Can solid-state sound really match that of tubes?
In general, stereo components use lower impedence noise filtered balanced bridging.
Why? Partly because the perceptions for signal to noise ratios are intermittently better.
People who are fidelity and sound reproduction enthusiasts key in on this feature.
In truth, higher impedence speakers or microphones/guitar pickups sound better,
but there is more possiblity of noise(s); hum, loudness variations and handling.
The signal is of higher strength while the signal to noise ratio remains constant.
The art of impedance matching is known as 'bridging' not to be confused with the
bridging the two channels of a stereo amp to one channel that is twice the power.
The weak signal in the 20 to 100 HZ range of tube preamps and power tubers is
a solvable common stage problem we want engaged to educate audio customers.
The myth exists, and partly correctly for midranged sounds, that tube amplifiers or
bass preamplifiers are the only choice available to professionals. The Kennedy amp
is unique in the market in adding a choice by trying to explain our quality differences.
We chose to use articles from respected publishers to build an industry background
and perspective to tell our story around. For what credibility it is worth, we present
some articles that balance technical experience and scientific facts about amplifiers.
Car stereo subwoofers are low (4) ohms for a reason: they work with current amp's.
A bass guitar amp is usually a voltage drive type amplifier. Think of a transmission.
When in low, power is transfered easily at slower rates through slow moving gears.
But a race car has no low gears, only it specializes in taking higher force into high
impedances at the expense of small damper control accurate focused performance.
The Kennedy Bass amp is expressly designed to drive hi voltage and high current.
It performs where circumstances and conditions give others no chance except to fail.
Volume cranked up in amp debate
By Brian
Santo
Escondido, Calif. - The analog world is being translated into bits faster than
you can say compact disk. Whether that's progress or an unfortunate progression
is open to debate-and nowhere is the argument more heated than in the music
industry, where many audiophiles still value vinyl and musicians treasure
tube-based amplifiers.
Both groups seek a quality of sound presumably unattainable by
the solid-state and digital counterparts of their analog artifacts. But those
holdouts may be left behind by the latest in technology.
Recently, Deja Vu Audio (Berkeley Springs, Va.) reported the
creation of a solid-state tube emulator that provides the sounds of several
classic electric guitars (see July 11, page 39). Other tube emulators have
preceded the Deja Vu design. The catch is that whenever attempts are made to
bridge the gap between tube and solid-state sound reproduction, subjective
criteria such as "warmth" often overwhelm scientific engineering
principles.
But one audio engineer and physicist contends that the physics
involved with the recreation of sound are no mystery and, furthermore,
pretending that audio reproduction is a black art only confuses the market.
That engineer, John Murphy of True Image Audio (Escondido), has designed a
number of tube and solid-state preamps and power amps for the
musical-instrument and professional-audio markets.
"Any product containing vacuum tubes is especially likely
to be surrounded by exaggerated claims of supernatural performance,"
Murphy asserted. "From an engineering point of view, there is nothing new
or mysterious about vacuum tubes. They have been in use since Lee de Forest
first inserted a control grid into a Fleming valve in 1906 to create the first
triode. Today, tube audio products are surrounded by such excessive
disinformation that the small, but real, sonic advantage that tubes offer is
almost lost in the hype."
When operated in a linear (or unclipped) mode, Murphy explained,
tube amps sound the same as their solid-state counterparts, provided that their
frequency response and group delay characteristics are well matched and their
distortion levels are sufficiently low. The audible difference between tube and
solid-state amps emerges only when they are clipped.
Murphy cited published results of several carefully conducted
double-blind listening tests confirming that even highly trained listeners cannot
hear the difference between tube and solid-state amplifiers when the amps are
operated in their linear range. "Only a handful of fanatics-but mostly
those with blatant financial interests-persist in making claims to the
contrary," he said.
Everything changes when you clip (overdrive) the amps, however.
"Then it becomes easy to hear the difference between typical tube and
solid-state amps. It is also easy to see the difference on an oscilloscope
trace," he said.
A typical tube amp (such as a pair of triodes in series) can be
seen to clip with a softly rounded waveform, while typical solid-state amps
(such as op amps) clip with razor-sharp edges.
