How to Choose the Right Tube Amp

Selecting the right tube amp to power your headphone collection can be a more complicated process than choosing a solid-state amplifier0. This is due to large variations between types of tube amplification and the real-world impacts on how these differences interact with various headphones. 

In short, tube amps are NOT all the same and NOT all tube amps will work properly with all headphones.

Headphone tube amps can be divided into two main categories: full tube amplification and hybrid amplification. Furthermore, each of these two tube amplifier varieties can be subdivided. Full tube amplifiers are separated by their construction, between those with output transformers and those that are output transformer-less (OTL). Hybrid amplifiers are more subtly sorted by their implementation.

But before we dive into the details defining the types of tube amps, any discussion of amplification needs to start with a review of the concept of impedance. This is key to understanding the differences between the amplifiers.

The Importance of Impedance in Amplification 

Impedance is one of the basic specifications of headphones (load impedance) and audio sources (output impedance). Calculating headphone load impedance is very complicated as it is made up of a combination of resistance, capacitance, and inductance, and is defined as a ‘reactive load’.

Impedance is different than sensitivity. Sensitivity (or efficiency) is a measurement of headphone volume at a specific power rating.

Impedance matching is the interaction between the source and the headphone and is key to high-quality headphone audio reproduction. The impedance values of the headphone and source should not be the same, and a general ‘rule of eighths’ has been adopted – that is, the headphone impedance should be at least 8 times greater than the source impedance. When this is ignored, issues such as higher harmonic distortion and noise, a low damping factor, and bass roll-off can occur.

The rule of eighths isn’t set-in-stone, and good audio results can be had with a smaller difference than 8, but the source and load impedance should never be the same, and the load impedance should always be significantly greater than the source. 

Amplifier requirements differ depending on headphone impedance. In general, low impedance headphones (< 50 ohms) require higher current sources, and conversely, high impedance headphones require higher voltage sources. Modern headphones are typically 32 Ohms or less due to the friendlier low voltage amplification requirements for portable devices.

Planar Magnetic headphones tend to break the rules. Unlike dynamic drivers, the impedance response of planar dynamic driver headphones is flat across all frequencies. Planar magnetic and dynamic headphones of similar ratings will often have quite different amplification requirements and usually planar magnetic headphones (although with low impedance and high sensitivity) can require robust amplification and respond better to solid-state amplifiers. 

The operating voltage and current (or bias) chosen for the transistors or tubes in amplifiers are optimized by the designer. They may be designed primarily for low impedance loads, high impedance loads, or a compromise of the two. Solid-state amplifiers typically have very low output impedance levels (with many nearing 0), while tube amplifiers can vary dramatically in output impedance depending on their design.

An amplifier may be incapable of producing sufficient power into a low impedance load if it is current limited, while it is able to make plenty of power into a high impedance load. Different amplifier designs will all have unique specifications and abilities.

Full Tube Amplification

With Output Transformers

A typical tube amplifier uses vacuum tubes running at high voltages to increase the amplitude (power) of an input signal. Tube amps were ubiquitous until the late 1960’s when the advent of solid-state components replaced the vacuum tube in most applications. This was due to the reduction in size, power consumption, distortion, and the cost of solid-state circuits. 

Tube amps were essentially dead. That is, except for audio fanatics. 

As discussed in the impedance section above, the high output impedance of pure tube amplification circuits is not well matched to the low-impedance loads common presented by modern headphones. A lower output impedance is required for efficient power transfer, and this is achieved using output transformers within the circuit.

The primary benefit of a tube amp that includes output transformers is the ability to pair with a wide range of headphones, but this benefit comes with a (literal) cost. Output transformers are expensive, and this is usually reflected in the high price of the amplifier. Also worth noting, is that the transformer also may have an audible impact on the resulting sound of the amp.

Output Transformer-Less (OTL)

Two very common tube headphone amplifiers are the Bottlehead Crack and Darkvoice 336. Both are examples of OTL tube amps and are very similar in design, with a smaller input tube and a large output tube. Despite their inherent limitations, both have many fans. Likely, (at least partially) due to their reasonable price, they are a gateway for many enthusiasts into pure tube amplification.

An OTL amplifier is often noted for having a very ‘tube-like’ warm sound and will usually have a high output impedance (often greater than 100 Ohms). This high output impedance SEVERELY limits appropriate headphone pairings to only a few models that have an impedance of 250 Ohms or greater (most of the available options are from Beyerdynamic, Sennheiser, and ZMF). 

