Can someone explain damping factor
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diyAudio Member

Join Date: Jan 2002
Location: Earth
Quote:
 Originally Posted by Merlinb 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.
Yes, a fraction of the speaker impedance but don't forget the transmission line effects of the speaker cable. The latter is a concern for wide-bandwidth NFB amplifiers. Also, noise pick-up.
I reckon for most home systems an output resistance of <0.5 ohm is fine.
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DF96
diyAudio Member

Join Date: May 2007
Quote:
 Originally Posted by planet10 The twiddles is the symbol for "proportional to”.
No it isn't. Twiddle means 'approximately equal to'. The symbol for 'proportional to' looks a bit like a Greek alpha. See Wikipedia.

DF is a measure of output impedance, scaled by nominal speaker impedance. It only has much relevance at low frequencies, so it is the low frequency output impedance which should be used. However, as output impedance is quite likely to be inductive it is really only the resistive part which counts. What DF you need depends on the details of the speaker design, and what sort of bass you like.

What DF you achieve depends on amplifier architecture and amount of feedback. You can't say '15dB of feedback gives DF=N'; you can say '15dB of feedback with this particular circuit gives DF=N'. DF beyond about 20 makes little difference.

 8th February 2018, 10:09 AM #23 AndrewT   R.I.P.   Join Date: Jul 2004 Location: Scottish Borders Damping Factor (DF) on it's own is not important. Source impedance/resistance seen by the speaker voice coil is what really matters. DF = Rated speaker impedance divided by the output impedance of the amplifier at the specified frequency. One has to add on all the other resistances between the amplifier output and the speaker driver terminals. Starting from the amplifier output one sees that the speaker current travels through the sequence: speaker leads1 , amplifier output inductor, speaker terminals1, speaker leads2, speaker terminals2, speaker leads3, crossover inductor, speaker leads4. One should sum all these resistances and add them to the amplifier's output impedance at the resonant frequency of the speaker driver to end up with the resistance seen by the driver. You will find that the "other resistances" swamp the amplifier output impedance. Let's assign some numbers to that list. speaker leads1 5mr speaker leads2 10mr speaker leads 3 20mr speaker leads4 15mr speaker terminals1 10mr speaker terminals2 10mr crossover inductor 25mr amplifier output inductor 12mr Amplifier output impedance @ ~50Hz. 30mr. Gives a DF = 8ohms/0.03ohms = ~267 total resistances seen by the speaker driver = 137mr (add on the other terminals in the route) Damping effect of all those resistances relative to the 8ohms speaker driver = 8/0.137 = 58 The DF of 267 bears no resemblance to the effective damping of 58 Ignore DF and look instead at milli-ohms of effective resistance in the whole route from amplifier to driver and back to the amplifier. Now as an exercise, see what effect on the effective damping there is by changing the amp's DF from 100 to 1000 I reckon that DF was invented by a copy writer to try to make his employer's amplifier look good. "see how good I am, I have made your numbers look bigger than all your competitors" __________________ regards Andrew T. Last edited by AndrewT; 8th February 2018 at 10:15 AM.
cnpope
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Join Date: Aug 2009
Location: College Station, TX
Quote:
 Originally Posted by AndrewT Damping Factor (DF) on it's own is not important. Source impedance/resistance seen by the speaker voice coil is what really matters. DF = Rated speaker impedance divided by the output impedance of the amplifier at the specified frequency. One has to add on all the other resistances between the amplifier output and the speaker driver terminals. ... ...total resistances seen by the speaker driver = 137mr (add on the other terminals in the route)
I agree with everything you say so far. But you also need to add in the DC resistance of the speaker voice coil as well. This is needs to be added in to the denominator too. And subtracted from the numerator. It counts just as much as the other series resistances or impedances, as adding to the effective source impedance. And it completely dominates the total source impedance.

