Tung-Sol.com:: FAQs Why is there a difference in Tube and Transistor sound?

FAQs

Question:
Why is there a difference in Tube and Transistor sound?
Answer:
Engineers and musicians have long debated the question of tube sound versus transistor sound. Conventional methods of frequency response, distortion, and noise measurement have always assumed linear (clean) operation of the test amplifier and have shown that no significant difference exists. In actual operation most amplifiers are often severely overloaded with signal transients. Under this condition there is a major difference in the harmonic distortion of tube and transistor circuits.

There are also significant differences in the construction of tube and transistor amplifiers that contribute to the sound. Tube amplifiers require output transformers to match the high impedance of the power tubes to the low impedance of the loudspeaker. The transformer has a natural high-frequency roll-off that makes the tube amplifier sound warmer. When the amplifier is overdriven the transformer also has a point of core saturation that provides a form of compression. This gives the tube amplifier sustain and a singing quality that sounds very musical. Transistor power amplifiers are either direct or capacitor coupled to the speaker load and do not have this natural compression. Negative feedback circuits used to reduce distortion in tube amplifiers are relatively simple. Negative feedback circuits used to reduce distortion in transistor amplifiers are more complex. The different types of negative feedback circuits in tube and transistor amplifiers react very differently to the harmonic content of the signal when the amplifier is overdriven. Tube amplifiers generally have a smoother, rounded waveform, where the overdrive of a transistor amplifier is more abrupt and resembles a square wave.

The harmonic content of an overdriven tube amplifier consists primarily of 2nd order and 3rd order harmonics with some 4th order harmonics. The harmonic content of an overdriven transistor amplifier is primarily 3rd order with suppressed 2nd order harmonics. 2nd and 3rd order harmonics are the most important from a viewpoint of electronic distortion. Musically the 2nd harmonic is an octave above the fundamental and is almost inaudible, yet it adds body to the sound, making it fuller. The 3rd harmonic is a musical 12th. Instead of making the tone fuller, a strong 3rd harmonic makes the tone softer. The odd harmonics (3rd, 5th, etc.) produce a "stopped" or "covered" sound. The even harmonics (2nd, 4th, etc.) produce a "choral" or "singing" sound. Adding a 5th to a strong 3rd harmonic give the sound a metallic quality that gets annoying in character as the amplitude increases. A strong 2nd with a strong 3rd harmonic tends to open the "covered" effect. Adding the 4th and 5th harmonics to this gives an "open horn" character. The higher harmonics, above the 7th, give the tone "edge" or "bite."

The basic cause of the difference in tube and transistor sound is the weighting of harmonic distortion in the amplifier’s overload region. Transistor amplifiers exhibit a strong component of 3rd harmonic distortion when driven into overload. This harmonic gives a "covered" sound with a restricted quality. A tube amplifier when overdriven generates a whole spectrum of harmonics. Particularly strong are 2nd, 3rd, 4th, and 5th overtones that give the sound a full-bodied "brassy" quality. Combining this reinforcing harmonic content with the compression and high-frequency roll-off of the output transformer in a tube amplifier is why your tube amplifier will "give it up" while your friend’s transistor amplifier will sound restricted and harsh.

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