Damping Factor >1000

[lumanauw]

Can anyone explain to me what is damping factor? I read in handbook that it has to do with output impedance/internal impedance. But how can internal impedance of a power amplifier be measured (or be calculated?).
At first I think it has to do with how many output transistors we use. More output transistors gives more damping factor. But I'm quite surprised when I look at a certain professional studio amp (that has damping factor >1000), it has a very few output transistor. So my assumption is wrong.
I tried to design an ordinary amp with 3 stages (differential, VAS, current amp), but it's goal is to pursue damping factor >1000. What is the key to pursue high damping factor in 3 stages power amp? Please someone share the knowledge.

[PaulHilgeman]

You really dont need one that high.

Anything greater than about 200 is fine, and it has been proven to be inaudible above that. I dont remember the sites that go over this, but they are out there.

[Steve Eddy]

Originally posted by lumanauw
Can anyone explain to me what is damping factor? I read in handbook that it has to do with output impedance/internal impedance. But how can internal impedance of a power amplifier be measured (or be calculated?).

Damping factor is simply the ratio of the nominal load impedance divided by the amplifier's output impedance.

So if you're looking at a nominal 8 ohm load and the output impedance of the amplifier is 0.1 ohm, the damping factor is 8/0.1 or 80. For a damping factor of 1,000, the amplifier's output impedance would have to be 0.008 ohms.

At first I think it has to do with how many output transistors we use. More output transistors gives more damping factor. But I'm quite surprised when I look at a certain professional studio amp (that has damping factor >1000), it has a very few output transistor. So my assumption is wrong.

No, you're on the right track. You can reduce output impedance by paralleling a whole bunch of output transistors. You can also reduce output impedance by way of negative feedback. Or both. So while the amp with the high damping factor may have just a few output transistors, it would likely be incorporating a high amount of negative feedback.

I tried to design an ordinary amp with 3 stages (differential, VAS, current amp), but it's goal is to pursue damping factor >1000. What is the key to pursue high damping factor in 3 stages power amp? Please someone share the knowledge.

Is the goal of a damping factor greater than 1,000 purely for the sake of doing it? If not, then I wouldn't bother.

se

[kiwi_abroad]

What's the point of a stupendious damping factor, implying very low output impedance, when your speaker cables will add around 0.05 to 0.1 ohms.
Oh, and don't forget the DC resistance of 4-6 ohms of your woofer, which is the 'real' resistance of the voice coil.

Adrian B.

[Steve Eddy]

Originally posted by kiwi_abroad
What's the point of a stupendious damping factor, implying very low output impedance, when your speaker cables will add around 0.05 to 0.1 ohms.

Yeah. A bit pointless for your amplifier's output impedance to be an order of magnitude lower than the resistance of the speaker cable.

Oh, and don't forget the DC resistance of 4-6 ohms of your woofer, which is the 'real' resistance of the voice coil.

But it's not the voice coil's DC resistance that really matters. It's the loudspeaker's impedance which for typical dynamic type loudspeakes will vary considerably. So if the amplifier's output impedance is high enough, it can result in some rather significant variations in the loudspeaker's frequency response.

Here's an example of how wild it can get using a SET tube amp driving Stereophile's dummy loudspeaker load which is 2uF in parallel with 8 ohms:



Though the more fundamental context of "damping factor" is about the amplifier's "control" over the loudspeaker at resonance. Dick Pierce *wince* wrote a nice article about this in Speaker Builder some years back.

There's copy of it over on diyspeakers.net. Just look under Articles.

se

[uvodee]

I overheard a teenager salesperson at Frys explaining to a middle aged couple that damping (he called it dampening) factor is from the utmost importance.
it can only be achieved in expensive stuff he said, it is the difference in travelspeed between high freq and low freq inside the system and must therefor be the same hence one uses damping factor to mark it. To bring the high freq speed down to the low freq speed is a complex issue and is very expensive!!!!!
I saw him selling a Sansui (if my memory serves me wright) with some spkrs ( i think Polk) and other stuff, probably around 3k ...... within a few minutes....

Jean-Pierre

[phase_accurate]

When it comes to taming a driver's fundamental resonance, Rdc of the driver is by far the largest resistance that comes into play.

Regards

Charles

[millwood]

Originally posted by uvodee
damping (he called it dampening) factor

I actually think dampening factor is the correct term rather than damping factor, if you think what this "factor" is trying to measure.

Originally posted by uvodee
I saw him selling a Sansui

I thought they had gone out of home audio.

[MarcelvdG]

A loudspeaker box designer can take the DC resistance of the voice coil and cross-over inductors into account, such that the quality factor or factors of the low-frequency resonance(s) have the right values and a nice response results when the box is purely voltage driven.

Any resistance from the amplifier is not taken into account by the box designer and increases the quality factors above the optimal values.

An equally valid way to look at it is to assume the loudspeaker to be designed to have a more or less flat response from the voltage between its input terminals to the sound pressure, and to calculate the response deviations in loudspeaker terminal voltage due to frequency-dependent voltage division between the loudspeaker on one side and the amplifier and cable on the other side.

Anyway, the difference in response between a damping factor of 100 and a damping factor of 1000 is negligible, unless there are very deep troughs in the impedance curve of the loudspeaker.

[ashok]

In connection with some discussion on this topic , someone mentioned a link to a test report on damping factors. I can't remember where I read this ( on the Net of course).
It said that at damping factors above 15 or so it becomes very hard if not impossible to hear any improvements. I would also guess that they assume that the real world impedance of the speaker system is reasonably uniform over its bandwidth.

( If we talk of the damping factor with reference to bass response only , the factor 15 makes more sense).

For systems with a very large variation in impedance ( over frequency ) a reasonably high damping factor will ensure that the ( electrical ) frequency response at the speaker terminals is really flat. With a low damping factor you will get a pretty variable response as can be seen in the wavy graph in the earlier post.
So the two factors are kind of tied to each other.

Can you imagine what must be happening at the speaker terminals with SE amps and similar circuits !

Most of the time we say something is good because we like it that way. That's good because it leaves the field wide open for DIY projects. We will never get to the end !
And all the time we are having fun !
Cheers.
ashok.