Damping Factor???

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  • 07-29-2004, 04:52 AM
    Damping Factor???

    Please help me!

    What does the Damping Factor of an amp mean? I use a NAD C370 and NAD C270 in a bi-amp configuration and B&W CM4 speakers. Iíve been told that B&W speakers need amps with a high Damping Factor, Why? What is this? The NADís have a Damping Factor of Ī150 I think, and wonder what this does to the sound. Iíve seen amp specifications where the Damping Factor goes over a thousand!!! :confused:

  • 07-29-2004, 05:57 AM
    Damping factor is an important principle of electrical power especially for operating electrical motors. Dynamic loudspeakers are linear electrical motors. It has to do with the control the source of power, in this case the audio amplifier output stage has over the motor's tendency to overshoot, resonate, or move on its own due to the mechanical inertia or momentum in its moving elements. Therefore, this pertains mostly to woofers. The moving mass of the woofer cone and voice coil assembly act as an electrical generator (all motors do, electric motors and generators are actually the same machine optimized for different purposes) generating a "back voltage" or "back emf" trying to drive current through the amplifer output stage. To the degree that the amplifier looks like a short circuit (zero output impedence) it surpresses this effect. The damping factor is technically defined as the speaker impedence divided by the source impedence. But the source impedence not only includes the amplifier output impedence, it includes the impedence of the speaker wire as well. This should explain many things you might not have understood up to this point. For instance, that is why heavier gage speaker wire is preferable to light gage wire especially for longer runs, why you will never see a resistor or an L-Pad (which puts resistance in series with the speaker) connected between a woofer and and amplifier, and why vacuum tube amplifiers which have inherently high output impedences tend to have boomier bass than solid state amplifers which generally have very low output impedences. The net effect is that any tendency for the speaker to have a false resonance in the bass region is exaggerated by using amplifiers with a low damping factor, thin gage wire, very long runs of wire, or the use of volume controls connected between the woofer and the power amplifier. On the other hand, for speakers which are inherently well damped mechanically, there may be no audible differences in this regard.
  • 07-29-2004, 08:25 AM
    I like hearing technical explanations for everything! :-) I don't know much about the damping factor, but I guess most DF >150 is pretty good, while DF>200 is even better. Anything over 400 is most likely undetectable unless you have very sloppy speakers.
  • 07-30-2004, 03:10 AM

    Thanks for the explanation. I'm not very technical when it comes to electronics, but I understand the basics of what the DF means, which actually leaves me with another question: Why, if the DF is so important doesn't the speaker manufacturer say what the minimum DF of the amp must be? Maybe he can add a list/table which says what DF the amp must have with ... lengths and Ö diameterís of speaker cable.

    Last week I bought 2 x 5m 2.5mm2 (QED's silver anniversary) speaker cable for my sisterís setup. Her amp is an NAD C350 with a DF of >150 and Iím looking for a new, affordable and good monitor speaker on stand for her. She now plays with DALI 5005 which I bought on sale a couple of years ago and of which Iím not happy about the way they sound. It would be of great help if the DF was added to the speaker's specifications.

    Thanks for replying,

  • 07-30-2004, 04:13 AM
    "Why, if the DF is so important doesn't the speaker manufacturer say what the minimum DF of the amp must be?"

    Good question. This is part of a much larger issue. Why doesn't the manufacturer list specific amplifiers that he feels his speaker would sound best with? Why doesn't he say what specifications an amplifier should have? To a minimal degree he does for instance by suggesting minimum and maximum power or what the impedence is so that if it is low, you don't buy an amplifier unstable with it. Perhaps for one thing, he knows that many people will use his speaker with amplifiers they already own and doesn't want to scare potential customers away. For another, many factors affect the sound the combination of speaker and amplifier choice will make, perhaps not all of them completely understood. In my experience different amplifiers do make speakers sound differently. Then you might ask why they aren't sold in matched pairs so that the amplifier is tailored specifically to a loudspeaker with active equalization included for it at the low signal level? For some reason, the industry didn't evolve that way. In many ways, pairing an amplifier with a loudspeaker is hit or miss. You'll see a lot of testimonials for that here and elsewhere describing how someone found a match made in heaven or something that just didn't work out. Then they go on to condemn one element or the other, usually the amplifier.

    The rule of thumb is that 16 gage AWG (American Wire Gage) copper wire is adequate for anything up to about 50 feet (16 meters.) To be on the safe side many audiophiles use 12 AWG which is heavier wire and is typical for the wire size used in the US for wiring a 20 amp power circuit to comply with the NEC (National Electrical Code.) You can research the web to find the equivalent wire gage in metric units. I'm sure any electrical supply house in Europe will have conversion tables as well. Good luck.