November 2016
Is there any "technical" difference between tube amp distortion and solid-state amp distortion? I have heard tube amps described as “warm” sounding but I can’t find any info as to why. Isn’t “clipping” just “clipping” no matter the device that is performing that function?
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The soft or “warm” sound of tubes relates to four properties.
Transistor amplifiers have much higher slew rates, higher frequency response, and hard clipping when the large signals cause the output transistors to hit the rail voltage of the power supply.
Solid state amplifiers are directly coupled to the speaker load, have a very high damping factor due to the negative feedback, and therefore produce much more accurate output than a tube type, amp. This is the reason for the harsher sound which is really more accurate than the output from a tube amplifier.
If you compare the FFT (Fast Fourier Transform) frequency domain traces of the two types of amplifiers, the differences will become readily apparent. A solid state amp will have many more harmonic components than a tube amp. The direct coupling of the solid state amp to the speaker, also eliminates the hysteresis from the transformer core.
The lowest distortion figures will always be obtained from a solid state amp, in the less than .1% range. Tube amps, conversely are in the 1% to 5% range depending on their design.
Your looking in the wrong place: Maybe not definitive, but look here: https://en.wikipedia.org/wiki/Tube_sound. Even harmonics are more pleasant to the ear.
Yes, there are technical differences. Both tubes and transistors are nonlinear devices, and the transfer curve for each is unique. The transfer curve defines how the output should respond to the input. Within a narrowly defined range of input values, the output values change in “mostly” linear fashion — in math terms we would say the function is monotonic. When your input values start to go beyond the linear operating region the output is no longer a simple (linear or monotonic) function of the input.
Every device, triode, pentode, JFET, MOSFET, BJT has a unique transfer curve. Imagine that the transfer curve for a triode is not a straight line, but more of a “lazy-S” shape — the middle section is pretty close to straight, but the top and bottom of the curve rolls over. As mentioned above this curve “maps” your input signal to the output signal; any given value of input is a point on the curve that defines the output. But when the signal is near the very top of the curve, the output signal change for a given change of input is diminished (like a demon turning down your volume knob). This results in a soft clipping effect if the top of the transfer curve is relatively smooth. Gentle excursions into nonlinear behavior in a triode tends to produce a nice mix of even and odd harmonics; and, if I recall correctly even harmonics lend warmth to the sound.
In the case of a BJT (bipolar junction transistor), the “lazy-S” curve looks more like a “Z” drawn backwards, where the extremes of the transfer curve don’t bend gently. Instead, they have sharp “corners” and tend toward a “flatter” transfer function at the extremes. When the input signal gets into this nonlinear region a large change of input signal results in almost no change of output signal, but can produce lots of harmonics — predominantly odd-harmonics.
The clipping is more aggressive at the extremes of signal input; almost an “all or nothing” affair. Contrast that to the tube clipping, which is more like “diminishing returns.” I have heard of an amplifier circuit that adds even-harmonic components of the signal. I haven’t built it, but the idea is that it would create a warmer sound.
Having said all that, clipping and harmonic content (warm versus cool) should not be thought of as synonymous. Clipping occurs at the extreme limits of signal input. Nonlinear transfer curves can create harmonics at any value of input signal. When you look in the mirror in the morning, you are seeing a “linear” reflection of yourself. At the carnival or fairground when you stand in front of curved mirrors that distort your reflection, you are seeing an exaggerated “nonlinear” reflection of yourself. While seeing such exaggerated nonlinearity is humorous, in the audio realm it would be intolerable to listen to... maybe.
Guitar effects pedals intentionally distort the signal, sometimes to an extreme that is almost unrecognizable — but, that’s an article for another day.
The answer is simple, really. Tubes don’t so much clip, as go soft, rounding the peaks off the waveform. Solid state, however, works all the way up until it hits the head; at which point it cuts it off sharply. The resulting distortion can be either modeled as odd harmonics for solid state, or even harmonics for tube amps. And it’s all in how they are perceived by the ear; the softer clipping that tube amps do causes it to be more perceived as being not quite as loud, whereas the hard clip of solid state tends to be rougher sounding. And to conclude - if your system routinely clips, you need to Tim Allen (MORE POWER!) it.
In this case, all clipping isn't the same. A transistor circuit is fine up to the power supply voltage, where it mows the peaks off square and flat. This produces a harsh distortion similar to the fuzz pedal for a guitar. A tube circuit starts to round off the peaks before they actually run into the power supply voltage. The rounder peaks account for the "warmer" sound.
It's not about clipping. Tubes and FETs have greater inherent 2nd harmonic distortion which gives them the warm sound. Look up the books and articles by Douglas Self to truly understand why modern bipolar output amplifiers are very hard to beat for sonic clarity. Some of the new Class D amplifiers (TI, others) are quite amazingly clear, too. I still use FET input op amps (LF412) when I want some of that tube warmth. But when I need absolute clarity, modern bipolar devices (LM833 and newer) for crossovers, and bipolar output stages, are (in my opinion) best. Read what Self has to say.
It has to do with the type of distortion between the two architectures. Tubes tend to have more odd-order harmonic (3rd, 5th, etc.) distortion; solid-state amps are relatively distortion-free, and any distortion they have is generally with even-order harmonic (2nd, 4th, etc.) distortion usually generated by their feedback circuitry. With clever signal filtering schemes, solid-state amps can mimic the warmth, etc. of tube amplifiers, without the power-wasting (i.e., heat) that tube amps have.
Simple answer: To the first order, they are pretty similar, however, typically tube circuits are operating at a much higher voltage — a transistor circuit operating at lower voltage, will tend to have higher harmonic distortion than a tube operating at a higher voltage. If you use a high voltage transistor then you can get harmonic distortion from a transistor which is comparable to (or better than) a tube, but you usually have a higher noise floor.