Clearly, the rule of eighths cannot apply with a source impedance of 120 Ohms as headphone impedance typically tops out around 600 Ohms for some Beyerdynamic models.

Commonly lauded is the pairing of the 300 Ohm Sennheiser HD650 with the Bottlehead Crack. Speaking from experience, they pair wonderfully. The HD650 sound significantly different than when powered by a solid-state amp and seem to gain improved dynamics and clarity when paired with the OTL amplifier. This may be due to a higher-voltage requirement at their exaggerated frequency spike around 100Hz. 

If versatility and range of appropriate headphone choice pairings are of primary importance, then OTL amplifiers should be avoided. However, their lower price point and unique sound qualities may make you willing to overlook these shortcomings when considering tube amplifiers. 

In general, do not buy an OTL amplifier for planar magnetic, or low impedance/high sensitivity, dynamic driver headphones.

Hybrid Tube Amplification

Hybrid tube amplifiers include a tube stage to create voltage gain paired with a solid-state output circuit to drive the headphones. This is an extremely versatile design, intended to achieve the low output impedance benefits of a solid-state amp, but with a bit of tube flavor added to the sound. 

The old adage of ‘you get what you pay for’ is so often true. There are very many inexpensive hybrid type amplifiers that use low voltage to electrify (and light up for aesthetic purposes) low-quality vacuum tubes. The effectiveness of this design is marginal and may only serve to add increased and unwanted distortion. In some cases, the tubes are completely separate from the audio circuit and are included for the tube ‘look’ only.

A properly designed hybrid tube amp will typically provide a lower output impedance with more current and power than an OTL amplifier. This usually contributes less even-order harmonic distortion than a purely tube-based amp, resulting in a less ‘tube-like’ sound signature. Whether that is a pro or a con, or even a worthwhile compromise, is entirely based on the preferences of the listener.

Tube Amp Sound and Appeal

Tube amplifiers are often described as having a more convincing sense of space and warm tonality than solid-state amps. If one considers the ultra-low distortion, sterile amplification provided from THX AAA solid-state amplifiers as one end of the audible spectrum, then the all-tube-based amplifier is at the opposite end, with hybrids somewhere in the middle. 

Measurements of tube amplifiers show more distortion than solid-state amps, but enthusiasts will tell you that this is where the magic happens. They claim that what makes a tube amp sound great is not adequately captured by the graphs. Many find the even-order harmonics created by tube amplification to be pleasing to the ear, more so than from ‘cleaner’ amplification sources.

Of course, the pleasing aesthetic aspect of tube amplifier glow cannot be ignored. 

Tube amps allow for further audio tweaking by tube-rolling, that is replacing tubes with equivalent models, or with the same tube made by different manufacturers. This allows for a subtle, undefined, and physical method of EQ’ing the sound to the listener’s personal preference. 

Amplifier Type Benefits and Drawbacks

Conclusion

Unlike the majority of solid-state amplifiers, not all tube amplifiers will work with most headphones. Output power is nearly all you have to worry about in solid-state pairings, so choosing a solid-state amp is a far simpler process all around. This isn’t to say all solid-state amps sound identical, just that they tend to have a far wider range of appropriate headphone pairings.

In fact, OTL tube amplifiers are the opposite - they are very limited in pairing options. Cost is also an important consideration, as tube amplifiers with output transformers tend to be expensive, with OTL and hybrid designs typically a bit less so. But remember, you never get something for nothing, and adding tubes to a super cheap solid-state amp doesn’t make a good hybrid amplifier. 

Do your research, and consider carefully what headphones you want to pair with a tube amplifier. Then make an educated decision on whether you can afford the tube amp you want, ensuring it is the kind that best suits your needs, or if you’d be better served by a solid-state amp instead.

Author Trav Wilson:  Audiophile or Audio-Phool? I don’t claim to have golden ears with magical properties, nor any ability to create music. But I do have a deep appreciation for music, founded at a young age, and curated over the years. I’m also unapologetically a gear-head and love lights, buttons, meters, switches, and especially things made from traditional wood, leather, metal, and glass materials. As with everything, take it for what it is: this is just one person’s opinion. Runs noteworthy.audio