This page approaches the subject of difference between Valve and Solid-state amps from the valve amp perspective.   The complimentary page (Solid-state/ Valve) in the Solid-state amps chapter, approaches the difference between these amps from a technical context (current drive, voltage drive) and includes how a solid-state amp can be designed to emulate a valve amp.

Solid-state amps have superior technical specifications compared to valve amps.   But when solid-state amps were first introduced it was noticed they sounded flat and lifeless in comparison to well made valve amps.   Also a solid state amp had to be twice as powerful as a valve amp to sound as loud  -  Why ?    Unfortunately little to no research was done.

Possibly the most controversial and misunderstood function within valve and solid-state amplifiers is Negative feedback.   Misunderstandings occur when technical functions become entangled within moral rules.   A simple example is believing left-hand drive vehicles are correct and right-hand drive vehicles are wrong.   Therefore, Sharia law must be imposed to force all countries that drive on the wrong side of the road to convert.   Valve amps evolved in a time when religious and moral values were inclusive within scientific and academic culture.   Almost all academics of technology stated that negative feedback is correct and no negative feedback was wrong.   Therefore negative feedback was imposed even though negative feedback was not required in a valve amp for it to function.

Solid-state amp

Negative feedback is an essential function of solid-state amps.   A conventional solid-state amp cannot function without Negative feedback, therefore there is no choice.

Comparator   A solid-state amp has 2 inputs, described as a comparator.   The +non-inverting input is connected to the incoming signal.   The -inverting input is connected to the output of the amplifier through R1 (negative feedback).   The comparator compares the signal at the 2 inputs, and forces the amp output to match the incoming signal, except for increased gain.

The gain of a solid-state amp is controlled by the ratio of R1 and R2.   Anything (including speakers) connected to the output has no effect on amp gain.   It makes no difference to the gain of the amplifier if there is a load, or no load, or what the load is.   The output Voltage will be the result of the ratio of R1 and R2.   If a screwdriver is placed across the output, a solid-state amp will attempt to provide the correct output voltage as a function of R1 and R2.   If the short circuit protection in the amp does not activate quickly, the amp will self destruct.

Zero output Impedance.   A solid-state amp gives the appearance to the speaker (load) that it has a short circuit is across it.   Any electrical signal that is externally applied to the output of the amp is fed back (feedback) through R1 to the -inverting input, which is amplified in the reverse direction (180deg) and immediately cancels out (shorts out) the signal form the external source.

The voice coil of a speaker generates electricity as it moves in and out in the magnetic gap.   The voice coil generates maximum electricity (Back EMF - Electro Motive Force) at fundamental Resonance Fs.   Back EMF from the speaker is fed back through R1 to the -inverting input, which is amplified in the reverse direction (180deg) and immediately shorts out (damp) the resonance of the speaker.   Zero output Impedance is described as 100% Damping factor.

Speaker Impedance.   Impedance is resistance R that varies with frequency and this includes all cone speakers.   The DC resistance of the voice coil in most 8Ω speakers is approx 6R.   Most cone speakers are 8Ω between 400Hz to 600Hz only.

At fundamental resonance (Fs) the speaker Impedance will rise steeply to 30R - 40R as in the above pic.   Above 600Hz the Impedance of a voice coil in most cone speakers will begin to rise due to its Inductive reactance   and may reach 20R to 60R at 10kHz.   The Impedance of a speaker varies by approx 400% across the frequency range.   Some cone speakers have an internal magnetic shorting ring which attempts to minimise Impedance variation, but at the cost of reducing efficiency.

With a solid-state amp, power to the speaker is dependent on the speaker's Impedance.   As the speaker Impedance increases, power to the speaker decreases, and vice-versa.   In the above pic the speaker Impedance at Resonance is 32R   therefore power decreases to 25%.   Only between 200Hz to 600Hz is the speaker 8Ω.   At 10kHz the speaker Impedance is 32R and the power decreases to 25%.   Across the frequency spectrum the average impedance of a speaker is above 8Ω.   Due to the changing Impedance of the voice coil, a cone speaker receives maximum power at mid frequencies but minimum power at bass and high frequencies.

Valve amp

A valve amp does not require negative feedback to enable it to function.   The primary purpose of Negative feedback is to quieten the amp and provide some damping to the speaker.   Only a small amount of Negative feedback can be applied, before a valve amp is caused to oscillate.

A valve amp with no negative feedback (Tetrode configuration only) will attempt to provide a constant current at the secondary of the output tranny.   As the Impedance of the speaker increases so does the Voltage from the output tranny.   An 8Ω speaker at Resonance FS is approx 20 to 30R.   The output Voltage from the valve amp will rise in an attempt to maintain constant current to the speaker.

