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Old 5th May 2004, 08:25 AM   #21
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Quote:
Originally posted by Joe Rasmussen

[snip]The output impedance, instead of approaching zero, is intentionally set to the impedance of the driver, - a damping factor of one or unity coupling. This requires realigning the bass enclosure... The impedance seen by the speaker driver is constant and equivalent to a fixed resistor. This is the essence of the 'hard amp' concept: the amplifier mimics a passive component under all operating conditions... This is very different from conventional [high feedback] transistor or PP pentode amplifiers, which enter undefined regions when the amp clips and feedback loses its grip on the amplifier... a 'hard amp' avoids gain transitions and treats back-EMF like a fixed low value resistor.[snip]

Hi Joe,

The trouble I have with the Olson hard amp concept is that if there is clipping or other non-linear behaviour like switching from clA to clAB or B, the output impedance will also be undefined. For instance, at clipping, the output level cannot increase so the output impedance as seen be the speaker looks like infinite. This is no different than with an amp with say .1 ohms Zout. So what's all the fuss about?

Jan Didden
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Old 5th May 2004, 08:52 AM   #22
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Quote:
then perhaps we can agree that any claimed benefits are at least overstated.
Definitely !!!!

But Olson's proposal is also an extreme case either. Me personally, I prefer reasonably balanced approaches. I.E. neither the amp with 0.000000001 Ohms output impedance nor the one with 6 Ohms.
An amp that has an output impedance that is consistent (i.e. independant of frequency, output voltage and output current) would be my choice.
It's output impedance can easily be taken into account when designing the box for a given target Qtc, as mentioned by Joe.

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Charles
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Old 5th May 2004, 09:31 AM   #23
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I agree that the term "damping factor" is misleading. I also agree the Zsp/Zout is not a good indication of the amps ability to "absorb" the back EMF generated by the motion of the speaker. However, all amps have this characteristic to one extent or another. How else can it be quantified?
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Old 6th May 2004, 08:31 AM   #24
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Quote:
Originally posted by roddyama
I agree that the term "damping factor" is misleading. I also agree the Zsp/Zout is not a good indication of the amps ability to "absorb" the back EMF generated by the motion of the speaker. However, all amps have this characteristic to one extent or another. How else can it be quantified?

Hi Rodd

That largely puts the finger on it: The ability of the amp to absorb back EMF is what is supposed to be behind the supposed logic of damping factor. But this thinking has a serious flaw:

The fact is that ALL amplifiers are poor in this respect because of the DCR or Re of the voice coil. Think about it. This severely limits peak current that can be absorbed back into the amp's output stage since it has go through anything from 3-4 Ohm (of a typical 4 Ohm speaker) or more. In effect, there is always a large value DC resistor beween speaker and amp!

Really, and this is an oft misunderstood concept, it is primarily the Qe of the driver, no matter what its Re is, that is the source of damping, not the amp. What the amp must do, or really not do, is erode that Qe. So ultimately it's the driver itself and the box alignment that dertermines damping, transient response etc. I know this because I've put in a lot of hard yards on this topic, also been able to consult some fairly enlighten sources on this. I might also add that there was a time when I didn't quite get it myself.

Now I am by no means relegating the importance of the amp. The amp must still cope with considerable back EMF, not so much for damping purposes but at least some. But the amp must be designed to have good bandwidth and not rely on feedback to get low Z out. Make the amp as open-loop linear as possible and the bass of a well designed speaker will be just fine.

We haven't even mentioned the potential danger of back EMF getting into the feedback loop of the amp... ?

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Old 6th May 2004, 11:48 AM   #25
moamps is offline moamps  Croatia
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Quote:
Originally posted by Joe Rasmussen
The fact is that ALL amplifiers are poor in this respect because of the DCR or Re of the voice coil.
Hi,
I don't think so. IMHO, almost ALL speakers are poor because have coils with relatively high and temperature dependent resistance.
Someone can find very good and short "DF problem" explanation on Thrue Audio site
http://www.trueaudio.com/post_013.htm
Author is John L. Murphy.

