Damping factor?

[JohnnieVan]

Hi guys, I have seen on spec sheets of amplifiers the term “damping factor” and would like to hear what its relevance is when choosing an amplifier also what the definition is. I know I can search it but I’d like some experienced human opinion. Is it just marketing fodder?

Thanks

 

[Stefan]

Jonnie, this is an interesting topic, and in my experience, it is not a marketing fodder. I leave the technical explanations to those who have a better handle on it. (aspects such as the damping factor of specific typologies such as single-ended designs and amount of feedback etc)

However, I do not believe there is a direct correlation between a high damping factor and sound quality. In some instances with specific speakers designs, amps with a low damping factor actually sound much better to my ears than those with a high damping factor. The inverse is also true, and certain speakers require the amp to control the bass system(driver and box alignment) of the speaker. Also, there is a view that after a certain point, a higher damping factor becomes almost theoretical.

As always for me one must view the amp and speakers as a system and not as independent components. The interaction between the amp and speaker is where the magic can be found.

 

[JohnnieVan]

Here are two examples from the Yamaha 90’s stable that have varied damping factor values

 

[Agaton Sax]

I am neither engineer nor amplifier designer. I am a lover of music reproduction so my facts may not be correct in absolute terms but close enough of how it will affect me.

With that caveat here goes: Damping factor is dependant on amplifier output impedance and speaker load . The lower an amp’s output impedance and hence higher its damping factor, the better that amp can deliver its power into a varying load. No speaker has an absolute flat load . In short the lower amp output impedance the the higher the damping factor hence the more accurate amp power delivery hence better sound . Right ? No. Take two girls. One is prissy and neat. Correct and accurate. The other is somewhat overweight ,blowsy and bralessly cheerful . Which do you prefer ? Low impedance a or less accurate but more lively higher output impedance b? We all differ but very few would prefer the extremes . Prissy, white frilly socks ,prune mouth a or fat , drunk tart, out of control b . We all (well most of us)would like something in between . I am so sorry if this sound sexist but I am to old to make this analogy fit the other sex.

So how do you manipulate an amp’s output impedance and hence damping ? You can use or increase negative feedback for a lower output impedance and get tighter , less free sound . You can also multiply output devices in parallel and (I think ) stick to NPN -PNP solid state output. Valves , single ended amps and simpler amps in general have lower damping. Which sounds better to you ?

Photorealism or Impressionism? On an emotional level both can satisfy but one enthrals even if it takes more effort . You choose . Thank goodness we are not al the same.

 

[Agaton Sax]

Duplicate . Deleted .Sorry.

 

[handsome]

Damping factor is the ratio of amp‘s output impedance (measured in ohms) to the speaker impedance (usually taken to be 8 ohms). The higher the damping factor the control the amplifier can exert on the loudspeaker’s cone(s). In basic terms if the amplifier has zero output impedance then any change of voltage at amp output will have maximum ”effectiveness” on the cone’s movement. If the amplifier has an output impedance of say 8 ohms then only half the voltage will reach the cone and be half as effective in moving it. Negative feedback reduces output impedance and thus increases damping factor. But too low an output impedance often results in the speaker cable having a higher resistance than the amplifiers output impedance so super high damping factors are probably irrelevant. Valve amplifiers tend to have low damping factors because it is hard to use lots of negative feedback around s valve amp’s transformer and the transformer itself add resistance. Single ended valve amplifiers are even worse as they often have zero negative feedback and thus often a damping factor of just 1.

Modern loudspeakers have very loose suspension (they are floppy) this allows their back and forth movement to be quite large and is necessary for a smaller cone to be able to reproduce more bass frequencies. Modern loudspeakers are much smaller than loudspeakers of yore. Such a loose suspension requires the amplifier to exert lots of control on the cone IOW have a higher damping factor. Old loudspeakers had larger cones with stiffer suspension (think pleated surrounds) that naturally have a lower amount of movement. They dont require high damping factors (valve amps - the tech of the time have low damping factors) since the speaker doesn't move as far and the stiffer suspension is more like a spring keeping the cone on the straight (and perhaps narrow ;-)).

No loudspeaker with a crossover has a flat impedance curve and an amplifier with a high damping factor will “ignore” these impedance variations and return a flatter frequency response. Amplifiers with low damping factors will do the opposite and their frequency response will vary according to the impedance curve.

Floppy modern speakers tend to have floppy indistinct bass when driven by amps with very low damping factors. High efficiency speakers like Lowthers, Altecs etc tend to sound “dull” with high damping factor amps, but very open and pleasant with low damping factor amps - especially single-ended valve jobbies - these loudspeakers have stiff suspension and limited cone movement, Lowthers move a maximum of a millimeter or so.

Obviously these are generalisations but this is how the mechanical (loudspeaker) and electrical (damping factor) interact. Damping factor is expressed as a number so obviously big numbers are better than small ones!

 

[ludo]

Damping factor is the ratio of amp‘s output impedance (measured in ohms) to the speaker impedance (usually taken to be 8 ohms). The higher the damping factor the control the amplifier can exert on the loudspeaker’s cone(s). In basic terms if the amplifier has zero output impedance then any change of voltage at amp output will have maximum...

