Sunday, November 20, 2005

Leo’s Phat Sound

Leo Fender – famous for his eponymous guitars and basses – had a background in radio. To complement his ‘radical’ electronic instruments, he built portable amplifier-speaker consoles like the Fender Bassman. He didn’t design his amplifiers in the 1950s with any thought of the controlled and sustained overdrive, nor any of the other vacuum tube amp qualities that bass players have come to crave. In fact his first amp designs were straight out of the RCA vacuum tube handbook!

With the help of field tests with musicians like guitar-pick-melting American surf guitar guru Dick Dale, Leo Fender created a physically and electronically reliable design to withstand use and abuse by touring performers. It just happens that the combination of his robust design, and the natural overdrive characteristics of tubes produces the trademark Fender sound we came to love.

Leo set the standard, and as electronics have evolved into more robust, compact and powerful transistor platforms, the holy grail of bass amplifiers has been to design a solid state amplifier that sounds like a tube amplifier. Unfortunately, early solid-state amplifiers were best noted for their harshness and limited dynamic range, caused almost entirely by the super clean sound and wide frequency response up to their maximum output, after which the onset of clipping occurred quite abruptly and harshly.

When an instrument amplifier is played near its maximum output, the attack of notes (that bit immediately after the note is struck) pushes the amplifier briefly into the clipping region, producing a "squashy, spitting" sound at the start of each note. If the amplifier is overdriven well into the clipping region, the type of overdrive was typically very dirty and unmusical.

Russell O. Hamm, in his seminal study “Tubes Versus Transistors
Is There An Audible Difference
?” found that Bass and guitar Amplifiers are often severely overloaded by signal transients (THD 30%). As noted, this occurs on the attack, so becomes a problem with nearly every note played on the instrument. Under this condition there is a major difference in the harmonic distortion components of the amplified signal, with tubes, transistors, and operational amplifiers separating into distinct groups.

This sound modification and creation process doesn't end at the speaker, either. It is a loop, where sound is fed back to the source. Performances are often played and recorded at medium to loud volume levels, where the acoustic energy from the speakers drives the already vibrating strings to modify the natural decay characteristics, and drive the string to produce acoustic feedback, a feature that guitarists and bassists learn to control with techniques.

The transistor characteristics which guitarists and bassists dislike are buzzing, white-noise sound and the lack of "punch." The buzz is directly related to the 'edge' (like the edge of a square wave) produced by overloading on transients (like the high amplitude attacks on individual notes). This white noise contains edge harmonics like the seventh and ninth that are not musically related to the fundamental. This is where we get the complaint that transistor amps emphasize harsh 'odd' overtones, whereas tube amps increase musical 'even' overtones. The ear hears these dissonant tones as a kind of noise accompanying every attack. The lack of punch is due to the strong third harmonic which is inaudibly "blanketing" the sound. This is correctable (theoretically) by using a large enough pad to prevent all peaks from reaching the amplifier's saturated region. Adding auxiliary peak indicators on transistor input preamplifiers can alleviate both these problems and the sound would be very close to that of the operational amplifier in its linear region.

Vacuum-tube amplifiers differ from transistor and operational amplifiers because they can be operated in the overload region without adding objectionable distortion. The combination of the slow rising edge and the open harmonic structure of the overload characteristics form an almost ideal sound recording compressor. Within the 15 to 20 dB "safe" overload range, the electrical output of the tube amplifier increases by only 2 to 4 dB, acting like a limiter (remember that doubling the power or loudness is equivalent to only a 3 dB increase). However, since the edge is increasing within this range, the subjective loudness remains uncompressed to the ear. This effect causes tube-amplified signals to have a high apparent level which is not indicated on a VU meter.

Tubes also sound louder and have a better signal-to-noise ratio because of this extra subjective head room that transistor amplifiers do not have. Tubes get punch from their naturally brassy overload characteristics. Since the loud signals can be recorded at higher levels, the softer signals are also louder, so they are not lost in tape hiss and they effectively give the tube sound greater clarity. The feeling of more bass response is directly related to the strong second and third harmonic components which reinforce the "natural" bass with "synthetic" bass. In the context of a limited dynamic range system like the phonograph, recordings made with vacuum-tube preamplifiers will have more apparent level and a greater signal to system noise ratio than recordings made with transistors or operational amplifiers.

