The electron tube in the recording studio

Introduction

Regardless of whether it is spoken factually, sung enthusiastically or performed in a virtuoso manner on instruments, every reproduction of sound events requires a recording or its preservation beforehand. For a long time, it was considered a dream of mankind to capture acoustic perceptions to be able to play them back at any time and any location. It was not until the late 19th century that the foundations were laid by Thomas Edison's phonograph and Alexander Graham Bell's telephone. The first working magnetic recording device called the "Telegraphone" was introduced by Valdemar Poulsen, a Danish inventor, during the 1900 World's Fair in Paris. As a result of Alexander Graham Bell's long-distance strategy, the tube amplifier was invented in the early 20th century for the rapidly growing telephone industry. Not only telephone technology but also radio ensured constant developments and innovations in the field of electroacoustics. With the components microphone, tape machine, mixer, tube amplifier and loudspeaker, the establishment of the analog recording studio (broadcasting) in Germany in the famous VOX building on Potsdamer Platz in Berlin was born as early as 1923. At the beginning of this era, the transistor had not yet been developed and so numerous electron tubes were used in the electro-acoustic equipment of the recording studio technology of the time.

Today's modern recording studio technology is largely dominated by the use of computers and the associated digital audio technology. Within the last 20 to 25 years, the word "digital" has almost enchanted almost all sound engineers and music producers and transported them into a fairytale world of technology belief. According to the motto: "If it doesn't say digital, there's old-fashioned sound in it!" In the period from 1960 to 1990, analog audio technology experienced its heyday with many wonderful recordings by great artists, which from today's perspective still sound incredibly good and often do so with me letting some tears of joy flow from his eyes while listening. The inevitable question arises: "The technology back then couldn't have been that bad, could it?" It's just strange that especially in Europe, tube and analog audio technology based on semiconductors has almost been lost from the memory of sound creators seemed to be. After all, it was German companies such as Telefunken, Neumann, Siemens, Elektro Mess Technik Wilhelm Franz KG (EMT), Eckmiller or Tonographie Apparatebau that, with an urge for perfectionism, got to the heart of circuit technology and sound aesthetics. So, it is not particularly surprising that as early as 1993 Bruce Swedien, legendary sound engineer of the Michael Jackson album "Thriller", stated in an interview that German and Swedish broadcast stations would all have to be crazy by almost thoughtlessly parting with this wonderful technology and simply exchanging it for "modern" equipment. Lucky for Bruce Swedien or Allan Sides (Ocean Way Studios, LA)! The majority of high-quality tube microphones and preamplifiers are now owned by American studios…

But something seems to be changing! The great guitar virtuoso and singer/songwriter Steve Lukather of the legendary band TOTO called one of his solo albums “Everchanging Times”. I met Luke a few years ago at the Frankfurt Musikmesse for a video interview and had the opportunity to talk with him in his straight robe beforehand. He told me that his guitar signal not only runs through Bob Bradshaw's CAE tube preamp but that he also uses an old, analog Neve console and numerous tube compressors (control amplifiers) and tube filters while working in the studio with tube devices. But the esteemed youngsters also seem to be changing their minds. As the long-time director and lecturer at the SAE and Abbey Road Institutes in Frankfurt, I have been able to pass on my experience as a sound engineer to many young people. Especially in the last few years, there have been more and more questions from students about the old tube technology. The motto of learning from history is still firmly anchored in the Abbey Road Institute's curriculum and is linked to the question: "How has music influenced technology and vice versa, how has technology influenced music over the decades?". This also leads to the question: "What can a young music producer incorporate from the knowledge gained from history into his current working methods?". An extremely exciting affair! Because it consists of the opportunity to develop your own "producer handwriting" in a special way. Because that is extremely important to clearly stand out from the sound of the often perceived and quoted “mush” of today's productions. In addition, some plug-ins can only be properly understood with sound knowledge of audio and studio history.

But who is this old, mystical lady in a glass robe? Does she really heat us up again with her donating, warmth-giving appearance? As the saying goes: "Those who thought they were dead live longer!", and that's a good thing! Actually, she never really disappeared from the recording studio and electric guitar world.