"Every engineering student who has studied Fourier analysis
knows why these two waveforms sound different: the harmonic structure,"
Murphy said. The hard clipping waveform of the solid-state amp has a different
harmonic content from the soft-clipped tube amp, simply because the waveforms
are different. While the harmonics from the solid-state amp have strong amplitudes
out to frequencies beyond the limits of audibility, the harmonics from the
soft-clipping tube amp fall rapidly in level with increasing frequency.
Note here that tube amplifiers are known for even harmonics, and that while the human ear is less sensitive to them as compared to odd harmonics, after more than a few harmonics, both odd and even harmonics are spaced less than a semitone apart and produce dissonant, dischordant harmonics. Most likely, this explains the reputation of tube amplifiers because they are better for amplification of distorted sound, creating less discord but greater percentage of noise. In truth a typical tube amp may have 1 or 2 percent Total Harmonic Distortion (THD) but sound better while clipping, due to large proportions of low frequency even harmonics and fewer higher frequency ones.
These 'hi freq' even harmonics seem to make most newer pedals sound 'transisted' or 'sterile'. Digital Signal Process (DSP) requires even more noise reduction compared to stomp-boxs with a small footprint and no additional carrying case. We think guitar stomp box style technology is slated for a comeback, but with a different set of rules. A More in depth analysis page gives more details (not our copyright again, but source is viewable in this page code): --> High even harmonics, clipping, and earlier solid state vs. tube amp, current drive, voltage drive stuff... From:http://users.chariot.net.au/~gmarts/amptypes.htm
Amplifier
debate rocks on
Those harmonic differences account for the "raspy and
obnoxious" sound of the solid-state amp in clipping, compared with the
much-more-mellow sound of the tube-amp clipping. A second, more-subtle
difference is that solid-state amps tend to have a fixed 50-percent duty cycle
as they clip, whereas most class A tube amps clip with a duty cycle that varies
as a function of the drive level.
Push-pull, class AB tube power amps tend to clip much like
solid-state amps, but they sound different because of their high output
impedance. In particular, tube power amps exhibit a peak in their frequency
response by as much as 10 dB or more at the resonance frequency of the speaker
they are driving.
"No wonder they are reported to sound 'warmer' than
solid-state power amps," Murphy said . "This aspect of tube power
amps is not seen in test reports, where reviewers use nice 8 dummy loads for
their tests. But measure the frequency response at the input terminals of your
speaker, and you will see this effect clearly."
As for class A tube preamps, Fourier analysis helps reveal the
harmonic structure of the clipped waveforms, Murphy said, noting that the
unclipped waves have no harmonics, except for residual distortion. For
instance, any square wave, regardless of its source, is composed of only the
fundamental and odd harmonics (first, third, fifth, etc.).
Square wave
To a first approximation, the clipped output of either type of
amp looks much like a square wave, and spectrum analysis shows that the
waveforms consist largely of odd harmonics. Even the tube-amp waveforms, with
their rounded shoulders, consist only of odd harmonics as long as the duty
cycle of the wave is 50 percent and the left half is an inverted image of the
right half (in other words, as long as half-wave symmetry is maintained). The
even harmonics are introduced only as the waveform deviates from a perfect
50-50 duty cycle. The graphs below show a summation of harmonics, not necessarily a single wave.
For our customers a professional courtesy, links to pedals with these
even harmonics:
Banzai Fireball Pedal
with one clean, one even harmonic solid state channel
Even though it is expensive, we think that cheap distortion pedals are designed for
low impedance amplification inputs with noise filtering added. Older pedals have too
much voltage/quantization sampling sibilancy and jack noise to use on our amplifiers.
Such control of even harmonics are important when using solid state amp's to amplify
distorted signals. Also of concern is recovery of dynamic range lost when more than
one pedal is used in a chain. While hard to find, dynamic range expansion is possible.
But we prefer active tone control pedals & preamps, with stage span cables after the preamp.
Good tone control reverses the effects of higher order dissonance especially (compressed) bass tones.