So why is this important? The sound of a tube power amplifier moving into and out of its clipping region is the sound that many bassists and guitarists crave. Even though it can be emulated at lower volume levels with overdrive preamps, digital modeling and even power attenuators, most musicians say it is not as good as "the real thing".

Additionally, musicians perceive vacuum tube based instrument amplifiers to be noticeably louder than the same power solid state amplifier. This occurs because of the logarithmic response of our hearing. Doubling power increases perceived loudness by 3dB. Most bass playing is percussive in nature – there is a strong but brief burst of volume as you strike the string, then the note gradually decays to a lower level.

Tube amplifiers exhibit a smooth transition into and out of overdrive, so for example, a 100 watt amp sounds only a little more distorted when trying to deliver 110 watts. This means that you can drive the amp quite hard with a "musically pleasant" amount of overdrive, that sustains a volume level at the amplifier's maximum power capability. As the note decays, the volume level changes little while the sound gradually cleans up (sometimes referred to as a "warm bloom").

On the other hand, solid state amplifiers are very clean right up to their maximum power output, then virtually switch to very high distortion levels above this point. This is hopeless when trying to play percussive sounds around the maximum power output, because the start of the note (the attack) is very distorted and unmusical, then it suddenly becomes clean as the volume dies (perceived as a lack of sustain). The only way to get a reasonable tone is to turn down and try to operate the amp always below its maximum power capability. Thus, the perceived loudness of a tube amp versus a solid-state amp. There are several things that happen at this point:

  1. A smooth transition from clean into clipping. This allows players to dig in for more aggressive sounds, or lay back for cleaner tones.
  2. Mild compression, as notes start clipping, and decay into the clean tone with very little volume reduction. These dynamics give an amplifier a 'feel' where the guitar and amplifier inter-react in what many players describe as an 'organic' way.
  3. Good tube tone. Whether you use a Fender, Marshall, Boogie, Vox or whatever, they each produce their own distinctive tube tone. The variations are due mainly to the output stage design (how "hot" the tubes are biased), the tubes used (6L6, EL34, EL84, 5881 are common), whether negative feedback is used, and different speaker types and cabinet designs.

Thus emulation of vacuum tube characteristics in more portable transistor circuits as been a preoccupation of electronic engineers. Early attempts were made with various stomp-box effects. These were all overdriven solid state pre-amps.

  • First we had the fuzz-tone (overdriven germanium transistors in the Hendrix era)
  • Then smoother and more subtle overdrive (soft-clipping designs) such as the Boss OD-1 and Ibanez Tube Screamers
  • And finally distortion with more sophisticated tone control to allow metal and grunge sounds.

All of these pedals are still available today. Probably the most tube-like are the overdrive pedals, although these only produce tube-like tones, not the dynamics.

Today we have even more sophisticated emulations. The Tech 21 SansAmp uses analog circuits to produce some great tube-like tones, and claims to include miked speaker emulations to further impart distinctive sounds. Since the Sansamp electronics are analog (here's the SansAmp circuit if you are interested) I would guess that the miked speaker claim is a bit specious or fanciful. I own a SanSamp Bass Driver, and have to say that it comes pretty close to tube sound at moderate input – but it still lacks something of the warm tube dynamic feel. Line 6 manufacturers their Bass Pod (which I also own; call me a gear junky) which digitally models amps with different tube emulations. In my opinion, the SansAmp has significantly better emulation.

How ironic that in an era of blistering computational power in cheap boxes like the Xbox, we are still trying to capture the sonic character of Leo Fender's classic early amps. Leo’s designs gave a characteristic full and punchy sound, suitable for many styles of the day, and later. At really high overdrive the sound becomes dirty, with bass in particular sounding flabby (which has its own enthusiasts as well). Steel and country players like the chime-like clean sounds, and blues players were quick to discover the classic way it breaks up when pushed hard. All hail Leo!

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