Before I tell you in detail about the use of tubes in the studio world, allow me to say something philosophical about recording studio technology and music production. Since my early youth, I have been dealing with audio technology, room acoustics, electrical engineering and the most important thing, music and its production. The interplay of technology and artistic expression is something wonderful. Nevertheless, concerning the end result of a music production, it is important to always be self-critical and to question again and again how far one uses the seemingly unlimited possibilities of modern technologies! Regardless of whether tubes, discrete transistor technology, operational amplifiers or 24-bit 192 kHz AD/DA converters are used, the decisive factor for the success of a production is the artistic energy of the moment that has to be captured. This circumstance, paired with years of listening experience and professional practice behind the mixing desk, often lets the famous spark fly. I don't see my studio equipment primarily as tools, but rather as musical instruments. I “play” on a mixing console like on my guitar. I see myself in such an environment as a conductor in front of a virtual orchestra. The levels of individual tracks naturally correspond to a certain almost "static" standard in terms of leveling, but mixing is done dynamically by moving the faders. The situation is different, however, when recording purely acoustic performances. It is important that what was heard in the recording room (primary room) is reproduced as authentically as possible in the control room (secondary room). The product should then sound as balanced as possible on all listening media at the end consumer. An extremely difficult process is to make an optimal microphone selection and set it up in the right place using the appropriate method (e.g., main microphone method using AB, MS, XY, ORTF, OSS, etc.). Music, its interpretation and the space of performance do not always fit together! The selection of the equipment to be used (microphones, microphone preamplifiers, etc.) and the recording process itself must also be made carefully. There is no question that the decision-maker has an immense responsibility about the end product. Last but not least, in addition to the musical performance of the artists, the characteristic handwriting of the sound engineer or the producer is created. But that's exactly what brings the tonal color, characterized by subjectivity and the energy of the moment mentioned above. But the dream of perfectly preserved music unfortunately remains, or thank God, an illusion! May the illusion nonetheless bring joy …

But why does the good old tube still exist in the recording studio? Why are so many sound engineers and music producers still using tube gear during the recording, mixdown and even the mastering process? Old equipment from the 50s and 60s coexists with many recent developments. These include microphones, mic preamps, DI boxes, compressors, and equalizers. Such studio tools equipped with tubes have a typical, colored sound of their own. It has to be said that they alienate the original in a certain way. But it is precisely this property that many sound engineers and music producers appreciate. Studio equipment is thus given the status of an artistic means. Believe me, numerous pop/rock recordings in your favorite playlists have been artistically twisted with several sound tools on the individual tracks of a multitrack production. A successful recording always consists of the composition, the arrangement, the quality of the recording musicians, the interpretation, the creativity of the sound engineers and music producers during the recording, the mixing and the mastering. Last but not least, the energy of the moment dominates. The chain described is therefore a total work of art!

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Tools of the sound engineers‍

Dive into the underworld of sound forges with me, where the alleged witchcraft originated. Discover the ingenious tools of audio technology, equipped with tubes. I will introduce you to exciting device concepts from the old days and today.
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‍Microphones‍

Microphones are often described as the extended ears of sound engineers. It is exciting that each microphone has its own sound. It is all the more interesting to be able to choose from several high-quality microphones during a production. Several excellent microphones are available at Studios 301 in Frankfurt. These include well-known manufacturers such as Neumann, AKG, Sennheiser, Beyerdynamic, Shure, Royer, Calrec, Brauner, Sony, Telefunken, Rode, Austrian Audio, Lewitt, Aston and Electro Voice.

A basic distinction is made between the recipient and transducer principles when it comes to microphones. The former includes the pressure transducers (omnidirectional) and the pressure gradient transducers. When it comes to the transducer principles, a distinction is made between voice coil, ribbon, low and high-frequency condenser, electret condenser and piezoelectric microphones. Another important criterion when considering microphones is their directional characteristics. In practical studio use, we use those with omnidirectional, wide cardioid, cardioid, supercardioid, hypercardioid, figure-8 and lobe pattern. Condenser microphones have different types of diaphragms. The two common forms are large and small diaphragm. But where is our tube microphone in this game? In principle, capsules for condenser microphones have a high internal impedance (Z = 200 MΩ to 1 GΩ with a capacitive component of 20 pF to 100 pF) which must be matched to a line impedance of approx. 50 Ω. A tube can be used for this, directly behind the capsule with a downstream transformer. To keep the ohmic load on the capacitor capsule as low as possible, a cathode amplifier using a triode is often used. There is also the option of equipping the line or symmetry driver with a triode as a voltage amplifier instead of the transformer.

Due to the different voltages required, tube microphones are fed with external power supplies, which are constructed much more complex than conventional phantom power supplies.