Tetrode circuit configuration, represents the Screens being connected to a separate filtered B+ supply (not Ultra-linear).   Regardless if the output valves are Tetrodes, Beam power Tetrodes or Pentodes.

If the speaker Impedance at resonance increases x 4 to 32R, then the output Voltage will rise x 4, and power is also increased x 4.   Between 200Hz and 600Hz the speaker Impedance will be 8R   and the amplifier gain will be normal.   At 10kHz the Impedance of the speaker is 32R   therefore gain and power will increase x 4.   Also any electrical signal from the speaker as back EMF is not fed back.   The speaker is not damped and is allowed to resonate freely.   The the output Impedance of a valve amp (Tetrode configuration) without negative feedback, is Infinite,   Damping factor is zero.

Applying negative feedback to a valve amp reduces the output Impedance and applies damping to the speaker.   Some early valve HiFi amps had a 3 position switch on the rear of the amp to allow the negative feedback to be changed to adjust the correct damping for the speaker.   Regardless of how much Negative feedback is applied it is not possible for a valve amp to achieve a Zero output Impedance (100% damping factor) similar to a solid-state amp.

Comparison

If an imaginary speaker had no internal Reactance and behaved as perfect 8R resistor across the frequency spectrum, there would be little auditory difference between a valve and solid-state amp.   All cone speakers are highly Reactive and the Impedance varies over the frequency spectrum (approx 400%).   Therefore, because Valve and solid-state amps operate in opposite ways to how power is delivered into a changing Impedance, the resultant audible difference is very noticeable.   With most cone speakers, solid-state amps appear to sound flat and lifeless,   the bass and high frequencies appear dead,   as if a blanket is put over the speaker.

• Valve amp       power is directly proportional to speaker Impedance
        therefore   power increases as the speaker Impedance rises.
  
  
• Solid-state     power is inversely proportional to speaker Impedance
        therefore  power decreases as the speaker Impedance rises.

When listening to a valve amp, the bass and hi-frequencies appear to spring to life with clarity and detail, or may appear to sound exaggerated.   If given the opportunity for comparison, every human with ears will hear the difference   and choose a Valve amp as the preferred listening option.   Every musician (guitar and bass player) instantly recognises the brighter response of Valve amps.   Musical instrument amps (Fender, Marshall etc) stayed with Valve technology for this reason.

But, some 2 - 3 way audiophile passive speaker boxes are deliberately designed to have a flat response with a Solid-state amp only.   The mid range speaker maybe 8Ω.   The tweeter maybe 4Ω, therefore x 2 more power is delivered to the tweeter.   When driven by a Valve amp, the 4Ω tweeter will have 1/2 the power in comparison to the 8Ω mid range speaker.   The high frequencies will sound subdued.   Speaker systems that are deliberately designed to have a lower Impedance at high frequencies to obtain a flatter acoustic response when drive by a solid-state amp, will not give a flat response when driven by a valve amp.

Ultra-linear.   The Screen acts as a second Grid modulating the gain of the valve in opposite phase to the Anode.   This return path acts as an automatic Negative feedback that removes most if not all residual distortion created by the output valves and output transformer.

This local feedback from the 50% primary taps to the Screen results in the speaker connected to the secondary being included in the UL feedback structure.   The output Impedance from the secondary winding tends to reflect the speaker Impedance.   A remarkable result is that this reflected Impedance tends to naturally maintain constant power to the speaker, over the frequency spectrum, providing the ideal damping that enables the speaker to give the most pleasing musical sound.

Amplifier history   overview

Until the mid 1960s the majority of audiophile Valve amplifiers were approx 15 Watts per channel.   Only a small % of audiophiles had valve amps 30 Watt and above. 16Ω was the standard Impedance for speakers and was later changed to 8Ω to suit solid state amplifiers.   Speakers were made as efficient as engineering would allow.   The average speaker was 6dB to 10dB more efficient than the majority of speakers today.   A speaker needing to handle power above 30 Watts was rarely required.   100 Watt Marshall guitar amps had 1 quad box consisting of 4 x 25 Watt Celestion 12in speakers.

High Q speakers   Speaker boxes were made as large as possible to achieve maximum efficiency at bass frequencies.   High efficiency speakers in very large boxes also means high Q (Fundamental resonance) which was tuned to achieve the maximum efficiency at the lowest musical bass notes.   Q is quality of resonance (energy stored / energy lost).   Many speaker systems had an excessively high Q of approx 2 to 4.   The average audiophile bass speaker system of today has a Q of approx 0.5 to 0.7.   Excessively high Q causes the bass notes to sound annoyingly resonant.   Excessive bass resonance is controlled by reducing the amplifiers output Impedance, described as Damping factor.

