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Damping Factor Specs


Several people have asked us about damping factor, as there are wildly different damping factor specifications stated for amplifiers.


A few companies have used damping factor as a marketing tool, and yes, with huge amounts of negative feedback, you can achieve very high damping factor. The negative feedback lowers the output impedance. However, huge amounts of negative feedback almost always yield poor sounding amplifiers, so this is not a good engineering choice from our perspective.


So what exactly is damping factor and how is it measured, and more importantly how should it be measured and presented ?


Damping factor is the ability of the amplifier to resist a change in its output voltage. The damping factor is expressed as a ratio of the rated loudspeaker load impedance and the output impedance of the amplifier.


For a typical 4-ohm, loudspeaker used with an amplifier with an output impedance of 0.004 ohms, the damping factor is 1,000. (4/0.004 = 1,000)



However, is that the full story? No.



First, the real measure of damping factor must include all the real contact resistances that are in series with the amplifier output impedance. As the speaker circuit is a series circuit, the correct measurement for damping factor must include the following additional resistances:


* The contact resistance of both the PCB tab and the wire connecting a class D module to the amplifier terminals. (The ICE class D solution rates the output impedance of their product at a tab on the PCB).

* The contact resistance of the amplifier terminal. If we were to measure the total output impedance at this point, it might be on the order of 0.02 ohms. The damping factor is then 200. (4/0.02 = 200)


The following is a quote from the B & O website regarding output impedance :


" The measurement is done at three different points to illustrate the impedance of connectors and wires. The curve showing the lowest impedance is measured directly on the PCB. The next curve is measured at the terminals of the ICE power application connector. The curve showing the highest impedance is measured at one pair of the four banana plugs on the ICE power application connector (10cm wire)."


Just a few little resistances added in and the damping factor is reduced from 1,000 to 200!


But, it does not stop there…



* The contact resistance of the speaker wire to amplifier terminal.

* The actual resistance of the audiophile speaker wire.

* The contact resistance of the speaker terminals on the actual speaker system.

* The contact resistance of the wire from the speaker terminal to the crossover network.

* The contact resistance of the crossover network terminals.

* The contact resistance of the crossover wire to the voice coil leads of the speaker.


Granted, the above are all very low resistances. But they exist and one cannot simply ignore them.


As noted previously, the actual damping factor is a series circuit, so the output impedance of 0.004 ohms is now in series with all the above, and the real series output impedance is now in the range of 0.05 ohms.


The new damping factor, same amplifier and real world example, the damping factor is now 80. Quite a difference from the initially quoted 1,000.


Now there is one hidden little gem, the DCR of the voice coil. For a 4-ohm speaker it is typically 3.2 ohms. As this resistance is additive with the others, as they are all in series, the total series impedance is now 3.25.


This yield a damping factor of 1.23 !



The point of all this is that damping factor is a misused specification.


It is often stated that a high damping factor yields better bass control. Well it is really not that simple. Things like the amplifier power supply and regulation, the current delivery of the amplifier, the drive into low impedances, etc. have a much bigger influence on bass control and slam, etc, then damping factor.


Also, keep in mind that there are some superb sounding tube amps that have low damping factors. 



Good Listening from Blue Rose / IQ.






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