What is a tube amplifier? What is the difference between OTL and Hybrid?

2023-07-03 16:36:00

It may seem strange to many today, but a tube amp can be used for different purposes. The reason why most people are not aware of this technology is the change in the standard architecture used in amplifiers, where thermionic valves, also known as “vacuum tube” or “vacuum tube”, were replaced by transistors, which are used in transistor amplifiers, also called solid state amplifiers (SS or Solid State). Previously, valves were used not only in audio, but also in televisions, radios and even computers, but due to changes in the technologies used, today there are few factories in operation, leaving NOS (New Old Stock) valves as an option. produced regarding 40 years ago or more that have never been used.

What is a tube amplifier?

Understanding what a valve amplifier is, how it works and what types. Source: Vitor Valeri

The tube amplifier is one that uses thermionic valves, also known as “vacuum tubes”, to amplify the power or amplitude of an electrical signal. The main feature of these amplifiers are the “lamps”, which can be attached at the top of the structure, as in headphone and speaker amplifiers, or internally, as in the case of guitar amplifiers, where the objective is to avoid the “vacuum tubes” from breaking, as they are made of very thin glass.

Used to amplify the power of an electrical signal, tube amplifiers serve, in the case of a digital audio system, to amplify the analog signal converted by the DAC (Digital to Analog Converter or Digital to Analog Converter). In case you didn’t know, the DAC is present in any device that plays digital audio files (example: music) and has speakers (drivers), because to produce sound it is necessary to convert the digital signal into analogue. However, for the driver diaphragm to vibrate and generate sound, an amplifier must be used, as the electrical signal emitted by the DAC, following conversion, is weak.

When using an electric guitar, which has pickups to transmit sound information through electrical impulses, it is necessary to amplify the signal so that the sound is produced by the loudspeaker of the sound box, since we are already dealing with analog signals, not being necessary the use of a DAC. To accomplish this task, you can use either a tube or a transistor amplifier, but if you’ve tried both, you should know that there are sonic differences between them.

Where are tube amps used?

Tube amplifiers are used today for applications involving audio reproduction. You can currently find them in sound systems for headphones, speakers and musical instruments such as guitars. However, a few decades ago they were also used in radios and radars both in homes and in military installations, where valves were identified as JAN (Joint Army Navy), which were produced with a focus on the use of the army and navy.

Valve radio from 1930. Source: uv201

tube amps for headphones

Due to the increased awareness of headphone amplification, this article will focus more on this area from here on out. Check below regarding the types of tube amplifiers (amps) for headphones.

Types of tube amps

There are two types of tube amplifiers: hybrid amplifiers and “pure” (full) tube amplifiers. These amplifiers can be designed to handle low impedance headphones, high impedance headphones, or both. In addition, depending on the design, the output impedance of the amplifier can vary greatly from model to model while solid state amplifiers (solid state or SS) generally have an output impedance close to 0 Ohm.

It is always desirable that the output impedance of the amplifier be lower than the headphone impedance. The interaction between these impedances is the key to good sound performance. In general, low impedance headphones (less than 50 Ohms) require higher currents while high impedance headphones need a higher voltage, measured in volts (V).

As amplifiers for portable devices cannot produce a high voltage, headphones should ideally have a low impedance. However, in the case of planar magnetic headphones it is different, because even with low impedance and high sensitivity, a more robust amplification is required.

As you will see below, different amplifier designs will suit headphones with different characteristics. Sometimes it may happen that they are not able to deliver enough power to low impedance headphones, but in high impedance there is already a great power production.

“Pure” tube amplifiers (full)

“Pure” valve amplifiers (full) use thermionic valves, also known as “vacuum tubes”, to increase the amplitude (power) of an electrical (analog) signal, raising the electrical voltage. As voltage alone is not capable of moving the loudspeaker (driver), output transformers (one for each channel) are used to convert the output voltage of the tubes into current at the headphone output. A solid state amplifier (solid state or SS) usually has a single power transformer and no output transformer, while tube amplifiers need a high voltage for the tubes and as they have a high output impedance, they require a transformer.

OTL (Output Transformer-Less) tube amplifiers

OTL tube amplifiers are a “pure” (full) type of amplifier. They are identified by having at least one large and one small valve. The smaller tubes are usually positioned at the front of the amplifier and are responsible for pre-amplifying the signal, raising it to line level. The larger valves, located at the rear of the amplifier, perform signal amplification, raising amplitude (power) to a sufficient level to make the diaphragm of the speaker (driver) phone vibrate and generate sound.

Pair of preamp tubes and power tubes from the OTL Woo Audio WA3 tube amplifier. Source: Vitor Valeri

Generally, an OTL tube amplifier has a high output impedance, most of the time being greater than 100 Ohms. Due to this characteristic, it is recommended to use headphones that have high impedance (250 Ohms or more) such as Sennheiser, Beyerdynamic, AKG and ZMF headphones. However, there are OTL amplifiers that are capable of handling headphones with an impedance of 32 Ohms, such as the Woo Audio WA3.

OTL Woo Audio WA3 tube amplifier and Sennheiser HD600 open-back over-ear headphones. Source: Vitor Valeri

It is interesting to mention that high impedance dynamic driver (speaker) headphones often benefit from OTL tube amplifiers due to the characteristic of impedance variation according to the frequency response. For the reproduction of sounds in the region of low frequencies (bass), for example, the impedance is usually high for dynamic headphones. However, in planar magnetic headphones, this does not occur, as there is no impedance variation in the entire sound spectrum (bass, midrange and treble).

Hybrid tube amplifiers

Hybrid valve amplifiers use a transistorized amplification circuit in the output amplification, that is, here the transistors are responsible for raising the signal power to the ideal level for the reproduction of sounds in the headphones. The tubes are only used to pre-amplify the electrical (analog) signal at the input.

xDuoo MT-602 hybrid tube amplifier. Fonte: Dobrescu George (AudiophileHeaven)

What makes hybrid amplifiers a “different animal” compared to OTLs is the difference in influence between the tubes used in the preamp section and the tubes used in the actual amplification (power tubes). The characteristic sound of tube amps is often shown by power tubes, which add a more natural tonality. However, this does not mean that it is not valid to change the stock valves (which come with the device) for better ones, as we explain in our article on how to choose valves.

The advantage of the hybrid tube amplifier over the OTL is its lower output impedance. This feature makes it possible, for example, to use in-ear headphones and magnetic planar headphones, which generally have low impedance and high sensitivity.

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