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Old 6th May 2004, 07:34 PM   #26
Sheldon is offline Sheldon  United States
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So for someone just getting into the technical stuff (only general college physics many moons ago), do I understand this all to mean that; if the impedence of the speaker system is fairly constant with respect to frequency and the output impedence of the amp is constant with respect to frequency, and the Q of the speaker system with a given amp is where it should be, then the absolute impedence of amp don't much matter? Or, put another way, it's the system as a whole that determines the outcome?

FWIW my pp ultralinear amp has an output impedence of 6 ohms but sounds just fine with my speakers. Very tight solid base to my ears.

Sheldon
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Old 6th May 2004, 08:17 PM   #27
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This is my DF " bibel"!

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Old 7th May 2004, 10:42 PM   #28
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Quote:
Originally posted by Sheldon
...if the impedence of the speaker system is fairly constant with respect to frequency and the output impedence of the amp is constant with respect to frequency, and the Q of the speaker system with a given amp is where it should be, then the absolute impedence of amp don't much matter? Or, put another way, it's the system as a whole that determines the outcome?

FWIW my pp ultralinear amp has an output impedence of 6 ohms but sounds just fine with my speakers. Very tight solid base to my ears.

Sheldon

Hi Sheldon.

Basically a pretty good summation. Congratulations, you seem to have gotten something that took me years to appreciate.

If you, as you do in your case, have 6 Ohm output Z and a bass alignment that copes with it, then the so-called DF means nothing. What the higher Z does is erode the Qe of the bass driver (not the total Qtc as some think), shifting the bass alignment to a different point, but one that may still sound audibly acceptable. In your case DF means nothing... your speaker still works, right, in fact your DF may well be less than 1 or no DF at all!!! What more proof is needed?

I still want to make clear the point that one of the two most imminent researchers, one of the founding fathers and pioneers of Thiele-Small parameters, the one and only Richard Small makes it abundantly clear that Qe (which is the dominant component of Qts) can be tuned by varying Re.

That meant DF was of no consequence to him. I argued with him, only later to realise how right he was!!!


Now how can you tune Qe? Add series resistance. That was what Small said !!! Just include output Z as part of the addition.

This also demonstrates that output Z now becomes part of Re, indeed, this may be a difficult concept to handle, but Re is really a loop. Low output Z is part of Re and vice versa, they are inseparable. The only thing to add is that output Z closes the loop but still has a value, but it can be several Ohms. DF at several Ohms, what kind of DF is that?

------------------------------------------------------------------------

Let us look at maths that don't lie, Let us set up a series of values and analyse the results of changing Re in its various incarnations.

8 Ohm (nominal) driver

6 Ohm DC Resistance or Re of voice coil

Qe = .4 (this is the key value that gets altered)

Qm = 5

Qts is calculated 1/(1/Qe+1/Qm) = 0.37

This value 0.37 is based on there being no added series resistance or Z. In fact it is the same as if the terminal were shortened. Re must become a loop or part of it.

Now what happens if we have an amp:

Output Z = 3 Ohm

This changes Qe proportionally, so:

New Qe = (Re+3)/Re times Qe = 0.60

Yes, Re has increased 50% from 6 to 9, and also Qe has increased 50% from 0.4 to 0.6 .

There is a proportional relationship between Qe and Re. If Re is increased by 50% then Qe also get increased by 50%.

Now we can recalculate Qts using formula 1/(1/Qe+1/Qm) = 0.54

That is a 46% increase in Qts.

BTW, We can deduce from this that speakers having low Qm (or high mechanical damping) are less affected by added Re than a high Qm driver.

Now these maths don't lie. What you have to ask is this: What effect does it have on the speaker that you have changed Qts from 0.37 to 0.54 - in case of some vented alignments, that could be awful, but a sealed box much less so. Also a Bessel like vented alignment should cope quite well.

