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WHY DO WE USE TRIODES
Audiophiles have been led to believe through published measurements at maximum output power and uneducated reviewers that single-ended triode amplifiers produce vast amounts of harmonic distortion.
As a matter of fact triode vacuum tubes are by far the most linear amplifying devices in existence today. They produce the least amount of distortion, and that distortion is predominately second harmonic, which is the least obtrusive type for the sound. By contrast, pentodes produce greater distortion, and the third harmonic tends to dominate. A transistor looks at best like a very bad pentode.
OUTPUT STAGE CHOICES : PUSH-PULL OR SINGE ENDED
To state the obvious, a single-ended circuit must operate in Class A. A push-pull amplifier may be Class A, AB or B. Class A indicates that each output tube handles the full cycle of the audio signal, while AB allow some of the devices to cut-off during a portion of the cycle while B only half the signal goes trough the active device.
Single-ended and push-pull circuits may be built with triodes, beam power tubes, pentodes, or the latter two in ultra-linear mode. We use exclusively Class A push-pull circuits for our output stages, there is a natural cancellation of even-order harmonic distortion products in this topology.
The cancellation is not complete, but it would be unusual to see large amounts of second harmonic distortion from a push-pull circuit. Note that a push-pull circuit has no significant ability to cancel odd-order distortion products. If low distortion performance is required, one must avoid the generation of odd-order harmonics in the first place. A good triode tube meets this requirement.
In single ended operation there is no mechanism to naturally cancel harmonic distortions and available power output is greatly limited. The full DC current for the output tube(s) flows through the transformer primary and strongly magnetizes the core of the transformer. Thus, much of the core's ability to couple the audio signal is used up by the non-audio DC current, and causes the core to saturate asymmetrically with audio signals. Adding parallel output tubes for more power directly increases the DC magnetization current, thus exacerbates the distortion problem. To deal with this an "air gap" may be introduced into the transformer core.
In most cases also a greater amount of core material is used, which in turn makes the whole unit larger. By increasing the size of the coils we soon become limited by parasitic capacitance and leakage inductance affecting the bandwidth of the transformer. The final result is either a higher degree of distortion (all harmonics with the second dominating, increasing with decreasing frequency), a measurably peaked frequency response, or both. As observed the best sounding single ended designs rarely reach above 20W and it is practically impossible to manufacture a transformer of utmost quality for more than 80W in single ended triode operation.
Since the distortion in the single-ended transformer is asymmetrical, a system based around this type of amplifier might be more sensitive to absolute polarity.
The same construction problems are true for the push-pull transformers but in that case the power limit for the same size transformer is 4 times higher. After 5 years of development we now make transformers with our own construction methods and materials allowing us to take this art to a completely new level.
ASSUMPTIONS LEAD TO WRONG CONCLUSIONS
Traditional theory gives negative feedback high marks. Consider that when the "error" signal is fed back into a non-linear amplifier, it multiplies the distortion order. For example, if an amplifier naturally produces second harmonic, negative feedback will create a second harmonic of that second harmonic, which is the fourth harmonic. If the basic amplifier has second and third, the fed-back amplifier will contain second, fourth, sixth, and ninth. As is well known, the higher orders of distortion are far more objectionable to the ear than lower orders, and odd orders more offensive than even orders.
Thus it may be possible to lower the level of distortion products and still have the distortion be more audible.
The application of negative voltage feedback also reduces an amplifier's measured output resistance, i.e., it raises the "damping factor." Here again, the measurement fails to capture the essence of things.
A negative feedback amplifier maintains better control over speaker motion because the erroneous motion creates a voltage (the back e.m.f.) which enters the feedback loop through the amp’s output terminals. The amplifier then cancels out this error signal, which corrects the motion of the speaker.
However, like many theories, this is an oversimplification and, in practice, the opposite result may be obtained. Quite often the motion of a speaker's voice coil former may not match the acoustical output due to cone break-up and the fact that the motion of the coil former is being sensed by the voice coil, which is a reactive element with phase shifts and delays. The back e.m.f. passes through a cross-over network, which will again alter phase and frequency relations. By the time the error signal reaches the power amplifier it is arguably an erroneous error signal. As the power amplifier attempts to correct for this signal, it may actually do the exact opposite with respect to the speaker's acoustic output.
To explain it in simple therms think of a feedback amplifier having 2 inputs. One for your signal and another for its own output. In theory whatever is different should be canceled but quite often on this second input you have a lot of other garbage that should not be there. Correcting for it is actually adding it to the signal, so here we would argue that is better not to use any feedback around the output of the amplifiers as you have no idea what will get there.
Small-Signal Distortion in Feedback Amplifiers for Audio
BRING THEORY TO PRACTICE - NO FEEDBACK
In general a signal passing through an amplification stage will have some distortion added to it. (In our case this will be almost only second harmonic.) And when that signal passes through the next stage it would be adding distortion to the signal and its distortion generating a minimum amount of higher (fourth) order distortion and so on. (Pretty much like the effect of feedback described above)
Consider now the following: the usual preamplifier (tube or solid state) has 3 stages - input buffer, gain stage, output buffer; then the simplest power amplifiers have 4 stages - input, phase splitter, driver, output buffer. And all of this is dependent on the signal amplitude via an extremely non-linear function.
In order to minimize this effect the preceding stage of any amplification stage should have at least 2-3 times lower distortion than the latter. (Would that be possible when we have 7 stages in the signal path?)
Minimizing the number of stages reduces drastically the order of distortion and its inter modulation products.
The sonic result is vastly improved transparency and speed. Our products feature the minimum sensible number of stages implemented with the most linear devices available operating as close to theoretically perfect operating conditions as possible.
THE POWER SUPPLY CONCEPT
Amplification stages are only half of the story. As seen on fig 1 the power supply is represented by a single capacitor or battery. This assumption alone has ruined many beautiful designs.
In theory a large enough capacitor is as close as you get to the theoretically perfect power supply. Well, not so in practice! You have a rectifier and the mains supply connected to it. For every cycle of the mains you have two things happening. You charge the capacitor through the diodes and the mains transformer for a limited amount of time by connecting it to the mains supply with all its noise and garbage and then you discharge it through the amplifier and load until the next charge cycle. So your power supply is constantly varying its value and for a portion of the time is connected directly to the polluted mains line. (don’t forget the fact that this capacitor is part of the signal loop!!!)
And this seems to be acceptable for all electronic design gurus?! Here is an example of a good amplifier design and its power supply.
By looking at the whole picture you see a coil (choke) separates the reservoir capacitor from the signal capacitor, thus preventing any noise from the mains line reaching the signal capacitor and as a side effect keeping a constant voltage across it. It is very unfortunate that few currently available commercial products feature similar topology. On first glimpse it looks simple yet it does all that is required by the PSU to approach the theoretically perfect with minimum component count.
