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Old 1st May 2004, 06:42 PM   #1
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Default Damping factor

Does anyone know how much of damping factor could I expect from bridged TDA7293 amp ?
I am planning to build two of these amps for my new active subwoofer project and the speakers I have need big damping factor to sound right.
I need about 120W of undistorted output per channel on 8 ohm speaker and a lot of damping factor.
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Old 2nd May 2004, 09:22 AM   #2
b<ben>n is offline b<ben>n  Australia
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this is probly a stupid question but what is damping factor and what dose it do.
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Old 2nd May 2004, 01:00 PM   #3
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It is ability of amplifier to control the speaker cone. It is the opposite of output impedance (DF=1/Zo). When DF is very low speaker cone will not stop moving when polarity of input signal changes. Than bass loses strength.

You can calculate DF of your amplifier with this formula:
DF = U1 / (U2-U1)
U1 = output voltage of your amplifier with speaker connected
U2 = output voltage of amplifier with same input signal, but speaker disconnected.
It is best to use sinewave signal generator for this purpose.

DF is important when using big speakers in big boxes (like subwoofers)
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Old 2nd May 2004, 01:17 PM   #4
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Quote:
Originally posted by b&lt;ben&gt;n
this is probly a stupid question but what is damping factor and what dose it do.
This is kinda one of my 'hobby horses' - if you know what I mean. Let me explain:

A value that is calculated by the ratio of the Load Impedance (suposedly the speaker) and the output Z of the amp. Say the load is 8 Ohm and the amp's output Z is 0.1 Ohm, ergo 8/0.1 = 80; the damping factor is 80. It is of rather inflated importance IMHO.

Solid State has high DP. Tubes, especiallly zero or low f/b has low DP. But is it related to sound quality? Not really.

I am highly suspicious about that DP measured under static conditions (which is the only way to measure it) has very little with DP under dynamic conditions. Rather than low static output impedance, what is more important is stable or predictable output Z ????

Besides, DP is often the end result of applying feedback, so more f/b and we get lower DP. Too good to be true? You bet!

These days few really mention DP. What is more important is perhaps how high output Z modulates the frequency response of loudspeakers. Here, strangely, even though Tubes are supposed to be pour in this respect, they usually shift the response around less than one might expect. Wonder why?

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Old 3rd May 2004, 04:43 PM   #5
gmarsh is offline gmarsh  Canada
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I've never understood the point of chasing damping factor.

The output resistance of an amplifier, the result of a finite damping factor, certainly does exist - but it's still incredibly small compared to that of the winding resistance of the speaker it's driving.

And amp resistance, speaker cable resistance and winding resistance produce the exact same result - they act as one big resistor in series with the reactive impedance* of your speaker which is the part of the speaker's overall impedance that actually produces music. All energy that goes into the winding resistance of a speaker only produces heat.

*eg, the L + L||R||C part of a Thiele-Small model.

so why isn't anyone trying to make 0.1 ohm speakers? hell, even going from an 8 ohm speaker to a 4 ohm speaker would be a gross improvement over trying to trim the output resistance of an amp from 0.02 to 0.01 ohms. And attempting to accomplish the latter by excessive feedback cound render an amp unstable and bad sounding...
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Old 3rd May 2004, 08:41 PM   #6
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Have you ever tried to knock on the bass speaker cone and listen the sound when speaker is disconnected and when speaker leads are shorted? You will hear a big difference in depth. The same is with amplifiers. Amplifier with low output impedance acts like a short circuit to speaker. In that case movement of speaker cone will follow input signal with no delay when changing directions.

The example of this situation are my sub and sub of my friend. This 2 subs use the same speaker in the same enclosure. One uses 120W amp with big damping factor, and other 120W amp with low damping factor. Sub which uses amp with lower DF makes much bigger amplitudes (about 50%), but with no audible benefit It sounds too soft and reaches it's bottom with only half available power applied. The other one wants to rip down my walls and is able to stand the full power from the amp without reaching the bottom.
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Old 3rd May 2004, 09:16 PM   #7
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Quote:
Originally posted by Cro maniac
It is ability of amplifier to control the speaker cone. It is the opposite of output impedance (DF=1/Zo). When DF is very low speaker cone will not stop moving when polarity of input signal changes. Than bass loses strength.