It improves signal to noise, eliminates cable losses, adds tube like resonance by boosting
certain wanted frequencies, whether they are the speaker resonance frequency or not, and
are the tone equalization of choice for any electric or metal sounding group and also vocals.
Here is a NOT underpowered clipping style pedal - it is using overvoltage instead of undervoltage.
This next sound file has unknown even to odd ratio harmonics but is rare and important in that it is not a noisier underpowered op and and diode type of preamplifier distorion pedal and can be assesed fairly because it is not just a recording of a clipping distortion pedal being fed into an tube amp in full volume concert overdrive as is common to get in example sound clip files. Notice the harder sharper edges and spits and ringing let it be known this is hard clipped overdriven tube like sound, not soft clip distortion.
The Tube Factor 290 volt (170 V-DC normal operation) pedal by Hughes and Kettner pedal.
Click here for a second Tube Factor sound byte
Click here for the JFET only type pedal driving a solid state amplifier sound
Recent update: This SD-2 guyatone recently became my favorite pedal after I hooked it up between a vintage Traynor Group One 1974 pre-amp and a crate solid state. The sustain is incredible and the raspy sound is replaced by heavenly smooth melodic tone. The physics may be related to even and odd harmonics or perhaps it is related to the power of active tone controls or just their voltage boost, or even the single pickup coil being removed from the interaction with the pedal. I do know that the true dual channel chorus helps on the crate and that it distorts nicely even as a stand alone amp, as crates are known for. I had originally favored this pedal merely on a theoretical basis, but as it turns out year later, I would be remiss if I did not make special note of it's usesful sound if used in this somewhat escoteric but nevertheless important configuration. I therefore continue to recommend it but not attached to the guitar as usual. Use it after a pre-amp or with bartolini on board amplified pickups if you must, but I have not tried them. I own many fine distortions, including the meteor and fireball and this combination turns out to be the best and also most inexpensive.
Note: An instrumentation amplifier is a differential op-amp circuit providing high input impedances with ease of gain adjustment through the variation of a single resistor. With any normal guitar unbalanced circuit, the variable resistor is on the guitar capacitor, with balanced design, the knob on the stompbox effectively controls wave phasing and swells by resistances changes to the voltage and currents by splitting the signal. This has led to 'true bypass' being a feature of newer guitar pedal stompboxes because splitting the signal lowers or eliminates the impedance.
"This is what I call duty-cycle modulation," Murphy
said, adding that many class A tube amps exhibit that characteristic. But most
solid-state and push-pull tube amps have perfect 50-50 duty cycles, he
explained, and therefore have no significant even-harmonic content in their
clipped waveforms.
When the tube amp clips, its duty cycle starts at 50 percent and
typically shifts to 55 percent (or even as much as 65 percent) as it is driven
further into clipping. That has the effect of adding even harmonics as the amp
is pressed further into clipping. Plotting the duty cycle vs. the input level
provides a kind of sonic signature of the amp. For a typical solid-state amp,
that signature is just a flat fine at 50 percent.
"But for some of the more interesting types of tube amps,
that signature starts at 50 percent, goes to maybe 55 percent and then back to
50 percent or even 45 percent," Murphy said.
Solid state
v. tubes: cranking the volume
"In response to a strong transient, these amps exhibit what
looks like 'dancing harmonics’ the spectrum analyzer. First the odds rise, and
then the evens rise and fall between the odds. When a guitar is used as the
signal source, the audible effect is a subtle, but musically interesting, sort
of 'reedy' sound mixed with an otherwise 'brassy' sound," he explained.
"Besides the obvious soft clipping, I believe this to be an
important reason why guitar players like tube amps. But so much for the truism
that says: 'tubes have even harmonics, and solid state has odd harmonics.' Bull
dung. The waveforms of both consist primarily of odd harmonics. Tube amps with
duty modulation just throw in a sprinkling of evens.
Further, Murphy contended, "the occurrence of those even
harmonics is not critically important , when you consider that most of the
guitar-overdrive devices in use by players today employ solid state diode
circuits, which exhibit soft clipping but with a fixed 50-percent duty
cycle."
