By Kevin Hayes

To obtain a damping factor greater than (or output impedance less than) a modest value, negative feedback must be employed. Negative voltage feedback works by taking a portion of the amplifier’s output, and injecting it back into the amplifier’s input such that it reduces the amplifier’s effective gain. The feedback signal contains both the desired output and any error the amplifier has made; thus, the feedback tends to reduce the gain and the error in equal amounts. From the 1940′s to today, many designers strive to put as much feedback into their designs as possible.

Traditional theory gives feedback high marks, but this analysis changes when we consider that the “error” signal is fed back into an amplifier that is non-linear and makes mistakes (otherwise there is no need for any error correction). Due to this, feedback may lower the overall level of distortion products, but it also multiplies the order of the distortion products. For example, if an amplifier naturally produces second harmonic, feedback will create a second harmonic of that second harmonic, which is the fourth harmonic. If the basic amplifier has second and third harmonic distortion products, the fed-back amplifier will contain the second, fourth, sixth, and ninth harmonics. As is well established, the higher orders of distortion are most easily heard and more objectionable to the ear than lower orders, and odd orders more offensive than even orders. Thus it is possible to lower the level of total distortion products and yet make the distortion more audible and offensive to your ear.

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. In the case of a feedback amplifier, better control of speaker motion is said to occur because the speaker’s excess motion creates a voltage (the back e.m.f.) which enters the feedback loop via the amp’s output terminals. The amplifier then acts in a manner opposite the error signal to correct for it. However, like many theories, this is an oversimplification and, in practice, the opposite result may be obtained. There are several reasons:

1) The motion of a speaker’s voice coil former may not match the speaker’s acoustical output due to cone break-up modes and room acoustics.

2) The motion of the coil former is being sensed by the voice coil. The coil is designed to be a good driver, but is a lousy sensor, primarily due to its high inductance, which will create phase anomalies in the back e.m.f.

3) The back e.m.f. may pass through a cross-over network, which will again alter phase and frequency relations.

4) A differing back e.m.f. from another driver may be summed in via the crossover, making a composite signal that does not match either individual driver.

5) The speaker leads may cause additional phase shifts.

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 wrong thing with respect to the speaker’s acoustic output. Subjectively, I have noted that high feedback amplifiers tend to give the bass a “one note boom” on certain speakers, and tend to create an electronic glaze in the midrange, possibly attributable to this process.

Another interesting thing is that the damping factor number cited in spec sheets lacks a context. In an actual system, the damping imposed upon a speaker is dependent upon the resistance of the complete connection loop, which includes not only the source impedance of the amplifier, but also the speaker cable and the voice coil of the speaker itself (and, of course, crossover elements where a passive crossover is used). The old Audio Cyclopedia (2nd ed.) on page 1120 goes through an example of this (which the author terms ‘true damping factor’) for a simple, single 8 ohm loudspeaker driver. To summarize:

Interesting, isn’t it? No matter what one does, the true damping factor of the system never exceeds 1.33. There simply is no point in pursuing an astronomically high number for the spec sheet, and doing so will cause one to add complexities that tend to reduce the overall sound quality.

With regard to damping, we believe the best approach is to design the amplifier to have as low an output resistance as possible in a static sense and use little or no feedback.

As we often say, in a battle between theory and the real world, the real world always wins. Or, as Daniel von Recklinghausen once said, “If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you’ve measured the wrong thing.”