In terms of sound, tube microphones are very soft and finely detailed. Voices can be reproduced very powerfully and with good throughput in the mix. The microphone method used is also decisive here. My favorites for vocal recordings are the Neumann M149, the Brauner VM1 and the unfortunately no longer available Sony C-800G, which was equipped with a steep HF/LF pentode EF94 (6AU6A). All microphones listed are representatives with large-diaphragm capsules. With the introduction of television, the TV stations demanded smaller microphones which, due to their size, should no longer attract the attention of viewers. That was the birth of the condenser small-diaphragm microphones with ½” capsules. Typical representatives from 1953 are the two Neumann microphones KM53 (omnidirectional pattern) and KM54 (cardioid pattern). The latter can be seen in action in old Beat Club recordings by Radio Bremen (German television and radio station). The special thing about these microphones was the tube used. This was the sub-miniature tube AC701 specially developed by Telefunken with a diameter of only 10.1 mm. It was characterized by high microphonic resistance, low NF noise and low distortion.‍

To this day, I still enjoy using the KM54 to record acoustic guitars using the 12-12 method. The microphone is aligned with the 12th fret of the guitar at a distance of approx. 12”; a special kind of sound, with a liveliness and freshness that I can't create so convincingly with other microphones.
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Microphone Pre-Amplifier‍

Every microphone needs a microphone preamp to bring the relatively low output level to the necessary line level. A gain of up to 70 dBr may be necessary here. Typical values are between 60 and 70 dBr. Both tube and semiconductor preamps are available on the market. Modern tube representatives are characterized by a warm sound. For vocal recordings in certain genres of music, you can definitely distort the tube a bit in order to tease out the typical sound characteristics. In today's everyday studio life, tube mic preamps are used e.g. from the German company Sound Performance Lab (spL), ART, DBX, Universal Audio, Gyraf, Tube Tech, Pendulum or TL Audio. But nostalgic elements are also used. The V76 microphone preamplifier developed by the “Institut für Rundfunktechnik (IRT)” is legendary and has excellent sound characteristics. This was manufactured by the company “Tonographie Apparatebau (TAB)” in Wuppertal and distributed by Telefunken. The four-stage amplifier was built with the EF 804 S and E 83 F pentodes. The entire technology, including the power supply, was housed in a metal cassette, the so-called “Dannerkassette”, in a rack. The circuit design of the amplifier was equipped with complex negative feedback. You may have listened to the vinyl pressing of Bert Kaempfert's original master tapes. These incredibly good-sounding recordings were made back in 1962/63, and it can be assumed that the V76 amplifiers were used.
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DI-Boxes

The abbreviation stands for Direct Injection. Such devices are used to connect unbalanced instruments (e.g., keyboards, electric guitars or electric basses) with a low output level or with line level to balanced microphone inputs in the studio or on stage. The advantage is that the signal can be transmitted over long distances (up to 300 m) largely independently of external interference. In the event of in-phase interference from interference signals, these are largely eliminated at the input of the microphone preamplifier by forming the difference. The actual symmetry driver of the DI box is constructed either with active electronics or passively using a transformer. The German manufacturer and tube specialist Palmer once brought an interesting tube DI box onto the market. At that time, I had a prototype from the chief developer Martin Schmitz in the studio and was enthusiastic about the extremely powerful sound when recording electric basses. This tube DI box is still one of my most valued tools in the studio today. It's just a pity that the responsible sales department didn't manage to place this excellent product on the market.

Mixing Consoles‍

If you look at pictures from well-known studios, you will still find large mixing consoles with up to 96 channel strips, on which three or four people can easily work next to each other, despite all the computer technology. Despite the considerable size, the inner workings of such analogue consoles look cramped but still tidy. Resistors, capacitors, switches, buttons, LEDs, potentiometers, semiconductors in the form of operational amplifiers, transistors and other ICs as far as the eye can see. Surely tubes have no place here, right? Not even close! The German manufacturer Solid Tube Audio has managed to build a complete inline console with tubes. It is called EVEREST C3 and consists of a total of 3120 (!) tube stages in 48 channel strips. Incidentally, the legendary TAB 76/80, as described above, are used as microphone preamps.

Another representative in the field of mixing consoles equipped with tubes is the VTC from the English company TL Audio. I had the opportunity to record drums with the console at Studios 301 in Byron Bay, Australia and was blown away by the unforgiving sound!
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Compressors

These devices are control amplifiers used for dynamics processing. As a result of level-dependent amplification, compressors change the program dynamics. Depending on various setting parameters, quiet passages are boosted and loud passages are reduced in level. Often frowned upon in high-end circles, and yet compressors are indispensable in certain stages of production! Here are some application examples for compressors:

