loudness, requiring 0.4 watts to provide twice the apparent loudness. As volume is turned up more, the ear will compress apparent loudness even more, requiring 4 watts to provide twice the apparent loudness. The human ear compresses sound pressure levels (SPL) on a logarithmic scale, increasing compression as SPL levels increase. SPL levels of 90 dB and higher fall in the ear damaging range.
Power Consumption
Thinking about a vacuum tube amplifier with higher power output? Besides the current drawn by the output tubes that produce power output, there are filaments drawing current. Higher power output tubes, besides being more expensive, have filaments that draw more current, increasing total power consumption. If you mostly run volume levels at a comfortable level, in the long term you would find yourself paying for high power consumption while running the amplifier at a comfortable volume level. For example, a 30‑watt amplifier running at 1‑watt power output. Instead of power output in watts, consider using sound pressure levels based on the sensitivity rating of the speakers being used. You can use the amplifier power required calculator at extron.com.
Single-Ended Triodes Sound Better
Of the various vacuum tube power output circuit topologies, the simplest, and arguably the most linear topology is the Class A single-ended triode. Single-ended amplifiers use one tube (or parallel tubes) to handle the entire 360 degrees of the audio waveform. Because there is no signal waveform splitting between alternating tubes, they completely avoid the crossover distortion and switching anomalies that can occur using signal splitters and two tubes in push-pull designs. Push-pull topologies cancel out even-order harmonics (which sound musical) and leave behind harsher odd-order harmonics (3rd, 5th, etc.). Single-ended triode designs let the rich even-order spectrum remain intact. An SE amplifier usually exhibits greater musical detail.
Why To Not Use Pentodes
A single-ended pentode/tetrode will produce more power output than a single-ended triode. However, triodes offer significantly better plate load matching characteristics than pentodes or tetrodes. Unlike triodes, pentodes inherently generate higher levels of 3rd and odd-order harmonics. These harmonics sound harsher and more fatigued to the human ear. Triodes have a much lower internal plate resistance. This low internal resistance yields vastly superior damping, more predictable load-line behavior, and far greater tolerance to the shifting impedance of real-world speaker loads. Pentodes naturally have a very high internal plate resistance. This results in a low damping factor, meaning the amplifier has poor control over the speaker cone's movement.
Why To Avoid Push-Pull Topologies
Beside the issues with allowing odd order harmonic distortions to pass while canceling even order harmonics, there is the problem of output transformer hysteresis. Push-pull amplifier output transformers use two alternating halves to push and pull current through a load. The current flows through a shared magnetic core inside the output transformer. Hysteresis is a property of an output transformer ferromagnetic core. When current forces a magnetic field in one direction, the core retains a tiny bit of residual magnetism even after the current stops. When the amplifier switches cycles from "push" to "pull", the new cycle cannot immediately alter the output smoothly. It must first supply enough energy to overcome and reverse the residual magnetic field left behind by the previous cycle.
For very small input signals near zero, the energy is sometimes insufficient to cleanly overcome the magnetic memory of the core, leading to a threshold effect where tiny signals are distorted or lost. Hysteresis introduces distortion such as odd-order harmonics which sound harsh or "edgy", and intermodulation distortion (IMD), Magnetic flux lags behind the current. This causes a phase shift to be introduced between the amplifier's input and output. This degrades the amplifier’s transient response. The amplifier is less able to handle sudden sounds like a drum hit and rolls off the high-frequency extension. While push-pull perfectly cancels out DC current through the output transformer primary, if the push-pull circuit (including the output tubes) is not perfectly balanced, a DC imbalance will occur, reintroducing saturation issues alongside hysteresis. To minimize push-pull hysteresis effects, output transformers with higher-quality cores must be used. By comparison, Class A single-ended output transformers are by nature biased for linear operation by continuous plate current that eliminates hysteresis issues.
Increasing Class A Power Output
It is possible to achieve higher Class A power output levels similar to push-pull by pairing two identical output tubes for about 2.89 times more power. For instance, a 2A3 specified as 3.5 watts as a Class A single-ended output, wired in parallel with another 2A3, would then provide about 9.3 watts output. The plate load resistance would be 1.25K ohms; half the value as specified for a single tube. When two vacuum tube triodes are wired in parallel, the plate load resistance is cut in half to maintain the same optimal operating conditions and match the halved internal plate resistance. The cathode and grid resistors may also need to be halved to keep the same bias point and proper grid-leak limits.
Low Frequency Scuttlebutt
Concerns over bass frequency distortion are a bit misleading. With a rare exception, in music recordings
there is little or no energy below 50 Hz. This is
based on many hours of observing music on lower frequency waveforms. Also, as long as they are sinusoidal
in nature, bass frequencies can tolerate some distortion before becoming
objectionable (2).
Over-specification of output transformer power ratings will help prevent transformer core saturation of
bass frequencies. What's important is that frequencies above 200 Hz are essentially true to the
original input signal and an amplifier with good transient response.
(2) This does not include video productions where bass
is exaggerated for effect.
Speaker Distortion
Speaker distortion is a phenomenon where speaker distortion increases as power to the speaker is increased. With
the advent of AI, you can search for detailed information on what causes speaker distortion and the amount of
distortion at low volume compared to high volume distortion. Besides speaker distortion, also search for dangers
of home audio high volume SPL levels of 90 dB and higher.
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