You can calculate DF of your amplifier with this formula:
DF = U1 / (U2-U1)
U1 = output voltage of your amplifier with speaker connected
U2 = output voltage of amplifier with same input signal, but speaker disconnected.
It is best to use sinewave signal generator for this purpose.

DF is important when using big speakers in big boxes (like subwoofers)

Yeah thats prety much what ive been told before too when i used to make diyamps about 8-10yrs ago. Basically the higher the DF the more responsive the speaker is, its rather important in subwoofers, especially when listening to high integral music like drum and bass. Running anything with high integrety and a low DF will destroy your speakers, especially over 100w continuos rms.

Trev
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Old 4th May 2004, 05:22 AM   #8
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Quote:
Originally posted by Cro maniac
It is ability of amplifier to control the speaker cone.
That's how it's commonly described. But it seems to me that it's not so much a case of the ability of the amplifier to control the speaker, but rather the ability of the amplifier to allow the speaker to control itself.

"Control" ultimately refers to the speaker's resonant behavior. If you have a speaker with a Qts of say 0.7, it'll have a Qts of 0.7 whether it's hooked up to an amplifier with a source impedance of zero Ohms or simply has its terminals shorted and no amplifier hooked up to it at all.

Similarly, if that same speaker has a Qts of say 1.2 when the source impedance is say 2 Ohms, it will have that same Qts if it simply has a 2 ohm resistor across its terminals.

Also, if you drove the speaker from an infinite source impedance (i.e. an ideal current source), its Qts would end up being its mechanical Q, Qms and would have that same Qts if its terminals were left an open circuit.

se
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Old 4th May 2004, 09:01 AM   #9
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Quote:
Originally posted by Steve Eddy


...not so much a case of the ability of the amplifier to control the speaker, but rather the ability of the amplifier to allow the speaker to control itself.

"Control" ultimately refers to the speaker's resonant behavior. If you have a speaker with a Qts of say 0.7, it'll have a Qts of 0.7 whether it's hooked up to an amplifier with a source impedance of zero Ohms or simply has its terminals shorted and no amplifier hooked up to it at all.

Similarly, if that same speaker has a Qts of say 1.2 when the source impedance is say 2 Ohms, it will have that same Qts if it simply has a 2 ohm resistor across its terminals.

se
Hi Steve

There are good reasons to have low Z when amp is driving a sub. But has it got anything to do with damping factor? I think you might on the right track here as I don't really think so either.

You mention a driver Qts of 0.7, but that is a component of Qe and Qm. Let's say the Qe component is 0.8, then a little playing with a calculator will tell us that Qm = 5.5

Now let's say that the source impedance is high at 1 Ohm. Measure the DCR (Re) of the driver, and let's say it's a 4 Ohm sub driver and Re = 3 Ohm.

The 1 Ohm is in series with the 3 Ohm and this erodes the Qe by the same ratio:

0.8 x (4/3) = 1.066

Now a bit more fingering on the calculator will tell us that the original Qts has eroded from 0.7 to 0.9 !!!

If the sub is sealed (good subs are) then the sealed box alignment has changed. If we put all the parameters into a suitable program, then analyse closely the cone amplitude/excursions, we see clearly that cone excursions at critical frequencies are now considerably larger.

What's this got to do with damping factor? Not much, and this is why: The output Z of the amp and the Re are in series, so in reality the best real case scenario is ONE - or unity damping factor.

So damping factor really is a myth. It's an arbitrary mathematical curiosity that has no real world import.

On the other hand, Low Z is important to keep bass alignments on track.

Oh, mea culpa, what is DP? Damping Phactor ?

Joe R.
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Old 4th May 2004, 10:27 AM   #10
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Quote:
Originally posted by gmarsh
*eg, the L + L||R||C part of a Thiele-Small model.
Just for the record, Small actually makes the assumption that the output Z of the amp is small compared to the Z of the speaker. His analysis does not extend to the case where the amp has a large output Z and the DF is "small".
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