• Fat and powerful sound with sum compression
• Accentuation of individual instruments in individual processing
• Adding warmth
• Compensation for dynamic attack errors in drums
• Loudness gain
• Reconstruction of lost dynamics
• Place instruments in the mix in a targeted manner
• Make programmed music livelier
• Effect compression (e.g., with guitars in jazz; Philip Catherine or Larry Carlton)
• De-essing of voices
• Automatic hiding of signals depending on another audio signal (ducking)
• Transient emphasis
• Apparent lengthening of decay processes in acoustic instruments

In the US, compressors have recently been affectionately dubbed the true rock stars of signal processing. The triumphal procession began as early as 1959 with the tube compressor model 660/670 from the American company Fairchild. Today, such a cutie can sell for as much as $25,000! One could say that this device revolutionized music production technology at the time. In English studios, the Fairchild compressors were stacked and euphorically applied to almost all instruments. George Martin and Geoff Emerick, for example, made extensive use of compressors in productions with the Beatles. Technically, the devices worked with a control triode according to the variable-mu principle. The 6386 from General Electric was used. It was an s.g. "Five Star Military Tube". At that time, such designs were significantly faster in the control times than solutions with opto elements. However, the variable-mu design does not reach the speeds of integrated VCAs (Voltage Controlled Amplifiers) or FET solutions. In summary, one can say that every compressor sounds different and therefore has its justification. Tube compressors are now available on the market with different circuit concepts. Well-known representatives are examples from the companies spL, TL Audio, Tube Tech, Summit Audio or Manley.
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Equalizer

Sound processing with equalizers in studios is as old as pop music itself. Filters based on the most diverse principles can be found in all recording studios around the world. Be it parametric filters based on the constant-Q or non-constant-Q process, pure coil filters or those using tube technology, they all have their own individual sound and thus determine the area of application. Tube EQs have a reputation for being very musical and silky. Take the bass drum of a metal production, for example. Here I mix i. i.e. R. three microphones in different positions, with targeted equalization in mono. A pressure receiver signal is processed at approx. 4.5 kHz with a peak filter, mixed with the basic sound and thus produces the typical kick sound. All other frequencies around the peak are reduced to the maximum with high and low passes. I achieve warmth and pressure by filtering a dynamic batter microphone at frequencies around 80 Hz (boost) and 400 Hz (cut), setting a medium Q value. Another microphone on the resonance head provides additional volume. Frequencies above 150 Hz are rigorously filtered out to largely suppress any room reflections.
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Plate Reverb

Effect devices are as much a part of music production as the salt in the soup. Tube use is rather less represented here. But there is one device that I still enjoy using to this day. This is the legendary 140 ST reverb plate from EMT. From 1957 this reverberation was the successor to the reverberation room, was considered unrivaled for years and can be heard impressively on numerous recordings from the 50s and 60s. Even today, this record sound is valued in recording studios worldwide. However, very few own the 190 kg (!) heavy device. To still have the legendary sound available, algorithms that emulate the "plate sound" have existed since the beginning of digital reverb devices. The Lexicon 224 X, which is now 40 years old and has produced countless hits, did this most impressively. There are still three old EMT 140 ST records in the Californian Record Plant Studios. This recently encouraged the company Universal Audio to record the corresponding transfer functions of the discs and to provide a plug-in for the common hard disc recording systems using a mathematical process (convolution).

The EMT 140 ST reverberation plate uses the vibration properties of metals in a targeted manner. A 0.5 mm thick deep-drawn steel plate measuring 1 x 2 m is spring-loaded into a tubular steel frame. The original signal to be reverberated is amplified so that the plate is excited to flexural vibrations using a special loudspeaker. Two piezoelectric accelerometer microphones positioned at different points pick up the reverberated signal. The signals obtained in this way are then added to the original in stereo. Similar to spatial reverberation, the reverb plate provides reflections that linearly get closer and closer together over time. In real space, on the other hand, there is a compression of the reflections with the square of time. However, our hearing does not perceive any difference between the two-time courses. If you want to change the reverberation time, you can do this by changing the distance between an insulating panel of the same size that is mounted in parallel. In this way, reverberation times of 1 - 4 seconds can be set.

The exciter amplifier and the two pick-up amplifiers are of course built-in tube technology and, as a special development, bear the radio station designation V54. The signals from the accelerometer microphones are brought to a line level of 6 dBu in four-stage RC amplifiers, each with 2 x EF 804 S and 1 x E 80 CC tubes. As usual in the good old days, all inputs and outputs are designed with floating transformer balancing. What is special about the amplifiers is that the developers managed to keep the k3 distortion products, which were rather unpleasant for our ears, extremely low at 0.6% given the conditions of the time.

