Paradigms in Amplifier Design
 

Background of Relatedness

For the last thirty years I have been working with low feedback and zero feedback amplifiers. Initially I was convinced that audio power amplifiers had to include certain elements (such as negative feedback) to control distortion. After some exposure to Robert Fulton of FMI (Fulton Musical Industries) in the late seventies, a larger picture began to emerge. Robert Fulton was emphatic that the quality of the topology came first, then quality of components, and if everything was right, the distortion would already be low. Negative feedback could be reduced or eliminated.

Back in the days when feedback was being experimented with, it was common knowledge that this was a compromise. In the succeeding decades it became accepted as a fact of audio design. More recently the negative feedback idea has been challenged by elements of the high-end audio community.

My own explorations resulted in an amplifier that was designed to reduce distortion in every way possible without feedback. In this way it was possible to examine the effects of feedback, since the amplifier was very functional without it. During the process, I also learned about common engineering practices that tend to hold back development in fields such as audio. In their recent book "Control Design And Simulation", Jack Golten and Andy Verwer discuss this phenomena in chapter two, with regard to applying mathematical models to the real world: "...mathematical models invariably involve simplification. Assumptions concerning operation are made, small effects are neglected and idealized relationships are assumed."

It is the mark of a good engineer to know when and which things should be assumed, neglected or idealized, and we see this in audio all the time. The problem here is human nature. We tend to stay within the limits thus set by the existing paradigms and to resist changes that threaten one's viewpoint of the world. When someone else creates challenges to the paradigms, it is normal also to try to protect one's world view by preventing the new idea from gaining ground.

As alluded earlier, negative feedback has been found to be an inexact solution to amplifier distortion. This is due to propagation delays (the very small but measureable amount of time it takes for a signal to move from the input of an amplifier to the output) which are a normal phenomenon of amplifiers. In order for negative feedback to work according to the math, it must be applied to counter the input signal in real time. Propagation delay in the amplifier circuit prevents this; the negative feedback will always be lagging the original input signal. This lag results in ringing effects and enhancement of the odd-ordered harmonics that the human ear/brain system uses to measure the loudness of a sound (specifically the 5th, 7th and 9th harmonics).

In the 1960s, General Electric's conducted a variety of tests, confirming that amounts of barely hundredths of a percent distortion were not only audible but also irritating to the human ear (conversely, they also found that the ear is quite tolerant of lower ordered harmonic distortion). In other words, a disparity exists between the mathematical proof for negative feedback and its actual application, and is an example of the engineering phenomena to which Golten and Verwer refer. Despite this, negative feedback is commonly accepted in the audio world, causing the reigning design, test and measurement paradigm to have a built-in weakness.

The Voltage Paradigm

The Voltage Paradigm is that reigning design, test and measurement paradigm for amplifiers and loudspeakers, and is seen in many audio magazines. It assumes that the voltage response of an amplifier is the only aspect of the amplifier that matters. If a speaker is being tested, the test signals will be voltages (the power of the test signal is not considered). The ideal Voltage Paradigm amplifier produces zero THD, has wide bandwidth and is "Load Impervious".

Being Load Impervious is not exactly what it sounds like! It is called load impervious as the amplifier will make the same voltage regardless of load; it has a "constant voltage" characteristic (hence the title 'Voltage Paradigm'). It works like this: if a 'constant voltage amplifier' were to produce 100 watts into 8 ohms, it could produce 200 watts into 4 ohms, possibly 400 watts into 2 ohms (and 50 watts into 16 ohms). In all of these cases the output voltage is about 28.28 Volts RMS. We are very familiar with these charcteristics, typical of solid state amplifiers. Under this model, to be 'load impervious', the amplifier will make different amounts of power depending upon the load impedance.

Loudspeakers designed under this paradigm are said to be 'voltage driven', as they expect the amplifier driving them will produce constant voltage despite the speaker's variable load impedance.

Voltage Paradigm amplifiers inherently employ a fair amount of negative feedback. However, as General Electric has proven, negative feedback is out of sync with the the rules of human hearing, due to added odd-ordered harmonic generation. It may seem like a subtlety, but how we detect loudness is arguably the most important rule of human hearing; in other words we are violating the most important aspect of hearing in our quest for 'good' bench specification!