Damping factor   Valve amps naturally function in quasi Current-drive having a high output Impedance with minimal damping.   A small amount of Negative feedback taken from one speaker terminal is fed back to the Cathode of the first input valve.   Feedback reduces the output Impedance and introduces Damping to control excessive Q Resonance of the speaker.   Valve amps are naturally noisy (hum) and negative feedback quietens the amplifier   and marginally reduces measured distortion.

Negative feedback   Academic engineers became obsessed with finding ways to increase negative feedback in Valve amplifiers to achieve a greater damping factor and lowest distortion figurers.   But Valve amplifiers will only tolerate a small amount of negative feedback before the amp is caused to oscillate, hereby restricting the amount of Damping that can be achieved.   When solid state amps arrived with the ability to have Zero output Impedance (100% Damping factor),   academic interest in Valve amps was discarded without further thought.

Music.   Pre 1970s many if not most large manufacturing companies maintained a strong class demarcation of management against trades and production workers.   This demarcation was also reflected in music.   Management including the academic class, was identified with Classical and Popular conservative music similar to Frank Sinatra or Julie Andrews   "The sound of music" etc   Middle class music was mostly mid range with minimal bass energy.   The working class identified with music of the devil, Sex drugs Rock'n'Roll, Jazz, Blues, Fusion, music with intense bass energy.

The rock recording industry noticed that the new Solid-state amps caused most speakers to sound flat and lifeless.   Without any standards in place, recordings had extensive EQ applied to boost the bass and hi-frequencies to compensate.   Many guitar and bass amp designers also noticed that the new solid state amplifiers (with 100% Damping factor) had a suppressive effect on the musicality of a speakers performance and stayed with Valve technology.   But academic engineers of HiFi amps were blindly obsessed with the almost perfect zero distortion figurers and 100% damping factor solid state amps could achieve.   The concept of allowing variable current drive to enable solid-state amps to emulate the higher output Impedance of a valve amp to manage speaker Damping, was negated.

Thiel Small parameters  www.wikipedia.org / Thiel Small  Thiel Small parameters for speaker design were developed in the early 1970s, but little interest was taken until computers were available in the 1980s.   Each parameter of a speaker is compiled into a singular complex algorithm that enables a speakers performance to be accurately defined.   The parameters assume the speaker is being driven with zero output Impedance (100% Damping factor) and will also optimise the performance of a speaker in a ported cabinet.   Thiel Small parameters provided a new way to manage speaker design and re-obtain the lost bass energy caused by the Zero output Impedance of Solid-state amps, but mostly at the cost of reduced efficiency.

High power Solid-state amps eventually became available at low cost and the latest chemical engineering of epoxy compounds enabled speakers to be designed that were capable of 100s of Watts.   With the applied use of Thiel Small design parameters   it was seen that by increasing the mass of the cone,   and decreasing the flux density BL by increasing the gap tolerance,   coupled with a stiff suspension   to enable speaker manufacturers to assemble speakers with wider tolerances, enabling mechanised production at lower cost   the end result is a speaker that is less efficient compared to previous times,   but with cheap high power Solid-state amps   the result can be accurately calculated to give the low frequency performance required.

The below pic is a generalised graph of an early design 50 Watt high efficient speaker (yellow) with a low mass cone and soft compliance, in a typical large sealed cabinet.   With zero damping, the speaker resonance Q2 is extreme +6dB.   With a set amount of damping provided by the Valve amps negative feedback, the resonant Q1.4 is reduced to +3dB at 60Hz, and is flat again at 40Hz and only down -3dB at 34Hz.   The music will sound deep and warm with extra responsive bass energy.

When the same high efficient speaker (yellow dash) is driven by a Solid-state amp (100% Damping factor) the speakers resonance is completely damped and the bass response begins to roll off from above 100Hz.   Bass energy at 50Hz reduced -6dB. The music will sound flat and shallow.

With a modern, less efficient, 200 Watt, Thiel Small designed speaker, with a heavy cone placed in a small ported cabinet, driven by a high power Solid-state amp (100% Damping factor) the response is almost flat down to 60Hz and reduced -6dB at 40Hz.   The modern speaker has an academically flatter response and the music sounds as we hear it to be.

By comparison, many earlier speakers in larger cabinets were 6dB to 12dB more efficient than most modern speaker systems today.   Many modern speakers now have to be driven with x 4 power to sound as loud.   High powered solid state amplifiers are now cheap and wasting power is symbolic of modern times.   History has been forgotten and there is little interest in reflecting on what was not understood during the transition from valve to solid-state technology.

All that was required was a simple addition to the gain management of Solid-state amps (in the right pic) to enable Voltage to Current drive adjustment so the Damping factor of speakers could be effectively managed.

sound-au.com   is a parallel site by Rod Elliott that has a detailed description of amplifier design, and is essential reading for those who require an advanced mathematical and electronic design explanation of amplifier principles described on this page.