The effective equivalent circuit, simplified, is an ideal zero-resistance voice coil in series with a total resistance R(tot), and then driven from a zero impedance source. R(tot) is the sum of all the things you listed, plus the DC ohmic resistance of the voice coil. And this last item, being several ohms, is far bigger than all the other contributions to the effective source impedance that you listed. In the numerator should go just the effective impedance of the ideal zero-resistance voice coil in its dynamical setting as it oscillates in the magnetic field.

Last edited by cnpope; 8th February 2018 at 10:30 AM.

 8th February 2018, 10:32 AM #25 AndrewT   R.I.P.   Join Date: Jul 2004 Location: Scottish Borders The back emf tries to drive the source impedance. Short the drivers input terminals for a zero effective source impedance and you have a fully damped voice coil. It's the same as regenerative braking of an electric motor. One shorts the motor for fast deceleration without any lock up. Slot cars were a great example of this. __________________ regards Andrew T.
diyAudio Member

Join Date: May 2010
Quote:
 Originally Posted by AndrewT {snip} ... I reckon that DF was invented by a copy writer to try to make his employer's amplifier look good. "see how good I am, I have made your numbers look bigger than all your competitors"
You are close, Andrew. DF was first proposed as a measure of amplifier quality in the late 1950's/early to late 1960's when the first transformer-less transistor amplifiers entered the market, vs transformer-coupled (mostly vacuum state) amps in competition. Some of the advertising contained long treatises on why the new type of amplifier was superior to the old, damping factor being one, a factor that was essentially ignored in previous marketing literature.
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Last edited by Johnny2Bad; 8th February 2018 at 10:45 AM.

 8th February 2018, 10:43 AM #27 globalplayer   diyAudio Member     Join Date: Oct 2014 Location: =) So how do you explain the big difference in (controlled) sound between a high df amd and a low df amp on high powered subs then? __________________ ◢◣◢◣◢◣◢◣◢◣
diyAudio Member

Join Date: May 2010
Quote:
 Originally Posted by globalplayer So how do you explain the big difference in (controlled) sound between a high df amd and a low df amp on high powered subs then?
It is "explained" by the choice of LF driver, in closed systems (powered subs).

Properly designed sub systems do not necessarily have problems with control, regardless of the DF. I do not know of sub frequency systems that fail with "control" due to DF alone.

Also it is easier to design a good bass amplifier, so why not low measured distortion with feedback / high DF, as the sonic issues (high order distortions, odd order distortions) are out of the operating frequency bands, while the advantages are just sitting there waiting to be used.

Same reason why sub amps are inevitably solid state; it's the proper approach to high power, which is needed as frequency and driver motor requirements move toward 1 Hz.

It is like a tractor vs a moto GP motorcycle. There is no point in designing either with the technology of the other.

In other words it is done because they can, without repercussions.
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Last edited by Johnny2Bad; 8th February 2018 at 11:00 AM.

cnpope
diyAudio Member

Join Date: Aug 2009
Location: College Station, TX
Quote:
 Originally Posted by AndrewT The back emf tries to drive the source impedance. Short the drivers input terminals for a zero effective source impedance and you have a fully damped voice coil.
It depends what you mean by "fully damped." It is certainly as damped as you are going to be able to get, given that the voice coil has its own ohmic resistance.

Consider an extreme example where we replace the copper wire of the voice coil by an equal number of turns of high resistance wire, so that now the measured resistance of the voice coil is, lets say, 100 ohms. To say that this speaker would be "fully damped" if one shorted its terminals together would be misleading, to say the least!

The point I am making is an elementary one, which is easy to see if you think about the equivalent circuit. All the items you listed, PLUS the DC ohmic resistance of the voice coil, should be added in the denominator.

diyAudio Member

Join Date: Jan 2002
Location: Earth
Quote:
 Originally Posted by globalplayer So how do you explain the big difference in (controlled) sound between a high df amd and a low df amp on high powered subs then?
Also, you would need to compare on the same amplifier to be sure. Eg: choose the high DF amp and listen, then add some output resistance and listen.
How controlled the bass sounds is also to do with things other than DF.
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