The Power Paradigm

The Power Paradigm assumes that amplifiers produce power and speakers are power-driven. Current produced by a power amplifier is not ignored and is considered in the amplifier's power response. Under this model, the ideal amplifier will make the same power into all loads, 4, 8 and 16 ohms. The typical amplifier, in this case, is a vacuum tube amplifier which usually makes its power into these loads via taps of its output transformer. There are a small number of transistor amplifiers that are designed with this behavior in mind also. Ideally amplifiers under this paradigm have little or no feedback. The Voltage Paradigm refers to such amplifiers as 'current source' amplifiers but the term is not accurate; in reality they are what they are called - power amplifiers.

Zero feedback power amplifiers have seen a resurgence in the last two decades, based mostly on their sonic character. Voltage Paradigm adherents will state that that character is based on distortion, but the truth of the matter is that what is really at the heart of it is the lack of distortions that humans find objectionable. In other words, this approach is based on the reality of the rules of human hearing, rather than bench specification.

In addition to a constant power characteristic, the ideal Power Paradigm amplifier will be low in objectionable distortions, while otherwise having similar qualities to Voltage Paradigm amplifiers if possible: wide bandwidth being an example.

Loudspeakers that operate under Power Paradigm rules are speakers that expect constant power, regardless of their impedance. Examples include nearly all horns (currently the Avantgarde Trio is the only known exception), ESLs, magnetic planers, a good number of bass reflex and acoustic suspension designs. Horns, ESLs and magnetic planers do not get their impedance curve from system resonance and so benefit from a constant power characteristic and indeed, many of these speaker technologies are well-known as good matches with Power Paradigm amplifier designs.

Distinctions

Speaker designers usually work with one of the two Paradigms by trying to achieve certain goals, such as creating a speaker that is 'tube friendly' or by default of owning a commonly accepted transistor amplifier. In other words this choice is not always conscious, despite there being very concrete rules that govern each paradigm. In some cases this is not important, but in the case of acoustic suspension and bass reflex designs, the impedance curve is often derived from a combination of resonant elements in the enclosure and crossover so proper performance may only be obtained in these cases by application of an understanding of these rules.

The Objectivist/Subjectivist debate has been raging in audiophile circles for nearly three decades. Objectivists operate exclusively in the Voltage Paradigm while Subjectivists tend to operate in the Power Paradigm.

In the world of speakers, efficiency of the speaker has been an issue that the Voltage camp has had to address, as the older Power Paradigm specification of 1 watt/1 meter was a 'chink in the armour'. The new Voltage Paradigm specification, Senstivity, illustrates the point: 2.83V/ 1 meter is the spec, resulting in a sound pressure expressed in db, just like the Efficiency specification. 2.83 Volts into an 8 ohm load is 1 watt (in other words the two specs have the same meaning if the speaker is 8 ohms). 2.83 Volts into 4 ohms is 2 watts. Thus, a speaker can have a senstivity rating that looks the same as the efficiency rating, but the speaker can be several decibels less efficient if the impedance is lower. This is an easy way to cover up how much power it really takes to drive a speaker, and also creates an expression that moves the efficiency issue into the Voltage Paradigm nomenclature. It would also seem to create a 'buyer be ware' situation: you have to know how to interpret the numbers to get to the truth of the matter.

The Tubes vs transistors debate also arises from the conflict of the two paradigms. Transistor amplifier designs operate almost entirely in the Voltage Paradigm whereas most tube designs are Power Paradigm technology. This appears to be the the main distinction that separates the Voltage Paradigm from Power Paradigm amplifier designers but the use of negative feedback is obviously another.

Specifications of amplifiers measured under the Voltage Paradigm will not tell you anything about the way that amplifier sounds. It is very easy to tell how an amplifier will sound using measurements based on the Power Paradigm as the measurements are made with regards to understanding and working with the rules of human hearing.

Any audiophile will agree that the most valuable thing they have with respect to their audio system is their own hearing. In fact human hearing defines the reality of audio. As these words are written, the high-end audio industry has been experiencing a shrinking market for over ten years. It is no surprise- in order for the market to expand, the industry has to touch, move and inspire the marketplace with the possibility of real music. It is my contention that if we are to make serious performance increases in the world of audio, the rules of human hearing can be our only guide.

Paper originally titled "Competing Paradigms in Amplifier and Loudspeaker Design, Test, and Measurement" and was written By Ralph Karsten, founder and owner of Atma-Sphere music systems, inc.