Hephaestus Audio

An amplifier’s damping factor is a rating that gives a feel for the amplifiers control over a load.  It is essentially a measure of the output impedance of the amplifier – a value that would be zero for an ideal amplifier (i.e. the ideal amplifier would be load-independent).

Like many measurements of an amplifier’s performance, this one is the victim of misinterpretation and measurement trickery:

First, the exact measurement point has a huge impact on the resulting value.  For example, taking a measurement right at the feedback points of the amplifier tends to give a high damping factor, as this is the actual point the amplifier is attempting to regulate.  Unfortunately, this is rarely where the user connects the load!  The user generally uses the externally accessible speaker connector, so this is the chain that results: the wires/traces from feedback nodes, the contact resistance of the speaker connector, and the external cables that connect to the load.  When this more realistic chain is considered, much more modest damping factors result, however they are much more representative and useful for the user.

Second, the bottom line is that if the amplifier’s effective output impedance is much lower than the minimum impedance of the loudspeaker being driven, there is little to be gained from further increasing the damping factor.  In fact, it may very well be detrimental, given that the typical means of increasing the damping factor is by increasing feedback.  If the feedback is increased excessively it may impact stability and result in lower-amplitude, but more objectionable, harmonics – this will be the topic of a future entry.

The audio industry is a small one, but luckily there are much larger industries that can be drawn upon both for inspiration and for the components particular to that industry.  For example, many of the MOSFETs and IGBTs available have been driven by the automotive and telecom industries, but they are also well-suited to audio.

Robotics is a good example of a field that can be used for inspiration.  For example, one particularly useful and powerful conecpt is that of “subsumption architecture” developed by Rodney Brooks.  This approach builds a robot up in functional layers: the innermost dealing primarily with functions such as movement and safety, and the outermost dealing primarily with high-level functions, such as path-planning or communication, that are sometimes able to override the lower-level functions.  As applied to audio amplifiers, this concept leads one to build a robust amplifier core that can drive the required load, protect against a short, etc. and then to augment it with layers of higher-level functionality and protection.

One of the earliest lessons I learned in amplifier design was the very fine line between reliability and usability.  If an amplifier is designed to protect aggressively against any possible deviation from the norm, then it is likely a user will trigger a protective measure during legitimate use.  Conversely, if the amplifier is designed solely with the idea that “the show must go on”, then it is likely the user will at some point do something the amplifier really should protect against.  If subsumption architecture is applied, then it allows for the best of both worlds: an amplifier that can protect itself from legitimate abuse via the inner layers, and an amplifier that allows the show to go on via the intelligence located in the outer layers.

The following are wonderful examples of audio companies using their websites both for commercial and for educational purposes.  My hope is to live up to the standard they set by helping to expand the body of knowledge in audio and by providing the sort of products my customers love.

Elliott Sound Products

Linkwitz Lab

Pass Labs

Rane

The following websites do not have a commercial angle to them, but they are indispensable resources.  They have been tremendously helpful to me in my products, projects, and overall audio knowledge.

Nutshell High Fidelity

Art Ludwig’s Sound Page

It is difficult to relate the intricacies of one’s approach to audio design, but these three articles do a reasonable job of summing up my design philosophy:

One of the most important requirements for designing a really good switching amplifier is Solid Execution.

A general critieria for designing good sounding audio equipment is given by the concept of Disappearing Derivatives.

Objectivists versus Subjectivists and the Engineering Perspective.