Can someone explain damping factor

[planet10]

Damping factor is a very misleading spec and not nearly as useful as knowing the output impedance (Z(source), the speaker impedance (Z(load)) and how well damped the LF alignmnet of the speaker is.

There is an appropriate output impedance for any speaker. Most modern commercial loudspeaker is designed assumming a low output impedance (ie voltage source), but in the diy world we have a lot more choice.

Since a speaker is driver is a current controlled device (not a voltage controlled one) there are advantages to amplifiers with high output impedance (current amp).

What is appropriate in the amplifier is very much dependent on the speaker to be used with it. They should not be considered separately.

dave

[Koonw]

This article may have the answers to your questions, noted the effect on frequency response of various damping. Once you have determined the critical damp factor of the speaker then you can adjust NFB for amp to lower or raise output impedance to match the critical damping so that the frequency response becomes flat. The output impedance of the amp and speaker damping factor need to be found out separately of course.

[dotneck335]

Originally Posted by CharlieLaub

If your driver has Re= 1 Ohm and your Damping factor is 15 then you can expect frequency response abnormalities. Why? Because the output impedance = 1/15 ~ 0.7 Ohms is on the order of the driver impedance. The amp and driver appear in series, and act like a voltage divider.

Methinks your math is in error. 1 ÷ 15 = 0.066Ω, not 0.7Ω.

[cnpope]

Originally Posted by planet10

Damping factor is a very misleading spec and not nearly as useful as knowing the output impedance (Z(source), the speaker impedance (Z(load)) and how well damped the LF alignmnet of the speaker is.

dave

Yes, indeed, I agree with you about that. But also, as I was saying, the purely resistive component of the speaker's impedance should really be adding into the denominator, not the numerator, in the ratio defining the damping.

One could roughly model the speaker as having an ideal zero-resistance coil (e.g. made of superconducting wire), in series with a resistor R(load). The measure of how well damped it will be is then given essentially by Z/R(tot), with R(tot)=R(load)+R(source), and Z being the impedance (highly frequency dependent) of the superconducting coil in its dynamical environment as it moves, on the speaker cone, in the magnetic field of the speaker magnet.

I just measured the DC resistance of one of my Lowther DX3 speakers, and it is about 7 ohms. Looking at the impedance plots for the DX3 there is a huge peak, of order 100 ohms or more, at about 60 Hz. Outside the range 30 - 130 Hz, the impedance is about 10 ohms or less. So outside the 30 - 130 Hz range, the resistance of the coil accounts for most of the impedance of the speaker. This means the "true" damping factor over most of the audio range will never be more than about 2 at the most, even if the source has virtually zero output impedance. And whether the source has 0.01 ohms or 1 ohm output impedance hardly affects the damping at all.

[Merlinb]

Originally Posted by EL34Dave

But how would one determine how much is needed?

Get a solid state amp (it has oodles of damping factor). Now add some resistance in series with the output, and listen. Do you like the sound? Try a different value. Eventually you will find a value that suits your speakers and your ears, and you can try to emulate that value in the next amp you build.

[globalplayer]

Originally Posted by EL34Dave

But how would one determine how much is needed.

On high powered subs about as high a df as you can get.
(at least 300, better 500 - 2000+).

Use "thick wire" also.

Some will say it doesn`t matter but clearly they didn`t experiment themselves.

[traderbam]

Originally Posted by Merlinb

Get a solid state amp (it has oodles of damping factor). Now add some resistance in series with the output, and listen. Do you like the sound? Try a different value. Eventually you will find a value that suits your speakers and your ears, and you can try to emulate that value in the next amp you build.

Except that you may be altering that amplifier's stability as you change the resistor, so what is heard may be a combination of the two. But I think this is a good way to see just how unimportant very high DF is.

[scottjoplin]

Thanks Koonw, that's a very interesting article that actually answers the question

[Merlinb]

Originally Posted by traderbam

Except that you may be altering that amplifier's stability

Yes I suppose so, although the resistance you use will probably be only a fraction of the speaker coil resistance, so I suspect the stability change won't be very much in most circumstances.

[globalplayer]

I am missing measurements of group delay, distortion and impulse response in that article.
FR alone isn`t telling the whole story.