In 1983, Murphy designed a tube-emulator circuit that, to his
knowledge, is the only solid-state overdrive device to exhibit duty cycle
modulation.
"I have worked with at least one well known guitar player
who sets up an array of tube-amp stacks on stage, only to use a small
solid-state pedal-effects unit 'stomp box,' as players say-for his actual
overdrive sound," he said.
'From the [perspective of the] audience, you would think he was
using the amps, but those are just for show. The advantage of the stomp box is
that it is reliable-no tubes to change, it's consistent and it usually provides
more gain or overdrive than a typical tube guitar amp. The stomp box drives
another guitar amp - tube or solid state - which then drives a limited number
of the speakers. Most of the amps on stage are just props without any
electronics or speakers."
The point, Murphy said, is that some professional artists would
just as soon use their solid-state pedals as their tube amps. They can get a
satisfactory overdrive sound from either. The pedal is simply more convenient.
"But ask a kid in the a audience," Murphy said,
"and he will insist that his favorite guitar player uses a tube amp,
because he saw it. Ha! A lot of really expensive tube amps are sold this way.
"As far as other characteristics of tube guitar amps are
concerned, I have found that the pre-clipping frequency equalization and
post-clipping EQ are absolutely critical adjustments. Once you have a
well-behaved clipper-even if it's just simple diodes, as in the stomp boxes-it
is the precise combination of pre- and post-clipping EQ that mostly determines
how an amp sounds. The 'secret' of the best sounding guitar amps lies in the
pre-clipping EQ response curve."
Subtle harmonic effects
If one could devise a solid-state amp that had soft clipping
along with waveform duty-cycle modulation, Murphy contends, the amp would look
substantially like a tube amp in the lab and would sound much like a tube amp
in the listening room-down to the subtle effects of the time-varying even
harmonics.
"From our knowledge of Fourier analysis, we can be
confident that the waveform tells the whole truth and nothing but the truth.
'The waveform contains no 'secret' information as to whether it was produced by
a tube amp, a solid-state amp, a digital waveform generator or hundreds of sine
wave generators operating in parallel, for that matter," he said.
"The mathematics of Fourier assures us of this. If we can make a
solid-state amp produce the same waveform as a tube amp when it clips -
including duty-cycle modulation - then we have successfully simulated the tube
amp with solid-state components."
Reproducing the tube amp
Murphy created his solid state tube emulator circuit in 1983,
when he was chief engineer for Carvin Corp. He claims his invention reproduces
the significant characteristics of a tube amp.
"This circuit was first used in a line of solid-state
guitar amplifiers by Carvin and introduced in their 1987 catalog of
musical-instrument products. That circuit continues in production today in
Carvin’s SX series solid-state guitar amps," he said. Carvin could not be
convinced to pursue a patent, and as a result, the tube simulator is now in the
public domain.
"Common diodes are employed to clip first the one half of
the waveform and then the other half of the waveform, but not at the same
stage," Murphy explained. That follows the way in which a pair of tube
triode stages, operating in series, clips only one half of the waveform at a
time. It is the independent clipping of the two halves of the waveform that
allows the duty cycle of the clipped wave to modulate away from 50 percent and
introduce the even harmonics.
"My invention employs op amps to buffer each diode-clipper
stage," Murphy said. "To more closely match the waveform of a 12AX7
triode clipper, my circuit also employs diodes in the feedback loop of the
inverting op-amp buffers to make the clipping a bit less soft."
Besides applications in guitar amps, the circuit could be
employed in the front end of any solid-state preamp or power amp to provide
controlled clipping characteristics that measure - and sound - very much like a
class A tube amplifier.
The next step in audio technology Murphy envisions, will be
vacuum microelectronics-thermionic emission with cold arrays of microtips based
on quantum tunneling as the electron source, and promising to provide triodes,
pentodes, and the like.
"Though this new technology is targeted at microwave
amplifiers and flat-panel displays, it is entirely possible that it will end up
in guitar amps and hi-fi gear," Murphy said. "Imagine that."
Reprinted from Electronic
Engineering Times, October 3, 1994
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