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8th February 2018, 12:00 AM  #1 
diyAudio Member
Join Date: Apr 2010

Can someone explain damping factor
I understand that it increases as GNFB increases. But how would one determine how much is needed. I have very efficient older JBL's with the 15" D130 LF drivers in smallish vented cabinets. Would these need more or less DF? I haven't done the calcs, but I believe that I am running about 15db of GNFB.

8th February 2018, 12:23 AM  #2 
diyAudio Member
Join Date: Feb 2004
Location: Hamilton, ON

Damping factor is defined by speaker impedance divided by amplifier output impedance. Since amplifier impedance is reduced when feedback is used damping factor increases. Damping factors over 15 or twenty is pretty much overkill. Almost all solid state amps meet this so i wouldn't sweat it.

8th February 2018, 12:36 AM  #3 
frugalphile(tm)
diyAudio Moderator

DF ~ 1/(output impedance)
Ideal output impedance is very much part of the amplifier/cable/speaker system. If the speaker has high mechanical damping higher output impedances are likely better, and underdamped speaker benefits from low output impedance. dave
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8th February 2018, 01:04 AM  #4 
diyAudio Member
Join Date: Jan 2017
Location: Melbourne

Some confusion about definition?
What about Damping factor  Wikipedia?
More importantly, why do we have to worry about this? Does a poor damping factor affect the sound quality or the amplifier stability? Regards ... 
8th February 2018, 01:17 AM  #5 
frugalphile(tm)
diyAudio Moderator

The Wikipedea definition only adds in the speaker impedance. Since that is far from flat the actual damping factor is a curve. The spec — useless IMHO — is typically grabbed at 1kHz and 8 ohms.
Whether high or low output impedance (or in between) is good is directly related to the speake rthat is being used with the amplifier. dave
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8th February 2018, 01:18 AM  #6 
diyAudio Member
Join Date: Mar 2007
Location: California

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.
I would assert that even with an 8 Ohm driver, if your damping factor is only 15 then you are still disturbing the frequency response by a little bit, but over a wide range of frequency. Is this then something terrible that must be avoided? Not necessarily. You just need to know what will happen when the amplifer's output impedance is in series with the load (the loudspeaker or driver). Remember, the driver presents a complex load to the amp. This load has an impedance magnitude that has one or more HUGE peaks unless the designer specifically added components to reduce this. Take a driver sans crossover. It might have Re=8 Ohms, but near the resonance frequency the impedance might be over 100 Ohms. At high frequency (depending on inductance) the impedance might slowly climb up to several tens of Ohms. The impedance is very "up and down". When you put this varying load in series with the amp's output impedance, and the amp's output impedance is not very small, what happens is that LESS POWER is delivered into the load when the load impedance is relatively LOW. Conversely, FULL POWER is delivered into the load when the load impedance is relatively HIGH. The difference between HIGH and LOW power in this case is the ratio of the amp's output impedance and the driver impedance as a function of frequency. Most of the time this is no big deal. It is only with tube amps (generalizing here) that this becomes an issue. Why? Because they often have noninsignificant output impedance WRT the driver. This essentially causes an "EQing" of the driver's output following the inverse of it's impedance curve. Some people actually like it, because it tends to accent low frequencies around resonance, and high frequencies where the impedance is rising. This just happens to "fix" some problems with some types of full range drivers. But in general it is not something good, because it is difficult to "control" from speaker to speaker, driver to driver, and amplifier designs will "sound" different if they have different output impedance. You are at the mercy of the impedance curve, or must implement lots of impedance compensation in your passive crossover. Using a very low output impedance amplifier (e.g. damping factor > 100) tends to take any concerns of this type completely out of the equation. Typically this means a solid state output stage. Last edited by CharlieLaub; 8th February 2018 at 01:21 AM. 
8th February 2018, 01:19 AM  #7  
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Join Date: Aug 2009
Location: Brazil

Quote:
Small DF will do a bass with little control as in tube amps.
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8th February 2018, 02:03 AM  #8 
diyAudio Member
Join Date: Aug 2009
Location: College Station, TX

Well, not really. The damping factor is defined as the dimensionless ratio of impedances, Z(load)/Z(source). Your definition (depending on precisely what is hidden in that twiddles sign!) seemingly has the damping factor being a dimensionful quantity with dimensions (ohms)^(1).
But damping factors are a bit misleading anyway. One would get the impression from the definition that if the source were "ideal," i.e. Z(source) = 0, then the speaker would be "infinitely damped." (For example, that if the speaker cone were pushed inwards a bit, it would take an infinite time to return to its equilibrium position.) But that, of course, is not true, and in fact is very far from true. The resistance of the load should really be added into the denominator of the ratio defining the damping factor. Resistance in the load has just as much effect as the same amount of resistance in the source, as far as limiting the damping is concerned. The situation is really quite a complicated one, when one looks properly at the dynamics of the speaker when it is driven by a source. A better measure of damping would be provided by something like the ratio Z(speaker)/R(total), where R(total) = R(speaker) + R(source). (Assuming, for simplicity, that the source (the amplifier) is purely resistive in its output impedance.) The question of how much of the speaker's impedance is accounted for by the pure resistance of the wire in the coil, versus how much is nonresistive reactance due to the dynamical motion of the speaker cone, is complicated. But quite a large proportion, depending on the frequency, is probably due to the resistance of the wire in the speaker coil, and that portion should be added into the denominator. 
8th February 2018, 02:07 AM  #9  
frugalphile(tm)
diyAudio Moderator

Quote:
Clearly DF=Z(load)/Z(source) implies DF ~ 1/Z(source) so exactly like i said. dave
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8th February 2018, 02:14 AM  #10 
diyAudio Member
Join Date: Aug 2009
Location: College Station, TX

Yes, obviously; that wasn't my point. The important point is that the damping factor, as a number, only has any absolute significance when the one impedance is divided by the other.

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