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Damping factor

kls9-x-over


This typical example gets the issue of Damping Factor into perspective. Amplifiers commonly have an output impedance of 0.1 Ohms. Loudspeaker cables will add 0.3 Ohms or so, and the low pas bass inductor of a loudspeaker – in this case a World Audio Design KLS9 measures 0.66 Ohms even with a Ferrite core (air cores have higher resistance). The bass unit 'sees' 1.06 Ohms when looking backward to the amplifier, giving a real life Damping Factor value of just 8. All the same, the amplifier still does apply useful electrical damping in subjective terms, our listening tests show.


WHAT IT TELLS US

This is an interesting measurement, seemingly of less consequence than bigger issues such as distortion, but with an obvious and simple link to sound quality. Damping factor is, indirectly, a measure of output impedance. If an amplifier has an output impedance of 1 Ohm and it drives an 8 Ohm loudspeaker, then the damping factor is 8 (i.e. loudspeaker impedance divided by output impedance). Of course, if a 4 Ohm loudspeaker is connected it becomes 4, suggesting damping has decreased, when in fact output impedance remains the same.


A loudspeaker is a standard reversible electromagnetic motor / generator. As a loudspeaker it is a motor: electrical current is driven into it and mechanical work is the outcome, in this case producing sound.


However, it is also a generator: tap the cone and the output terminals will produce a voltage. If you try and draw infinite current from a motor it will stall trying to deliver it, much like a car might stall if asked to do the impossible, like push down a big brick wall. Place a shorting link across the loudspeaker terminals and this locks the cone; it has stalled. Energy stored by the reactive mechanical elements is returned from the generator and dissipated as heat in the shorting link.


An amplifier presents a low output impedance to a loudspeaker and it similarly will stall the bass cone if a signal abruptly ceases, causing it to stop rapidly, rather than flap back and forth in uncontrolled fashion as it settles down. An amplifier with low output impedance grips the bass cone and produces ‘tight’ sounding bass, free of waffle and overhang it’s said, and our listening tests confirm this. High output impedance (low damping factor) valve amplifiers in particular produce soft and bloated bass with weakly damped loudspeakers. But audio is never simple and the picture turns out to be a little more complex, albeit interesting for what it suggests.


Let’s look at the apparently damning condition of no bass control met in a Single Ended amplifier and the sloppy bass that it promises. Loudspeakers are damped mechanically by the air load, magnetically by the motor and electrically by the amplifier. A loudspeaker can be acoustically over damped, in which case no extra electrical damping from the amplifier is necessary. Switching from a well damped solid-state amplifier, Musical Fidelity’s AMS50, to a lightly damped valve amplifier, an Icon Audio MB845 with a large number of loudspeakers under review shows that the latter consistently has bigger, softer bass. However, this better suits over damped ‘tight’ sounding loudspeakers so a Single Ended amplifier with no feedback and little bass control needs to be matched with a well damped loudspeaker. In the extreme case of a Tannoy Westminster Royal SE horn, acoustic damping is so great, electrical damping is of little consequence and other properties of an SE, namely low level linearity, come to the fore. So don't judge an amplifier by its damping facto; it's 'horses for courses' in this matter.


Most loudspeakers are a little under damped, their bass peaks up a few dB and it sounds a trifle soft, if ‘enthusiastic’, and in this case a transistor amplifier with a good damping factor is the best match. When loudspeakers move from design to initial prototyping, they will be driven by a transistor amplifier during listening tests and bass balance will be set, wittingly or unwittingly, to suit a well damped transistor amplifier.


A damping factor figure of around 20 is widely accepted as the break point for audibility with most loudspeakers and broadly speaking our listening tests confirm this. Transistor amplifiers exceed this figure by a good margin and there is little difference between them in perceived bass control (if not in bass quality). Naim amplifiers are an interesting exception with their DF of 15.


In conventional loudspeakers, a bass inductor lies in the signal path between amplifier and bass unit and limits the influence of high damping factors. A resistance of 0.3 Ohms will mean a DF of 27 is the best an 8 Ohm bass unit will see, or just 13 with a 4 Ohm bass unit (which most are nowadays). This makes high damping factors academic. Our real life example above shows even these figures seem a little high.


Valve amplifiers rarely exceed a Damping Factor of 20 and zero feedback Single Ended designs below 5 quite obviously impose little control over a loudspeaker’s bass cone.


So Damping Factor does affect bass quality, but there’s little difference between a majority of transistor amplifiers because their use of feedback results in low output impedance, around 0.1 Ohms, and a DF of 40 or more. Very high DF suggests an amplifier uses a lot of feedback.



HOW WE MEASURE IT

We measure the voltage driving an 8 Ohm and then a 4 Ohm load, with 40Hz sine wave at around 4V. Damping Factor is then derived by the formula -


2- V8/V4 / (2xV8/V4)-2  where V8 is voltage across 8 Ohms and V4 voltage across 4 Ohms.


 

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