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A little bit of theory must be

Before I get into the nitty-gritty of tube sound, let me say at this point that people's perception of sound is a purely subjective matter. Among other things, the listening experience, the listening conditions in general, the mental state and the hearing-physiological conditions contribute to this perception. Right or wrong, good or bad is therefore difficult to define. Only statistical evaluations of scientifically supported hearing tests allow objective conclusions. All in all, everything is a matter of taste and whatever you like is allowed!

About the sound properties, there is a striking difference between tube and semiconductor technology in the type of distortion that occurs depending on the level control. For example, if the modulation with a sinusoidal audio frequency signal exceeds the symmetrical operating voltage limits of a circuit with bipolar transistors, the signal is cut off at the top and bottom, parallel to the time axis; this process is called clipping. If you now look at the deformed signal in the frequency range, you can see odd-numbered spectral components. A pure sine tone viewed in the time domain results in a single spectral line in the frequency domain at the point of the respective fundamental frequency. In the case of clipping, on the other hand, odd-numbered harmonics (partials) with 3f, 5f, 7f ... are added. Let's assume that our semiconductor amplifier is clipped with a sinusoidal tone of 1 kHz, then odd-numbered harmonics arise at 3, 5, 7 ... kHz. These are parts that were not previously included in the original! This can be demonstrated mathematically using the Fourier analysis. The results can then be converted into distortion reduction or THD+N (Total Harmonic Distortion + Noise). If we now replace the measurement signal with the bowed note of a violin, causing the input stage to clip due to excessive modulation, then odd-numbered harmonics are added to the spectrum of the violin sound with its characteristic formants. This primarily means a change in timbre. The auditory impression of such distortions can be described as harsh or shrill. Other reasons for the deformation of signals in the time domain are non-linear amplification characteristics and so-called Crossover distortion (deviation from the zero crossing of the signal) in push-pull circuits. Incidentally, in the recording studio, reaching the clip limit is usually at +22 dBu (according to IRT specifications). That corresponds to a voltage of almost 10 Veff. In the meantime, some manufacturers have even begun to produce devices with a clip limit of up to +28 dBu. The reasons for this are a higher dynamic range and an associated improved signal-to-noise ratio.
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Is everything different with a tube?

Tubes are usually operated with much higher operating voltages and thus allow excellent headroom. The overdrive behavior in the limit range sets in softly and slowly. A sine tone does not clip directly, but when viewed on an oscilloscope, it becomes somewhat more rounded "bulky" at the top and bottom. This is primarily due to the special type of tube characteristic (not constant slope). The question of the type of harmonic spectral components is interesting at this point. As a result of this special deformation of the signal, even-numbered harmonics rather than odd-numbered ones are produced here, i.e., those with 2, 4, ... 2n times the fundamental frequency. This essentially applies to single-ended circuits (class A mode). Depending on the circuit concept and the choice of components, however, tube devices also produce k3 and k5 products!
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A little criticism must be

But beware! As the saying goes: "All that glitters is not gold!" Despite all the nostalgia and enthusiasm for audio technology of the past, one should also think about the weak points of the use of tubes. Not every device equipped with tubes is a good device. However, a studio device that is generally considered to be bad may be just right for a very specific sound in a title! On the playback side, i.e., in the hi-fi area, you can hear many different voices and opinions on the use of tube amplifiers. The same applies here: "Listen to what you like!" People who love technology like to discuss a lot about this or that electrotechnical property of their devices and sometimes completely forget the music. Nevertheless, at this point, I would like to make a few critical comments, especially about tubes in the hi-fi sector. I also feel that tube amps have an "airy" sound to them. The reason for this is the high output resistance and the associated damping factor. As a result, a bass chassis is not dampened to the required extent and thus tends towards uncontrolled decay processes. Rock guitarists who play with tube amps love exactly this effect, which is possibly also supported by non-existent overall negative feedback. A good example of this is the AC30 combo amplifier developed by the English company Vox at the end of the 1950s. This amp really let it rip with muted staccato/single notes, as the built-in Celestion G12 speaker produces a heavy hub.
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The Final Chord

I hope you were able to take a look through the tube with me and have now gained a little insight into how things work in music studios and what equipment is used. It is nice to see that the old lady in the glass robe has never let herself be pushed aside over the years. Young developers in the field of digital audio technology have the opportunity to learn immensely from the old technologies, which were conceived and crafted with great attention to detail! In addition to modern, digital audio technology, we at Studios 301 still swear by tube technology. Both historical and tube devices from today's development and production are used. Of course, the Studios 301 in Frankfurt has numerous devices and microphones that are equipped with good old tubes.

Conclusion: “Courage to use tubes, because it's worth it!”