There really is nothing missing in this analysis. This explains why some amps, especially zero feedback types like SET, with typically 3 Ohm output Z and thus hopeless DF and often lower than 1, can still sound OK.

So forget DF, it's the bigger picture that matters!!!

Joe R.
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Old 8th May 2004, 01:44 AM   #29
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Quote:
Originally posted by Joe Rasmussen

Now these maths don't lie. What you have to ask is this: What effect does it have on the speaker that you have changed Qts from 0.37 to 0.54 - in case of some vented alignments, that could be awful, but a sealed box much less so. Also a Bessel like vented alignment should cope quite well.

There really is nothing missing in this analysis. This explains why some amps, especially zero feedback types like SET, with typically 3 Ohm output Z and thus hopeless DF and often lower than 1, can still sound OK.

So forget DF, it's the bigger picture that matters!!!

Joe R.
Joe,

Nice analysis. It shows that as long as the output Z of an amp is >0, it will have an affect (mathematically) on the Qts of the speaker in its box (ie. the "damping" of the the system). As the amps output Z becomes larger (>1 - 2 ohms), it begains to have a significant affect on Qts and by extension again, the overall damping of the speaker.

So, semantically speaking, "Damping Factor" would appear to be the correct term to give the ratio Zsp/Zout.

However, it has been the experience of most here that well controlled bass can still be had using an amp with relatively high output Z. So that would mean, as you have shown, that dramatic changes in Qts will have little affect on the perceived "tightness" (cone control) of the bass speaker. Where does that leave us?

Why does one speaker/amp combination sound tight yet by just changing the amp, the same speaker will sound loose and flabby? What is the mechanism that imparts the character of "controlled bass"?
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Old 8th May 2004, 01:16 PM   #30
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Quote:
Originally posted by roddyama

However, it has been the experience of most here that well controlled bass can still be had using an amp with relatively high output Z. So that would mean, as you have shown, that dramatic changes in Qts will have little affect on the perceived "tightness" (cone control) of the bass speaker. Where does that leave us?

Why does one speaker/amp combination sound tight yet by just changing the amp, the same speaker will sound loose and flabby? What is the mechanism that imparts the character of "controlled bass"?
That is a very good question. Yes indeed!

I don't think it is down to one or two factors, but there are at least some I can think off. The first that comes to mind, and ain't a new thought, that the open-loop characteristics must be good and especially linear. These have more stable and reliable output Z, even if they are higher. The other is dynamic stability. Have you ever seen Fletcher-Munson curves? They really also reveal that in the bass we have very little dynamic compression when compared to mids. So we seem sensitive to distortion in the mids but sensitive to dynamic/transient behaviour in the bass (and also, but not quite to the same degree in the treble). I believe this is the key to understanding why some amp really do the trick in the bass. I have heard an amplifier with 2-3 Ohm output Z and only 18 Watt, that absolutely killed 100 Watt plus SS amps, not only in bass quality but also seeming a lot more extended. You might want to read the review of the amp on enjoythemusic.com - here is a part quote:

"The bass was tight and strong... and was the tightest I had ever heard from my system. Remember, I am not measuring the bass reproduction of this amplifier to another tubed one, but my Distech Monoblock 140 watt per channel solid-state amplifiers. Then the tympani kicked in and I heard the most authentic reproduction of tympani I'd ever witnessed. So they did do the bass better than any amplifier here."

In fact this amplifier got one of their Best of 2003 Awards. Oh, I must also confess, I had a hand in this design.

Now you may have an idea what I'm listening to. So how did we get this truly great bass quality. By enhancing transient stability. The key was the way the power supply interfaces with the power supply, actually by fixing and stabilising the surrent. Under transient conditions the current doesn't change. Yes, it is Class A, but with a difference, the current is regulated! That's right, not the voltage but the current regulated. You get this huge power supply isolation.

I think I've said enough....

The complete review:

http://www.enjoythemusic.com/magazin...achapter52.htm

Joe R.
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