Tube Compressor

Understanding Tubes in Audio Gear

First, one should understand why tubes exist in audio gear. On the most basic level, tubes amplify electricity. Before transistors were introduced into the market in the 50s, electronic amplifiers used vacuum tubes. In audio, vacuum tubes lend a characteristic sound as an audio amplifier. So, they often smooth out harsh-sounding audio.  

Tube Compressors

Tube-based ( or valve-based) compressors were the only choice of machine-modifying gain in the early days of audio. The principle on which they were working was that the level detector circuit sends the voltage to the tube. This voltage directly drives the gain of the tube. Fairchild and Altec compressors were the early devices built on this principle. Today, Manley makes a state-of-the-art update on this approach. Vari-mu (short for Variable Mu, also a Manley trademark) designs use a vacuum tube instead of a transistor, where the variable voltage input changes a tube's bias instead of a transistor's gain. It has a somewhat different action, generally claimed to be even smoother and more "creamy" or "organic."  Other known vari-mu-style examples are the Markbass Compressore and the Lightning Boy Opti-Mu Prime. The sometimes musical and appealing distortion introduced by the tube-based gain state is a part of the tube compressor sound. In addition, the tube has a reaction time as its gain changes by the level detector circuit. Any increase or decrease in the gain is not instantaneous. This gives tube-based compressors their unique attack and release time properties. The rate at which the gain increases or decreases significantly influences the overall quality of the compression effect.  

Application in Various Devices

Besides compressors, tubes can be found in various devices, such as microphones, guitar amps, equalizers, etc. When discussing hardware units, it is essential to understand that tubes need some time to warm up. Guitarists are primarily aware of this since this kind of amps is common. As the tubes warm up, they increase the temperature. They need a certain amount of time to warm up to become stable. Before they completely warm up, tube-based audio processing devices can give you fluctuating outputs. Therefore, this can mess up your sound. In order to be sure of the full potential of tube-based gear, one should power it at the beginning of the session and give it 10 to 15 minutes to warm up. On the other hand, it is also desirable to let tubes cool down and then move them if you intend to. This is because the tubes are much more fragile when hot.  

Common Usage

Tube or valve compressors are mainly used after recording in order to add warmth to drums, vocals, and basses. This effect is caused by a small amount of second-order harmonic distortion that occurs in the final gain circuitry of the compressor. The distortion is the result of the random movement of the electrons. In the cases of valves, this occurs at exactly twice the frequency of the amplified signal. Although this distortion only contributes 0.2% of the amplified signal, the human ear (subjectively) finds it appealing.