raising driver Qts - you can't tuna fish

[CharlieLaub]

Originally Posted by CLS

OTOH, eventually, Qts is not a keypoint. You may adjust it to whatever you want, if you are willing to leave the mainstream camp of "amps should have low output impedance".

Any woofer with decent Xmax and proper fs can be tuned to fit OB well. Low Qts (or Qes) of the driver is not a problem at all. Once it's working with high impedance amp, the system can be tuned to a proper Q.

Again, Nelon brought that up with a series of tests on fullrangers. I made a thread about that for OB bass purpose, too. Still largely ignored. Sigh~

I thought I would give this topic its own thread rather than bury it this inside that long, long NP slotted open baffle thread, where the post by CLS is located.

I used to espouse this same thing - that you can "adjust the Qts" of a driver by playing with the amp's output impedance. Recently, however, I (finally) arrived at the conclusion that this is not "physically" increasing the Qts. You can "effectively" increase the Qts by:

(A) by adding series resistance, or
(B) by increasing the amplifier's output impedance.

But what is really happening???????????????

For (A), it should be easy to see that a resistance in series with the driver is creating a voltage divider. We all know that the impedance of a driver varies with frequency, that there is a "peak" around resonance, and voice-coil inductance causes a slow rise at higher frequencies. When the impedance is high, e.g. very close to resonance, the voltage division directs proportionally more power to the driver than when the impedance is low(er), e.g. in the low-to-middle part of the passband. But don't forget that at high frequencies the slow impedance rise in the driver also causes the voltage divider to increasingly direct the power to the driver and away from the series resistor.

We have a very similar situation in (B). In this case, it is the amp's output impedance that is in series with the driver, but the same argument applies.

In my mind, this is NOT increasing the Qts of the driver. The driver is still as before, only the "apparent" behavior has changed. In either case, there has effectively been a change in the voltage reaching the driver, so that it is no longer frequency independent. You could call it "frequency shaping via output voltage manipulation" or you could call it a "tone control".

Another interesting example of this is when feedback is used around a driver, such as that employed by Stahl in his "ACE-BASS" system. For years when I was younger, I stared at the patent and his JAES (?) publication trying to figure out what the heck he was talking about when he said "the parameters of the driver have changed, in fact if an engineer measured the amp+driver they would measure the new (ACE-BASS imposed) parameters". Wow! I wondered why everyone didn't use this approach? It seemed you could basically dial in whatever driver parameters you wanted, and somehow the driver would be magically transformed in to a high efficiency, low resonance, bass machine! It was like Hoffman's Iron Law was lifted! But I finally realized, after downloading the very nice, and interesting program Basta!, that what is actually happening is the amp is called on to deliver electrical power (e.g. with some magnitude and phase) that is crafted to cause the response to take on that of the "new, imposed" driver parameters, but the DRIVER STAYS THE SAME!!! Thus, no magic. It's just another fancy "tone control", really. In fact, once I could see the amplifier demand when the ACE-BASS circuit was engaged in Basta! I realized that I could just as effectively shape/boost the response using the standard line-level filter circuits (placed before the amplifier), without resorting to a dicey feedback resistor, and all the problems that it introduces. I can't tell you how many hours I spent thinking about thermal tracking of teh driver's Re... Now, about the only thing left over about the ACE-BASS circuit that seems interesting is the claim that some orders of distortion related to the suspension compliance are reduced if the imposed compliance swamps the true compliance.

Anyway, since it took me awhile to wake up to this reality, that the laws of physics can't be broken, I thought I would share my thoughts on this here in hopes that everyone will chime in with their usual uplifting comments and such.

-Charlie

[speaker dave]

I've got no problem with manipulating Qts, other than not feeling great about wasting power in resistance. If it is done with feedback being used to raise the output impedance then I guess I have no objections at all. I know people will jump in with the horrors of loosing the magical properties of amplifier damping but in the end it is simply providing an EQ curve based on the impedance curve.

I've also mentioned on a thread or two that I've done distortion tests with variable output impedance amps and at low frequencies there was no difference in distortion level with high or low driving impedance, as long as the comparison was otherwise equalized to the same response curve.

With regards to thermal issues, constant current means constant SPL so you will actually be better off with higher source impedances.

Uplifting enough?

David

[bwaslo]

Another way to increase Qts is to add mass, which also decreases fs. Hurts efficiency, though.

[forr]

Hi Charlie,
The driver can't be separated from what it is driven by : the amplifier. They form an indissociable whole. As all amps are different from the point of view of ouptut impedance (most often by fractions of ohm but not always), you can't refer to standard driving conditions implied by saying "THE DRIVER STAYS THE SAME". What you may mean is that the response to the voltage across the driver's terminals is not affected by the driving impedance. If there is no significant voice coil heating, this is right.

[CLS]

So I usually use the term "system Q" for describing the behavior of amp + driver (or + associated loads).

In fact the system Q is what listeners are facing eventually. Say, many ported boxes are designed to be Q=1.1 or so to get the loudest boom, thus the somewhat inferior impressions in subjective sound quality (transient response). However the woofer in that box might have a Qts of lower then 0.3.

Oh no, I'm drifting off topic again. But actually not. The drivers are never working alone. In the ported example above, the acoustic load of the box largely affects the 'system Q' -- or usually named as Qtc.

While in the case of OB, the air (acoustic) load is too little to affect system Q, so we look for another partner -- driving amp. It's just another factor in the chain.

As to the comparison of case (A) series resistor and (B) higher Zo by feedback, "feelings" aside, I guess there's no reason against (B). The characters of much less waste in efficiency, and lower cost of components...etc are just an obvious choice in the engineering POV.

[woody]

Speaking of OB's didn't Martin King's H baffel lower the driver's fs and raise the QTS ? Of course it also lost some sensitivity to.

[CLS]

Yes, H baffle loads the woofer slightly more than flat OB, but still too little to effectively alter the whole thing.

In my trials of slot loaded OB (as proposed by Nelson Pass), the measured system Q is 0.35 (raised from 0.28 of nude driver in free air). For a low Q woofer like that, such acousitc load is not enough.

[MCPete]

I used to espouse this same thing - that you can "adjust the Qts" of a driver by playing with the amp's output impedance. Recently, however, I (finally) arrived at the conclusion that this is not "physically" increasing the Qts. You can "effectively" increase the Qts by:

(A) by adding series resistance, or
(B) by increasing the amplifier's output impedance.

But what is really happening???????????????

About a year ago I went so far as to travel to the library of a local university to photocopy an article by W.J.J. Hoge in the Aug. 1976 issue of AUDIO magazine. That article was given as a reference by Dickason in his Cookbook for the method of increasing Qes by the connection of a resistor in-series with a driver. So I thought that Hoge's article would provide an explanation about how it works. Unfortunately the article doesn't explain it at all and barely makes a reference to the technique.

If anyone can explain how this works, I'm all ears.

Regards, Pete

[CLS]

Thiele/Small - Wikipedia, the free encyclopedia

In which you may see the equation of Qes. "Re" in that equation means DCR of the voice coil. To get a 'real' Re (and Qes), one should add all resistances in series with voice coil onto this value, i.e., resistances of all wiring, connectors, and output impedance of driving amp.

In 'normal' situation, those resistances are very small, so normally ignored. Now if driven by a high output impedance amp, than the effective Re would be changed significantly, thus altering the Qes.

In some speaker design software, there's also such a variable for the simulation. For example, WinISd and hornresp have it. Other than those who are using tube amps might set 2~3 Ohm for this, I guess most poeple just leave it as default 0, and almost nobody would put a number >10 in that blank. Hehe...

[Pano]

Yes, Unibox also takes this into account. It changes the effective Qt of the driver. Important to know if you have big coils and/or a high impedance amp.