As so often in matters audio, this sounds right, but it isn't. No offense to Handsome.

To illustrate. At the resonance freq of a speaker driver, we can easily have 5 times the impedance compared to the nominal (say 8 Ohm) impedance. So at that freq the amp with a damping factor of 320 has a damping factor of 1600. What does it do to control the movement of the speaker cone? Nokkol niks. Zip. Our only chance of taming the movement of the cone at resonance is correct speaker box volume and port tuning. The amp won't help us at all.

The amp will control cone movement better if the amp is a current source. Like a Howland current pump. But I don't think any standalone commercial amps are current pumps, as they have to be designed with the speaker impedance in mind, to compensate for the above mentioned driver resonance for instance. They go together and you can't mix and match amps and speakers then.

Strangely, an ideal current pump has an infinite output impedance so the most rotten damping factor possible. Could work well in audio as the cone movement (the force applied to the cone) is actually determined by the current in the voice coil, not by the voltage across the coil. The current pump supplies whatever voltage is required to establish the desired current in the voicecoil. My understanding is that this lowers driver distortion appreciably. I've toyed with the idea but never tried it in a power amp. On a much smaller scale, it has made the summing buss in daisy-chain comms systems a total joy though

So my vote goes to "Damping Factor in an audio power amp is complete nonsense." Whether there are formulas and measurements to determine it accurately or not. I love measurement, but more as a sanity check. And sanity is often a slippery idea.

Use the amp that has enough power and that sounds right with your speakers. Understanding why it works is probably impossible. Too many notes. Just fun to try.

 

[naboo]

For what it's worth...

 

[naboo]

As so often in matters audio, this sounds right, but it isn't. No offense to Handsome.

To illustrate. At the resonance freq of a speaker driver, we can easily have 5 times the impedance compared to the nominal (say 8 Ohm) impedance. So at that freq the amp with a damping factor of 320 has a damping factor of 1600. What does it do to control the movement of the speaker cone? Nokkol niks. Zip. Our only chance of taming the movement of the cone at resonance is correct speaker box volume and port tuning. The amp won't help us at all.

The amp will control cone movement better if the amp is a current source. Like a Howland current pump. But I don't think any standalone commercial amps are current pumps, as they have to be designed with the speaker impedance in mind, to compensate for the above mentioned driver resonance for instance. They go together and you can't mix and match amps and speakers then.

Strangely, an ideal current pump has an infinite output impedance so the most rotten damping factor possible. Could work well in audio as the cone movement (the force applied to the cone) is actually determined by the current in the voice coil, not by the voltage across the coil. The current pump supplies whatever voltage is required to establish the desired current in the voicecoil. My understanding is that this lowers driver distortion appreciably. I've toyed with the idea but never tried it in a power amp. On a much smaller scale, it has made the summing buss in daisy-chain comms systems a total joy though

So my vote goes to "Damping Factor in an audio power amp is complete nonsense." Whether there are formulas and measurements to determine it accurately or not. I love measurement, but more as a sanity check. And sanity is often a slippery idea.

Use the amp that has enough power and that sounds right with your speakers. Understanding why it works is probably impossible. Too many notes. Just fun to try.

I agree that out of the many variables, this is just another and not likely to influence the sound you hear too much.

Also, my understanding is that the damping factor must be considered with a speakers stiffness etc. to confirm suitability.

So just for me to understand better, what about an open baffle speaker, where there is no box? Only certain speakers are suitable for this application, but I have never given consideration to the amp.

Perhaps it is a worthwhile experiment to do speaker measurements eg spectral decay on OB speakers with amps with very different damping factors to determine whether it has an influence?

Would love to hear from more experienced speaker builders... @BrianB ?

 

[handsome]

Damping factor is just a convoluted way of stating an amplifiers output impedance. Modern solid state amplifiers are voltage sources and thus have a theoretical output impedance of zero. Damping factor does have an effect on sound dependent on amplifier and speaker but, once damping factors are higher than ~60 it is thought to be irrelevant. Many Stereophile reviews used to show the result of a speakers uneven impedance in combination with an amplifier's output impedance - the lower the damping factor the more the frequency response would be affected. How this actually sounds of course is less defined. I have a valve amp with a ~4 ohm output impedance that sounds lovely with a lot of speakers but with a pair of small PMCs it sounded distinctly shit - ill-defined and undynamic. Those same speakers with a Naim amplifier are very dynamic and clear - but these are just the results of my ancient and rather hairy ears of course. Interestingly, back in the day, several valve amplifiers used to offer variable damping and the control was often used as a kind of tone control. Since damping factor is the ratio of amplifier impedance to speaker impedance obviously it would vary drastically especuially at resonance - hence my statement that it is just a convoluted way of stating output impedance. Current drive is an extremely logical way to drive a loudspeaker but unfortunately in order for it to work the amplifier must be matched to a given speaker. Voltage drive allows for anything (hello Krell!) to be driven and thus is a more practical route to take especially for mass-produced or commercial products. Current drive would also require (I think) very, very high supply voltages - and if you have those, a swift kick in the arse would be the most fitting response for the rest of the amplifier not having any valves in it!

 

[charles]

From the Sansui BA303 tube audio power amplifier's service manual: