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Theory of the Amplifier / Speaker Interfaceby Thomas Dunker (Horn Website) reproduced from the Joelist, thanks Thomas ! As most of you are probably aware, I have spent a great deal of mental effort trying to get a better grip on amplifier-loudspeaker "interface" problems. The inspiration has come from various old papers, from native Norwegian speaker veteran Ragnar Lian (co-founder of Scan-Speak back in the 70's), Kurt Steffensen, Poul Ladegaard and others. The basic conflict of interest is that (electrodynamic) speakers are inherently current controlled transducers, the force accelerating the diaphragm(s) and thus controlling the air pressure fluctuations we call sound, is ideally related to current through the formula: F = (Bxl)*IIn other words, we ought to be measuring the distortion of the CURRENT flowing through the speaker's voice coils (and crossover components in the case of passive crossovers). Instead, distortion in amplifiers is invariably measured as voltage amplitude distortion. By Ohm's law, the current in a speaker voice coil is the amplifier output voltage divided by the speaker impedance. Therefore, the current can only be linearly related to the voltage if the speaker presents a purely resistive load to the amplifier. When, as is usually the case, the speaker impedance varies with frequency, amplitude, voice coil temperature etc., these dynamic impedance fluctuations translate directly into distortion of the driving current, as I = V / Z. In addition to this, several of the properties of the voice coil, magnetic flux path and ferromagnetic/magnetic materials in the drivers are unlinear and further distort the current. The negative feedback often used in power amps is totally blind to all of these problems, and with the reliance upon "damping factor" (electromagnetic-mechanic damping at resonance), huge back-EMF currents (at high voice coil velocities near resonance) are dumped back into the output of the power amp and sneak into the feedback loop and effectively hamper the intentional action of the feedback, which is to maintain perfect voltage amplification and low output impedance. This was thoroughly investigated by Matti Otala and others in the 70's and has been the basis of groundbreaking articles by Lian, Ladegaard, and our own Kurt Steffensen. The idea that a power amp must have a near zero output impedance is not in agreement of the speaker's demand for undistorted driving current. As long as the speaker impedance is complex and dynamically unpredictable, a "perfect voltage amp" will always deliver distorted current, with the exception of pure sine waves at low power. A music audio signal is in itself complex and unpredictable, and as the signal amplitude modulates the nonlinearities of the speaker (and amplifier), we see that the "normal" situation is really an unhappy marriage of amps and speakers. Most speakers are designed assuming a low amplifier output impedance, and these will show large frequency response and damping problems when driven by a linear current amplifier. (Output current proportional to input voltage regardless of speaker impedance). This is not to say that speakers can not be made to function well with such amplifiers. When we consider a simple amplifier such as an SE triode amp, we immediately realize that the triode is a voltage controlled current regulator. A transistor is similarly a current controlled current regulator. At any rate, these devices can be used to linearly convert a voltage signal to a proportional current signal. In the absence of global feedback, the simple SE triode amp's "damping factor" is given as the ratio between the output transformer's primary impedance and the output tube's internal plate resistance. Typically this yields a "damping factor" somewhere between 1 and 4. The output transformer itself only transforms the impedance and current to match the requirements of the speaker (plus removes the DC component of the tube current). Because the amp-speaker interface has been poorly understood by a lot of audiophiles, a lot of misconceptions have arisen. For instance, it is often assumed that negative feedback is solely responsible for "unnatural sound" when it is the situation in which feedback is used and when it is is used for the wrong reasons that it does not even do what it was supposed to do. Most feedback amps work extremely well when they dump their output signal into a resistive load, when measurements are taken and show ultra low distortion. However, when a real speaker is attached, the distortion is anyone's guess. Different speakers will affect the amp's distortion differently, for instance, all because of the wildly different impedance characteristics of loudspeakers. A whole war ensued between "objectivists" and "subjectivists" when it was found that measurements did not agree with subjective results, all the while when the measurements were done under extremely unrealistic and irrelevant conditions (resistive loads, low power levels, continuous signals). A tremendous amount of energy has been wasted in that "war" and a lot of the fundamental problems remain unsolved, although the knowledge and technology is there. Negative feedback is being blamed for problems resulting from not NFB itself, but for the reason that NFB is used to make sure the amp at all times disregards anything but the output voltage of the amp rather than the current which is what the speaker transduces into sound. We do not hear the voltage signal across the amp's output terminals, we hear the current through the cables, crossovers and voice coils. It is difficult to find a single commercial amp or speaker manufacturer that is seriously concerned with this problem. We have been making voltage power amps for half a century, and they still don't suit the speakers any better. People want small, cheap speakers with a lot of bass, and therefore most of the audio world is stuck with high-Q vented enclosures that would never cooperate with amps trying to deliver undistorted current. The attitude of amp builders is often that, "I do amps, let the speaker people do the speakers" - well, the result is neither amp builders or speaker builders can do anything to the situation when they're both forced to follow the conventions. So how will a high output impedance power amp sound - that depends on the speaker. A speaker does not make sound unless driven by an amp. I believe we could solve a lot of problems by going over the power amp and speaker interface with a more holistic approach. Maybe with the right amp your old speaker is wrong. Maybe you'd write off an excellent amp because you insisted not to part with or modify your "reference" speakers. We have here a situation where two variables are mutually dependent and mutually affect each other's potential performance. As people rediscover efficient speakers with inherent damping, the success of simple triode amps is explainable. But how many times have I had to read reviews of 7W triode amps driving tiny, inefficient speakers normally powered by enormous arc-welder type power amps, and the guy being all disappointed because the amp runs out of steam and lacks "control" in the bass region - as if anything else could have been expected! Apparently the audio industry believes in what it is doing, even when the technology is full of mistakes and misconceptions that can not be defended from a technical point of view. It is truly strange that there are few if any conceptual differences between no-fi department store systems and "high end" systems when it comes to the amp-speaker interface. All the same problems are evident. And then they tell us that by now things are so perfect that it's the recording media that need to be improved - as if that was the last frontier! Oh, well.... Thomas Top
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