Yeah. Thanks for the reality check.
In the real world of today's production drive units, Qms is rarely if ever low enough, so we must plan on at least a little amp damping.
I guess I'm always thinking in terms of starting with a clean sheet--a hypothetical new driver designed for current drive, so I can have whatever Qms I want. In that utopian scenario, I'm not concerned with amp damping, so I can let Zout be as high as it needs to be to not compromise other amp parameters.
I really must build this driver...
Bill
/his head in the clouds and his feet not far beneath.
Ah. An honest-to-goodness POWER amplifier. Not what I linked to . . . unless . . . I suppose it's too much to hope that your load will be purely resistive? 😉
Of course not. If it was resistive, I could drive it with either a voltage or current amp, as the other is proportional.
So I need the product of the current and voltage samples for feedback.
So I need the product of the current and voltage samples for feedback.
Why do you need the PRODUCT ? The SUM will do as well. Remember that an amp is usually fed with a VOLTAGE at it's input that is proportional to the sound PRESSURE. We want the sound pressure to be proportional to the input voltage in the end, so don't make things more complicated than actually needed.
regards
Charles
I'm driving plasma. The temporary pressure increase (creating the compression wave) is directly proportional to the temporary power increase. This is described in more detail in Hill's patent. (temporary because in long power increase the plasma will eventually achieve equilibrium with atmospheric pressure and increase volume instead)
Prune said:
Supose this runs like a klass A singel ended, and have kind of bias. since -U*-I = P , but you really want a negative preasure (everything else in phase). You end up with sinking and sorsing power to the plasma driver.
If current driven the speaker-element would probably best have : 0.5<Qm<1
http://www.seas.no/seas_line/midrange/H422.PDF ~ Qm=0.79
Question:
Can we relay on greater mecanic linearity than electrical feedback ?
Prune said:
I believe that the voltage gap on the Hill Plasmas remains
fairly constant, being the ionization voltage of the helium,
and so your power should be quite proportional to current.
BTW, I've owned two sets of these, and modified one set to
use his tubes as cascodes for a low impedance input, that is to
say, I drove the Cathodes with a power amp in series with a
resistor (could have used a current source). The result was
a real improvement IMHO, and we used it at several shows.
Yep - a Qms/Qts of 0.5 would be great for dipole or IB applications, and definitely suitable for a bass driver.Bill F. said:
Konnichiwa,
Allow me to disagree, as a dipole application reduces the total Q below 0.5 anyway.
Strictly only in anechonic (torture) chambers.
What really matters is the behaviour of the speaker in an acoustically small room (unless you are rich enough to own large volumes of space). And in this case any such concepts as Qs = .5 or WHATEVER are easily and immediatly illustrated ad absurdum anyway.
The biggest problem with all speaker pseudo science is that it reliably and regulary omits the most significant variables in the equation (hence pseudo science).
Sayonara
454Casull said:
Allow me to disagree, as a dipole application reduces the total Q below 0.5 anyway.
454Casull said:
Strictly only in anechonic (torture) chambers.
What really matters is the behaviour of the speaker in an acoustically small room (unless you are rich enough to own large volumes of space). And in this case any such concepts as Qs = .5 or WHATEVER are easily and immediatly illustrated ad absurdum anyway.
The biggest problem with all speaker pseudo science is that it reliably and regulary omits the most significant variables in the equation (hence pseudo science).
Sayonara
I think the Qts lowers just a tiny bit, but the difference is basically negligible... I think I saw a mention of it on SL's site.
Please remedy my ignorance. 🙂 How does room volume affect the behavior of the cone?
Konnichiwa,
This appears to be an interesting subject and the view(s) promoted by my compratriot SL (Who I very much respect especially for his work on speakers) are far from being the last word.
It does so by introducing a reverbrant field subject to resonance and by moderating the radiation angle. If I design a speaker based on free or halve soace operation it MUST SEVERELY deviate from frequency response linearity in an acoustically small room at low frequencies. If I then (as common in "HiFi" speakers) ignore directivity vs. frequency we have a veritable disaster at our hands and ears. Not that this ever stopped anyone.
Sayonara
454Casull said:
This appears to be an interesting subject and the view(s) promoted by my compratriot SL (Who I very much respect especially for his work on speakers) are far from being the last word.
454Casull said:
It does so by introducing a reverbrant field subject to resonance and by moderating the radiation angle. If I design a speaker based on free or halve soace operation it MUST SEVERELY deviate from frequency response linearity in an acoustically small room at low frequencies. If I then (as common in "HiFi" speakers) ignore directivity vs. frequency we have a veritable disaster at our hands and ears. Not that this ever stopped anyone.
Sayonara
Kuei, I have neither the thorough experience nor the undeniable wisdom that you possess - if you could, try to PLEASE explain those statements that seem unclear at first glance... For example, I don't know how a reverberant field modulates the radiation "angle"...
Howard (completely sincere)
Howard (completely sincere)
Konnichiwa,
Are they? I would think they are about as clear as possible. Any reproduction is a filter. If you design the filter chain based on incomplete variables that will alter the filters function after the filter has been applied you reproduction will deviate from the desired response. The room is part of the filter.
It doesn't. Please read again what I wrote, reading "The room" for it:
In other words:
"The room alters the system Q by introducing a reverbrant field and resonances"
"The room alters the system Q by altering the radiation angle"
In addition:
"A baffle small compared to the wavelength radiated alters the system Q by destructive interference"
The net effect of this that the actual speaker system must account for mutiple variables where the Qt of the Driver must be adjusted to give a desired filter function after all variables have been accounted for, hence the statement "Yep - a Qms/Qts of 0.5 would be great for dipole" is a priori false as it does not account for key variables in the equation.
Clearer? And please try to read what I am actually WRITING, which will keep such misunderstanding as the above: 'I don't know how a reverberant field modulates the radiation "angle"..' to a minimum and saves me to having to restate the same thing again.
Sayonara
454Casull said:
Are they? I would think they are about as clear as possible. Any reproduction is a filter. If you design the filter chain based on incomplete variables that will alter the filters function after the filter has been applied you reproduction will deviate from the desired response. The room is part of the filter.
454Casull said:
It doesn't. Please read again what I wrote, reading "The room" for it:
Kuei Yang Wang said:
In other words:
"The room alters the system Q by introducing a reverbrant field and resonances"
"The room alters the system Q by altering the radiation angle"
In addition:
"A baffle small compared to the wavelength radiated alters the system Q by destructive interference"
The net effect of this that the actual speaker system must account for mutiple variables where the Qt of the Driver must be adjusted to give a desired filter function after all variables have been accounted for, hence the statement "Yep - a Qms/Qts of 0.5 would be great for dipole" is a priori false as it does not account for key variables in the equation.
Clearer? And please try to read what I am actually WRITING, which will keep such misunderstanding as the above: 'I don't know how a reverberant field modulates the radiation "angle"..' to a minimum and saves me to having to restate the same thing again.
Sayonara
power feedback
I've read the full discussion (very interesting, thanks to all) and I've been surprised having not found any mention to Stanley White's powtron concept.
A loudspeaker can be controlled at low impendance (voltage) or high impedance (current) but the real phisical quantity that must be forced to follow the signal is the electrical Power, ie the product of current and voltage.
Stanley White Powtron amplifier, described in his site (I do not remember, google gives it shortly) works on this concept, having two feedback loop, one for current and one for voltage applied in a clever manner on the input phase inverter stage.
Any comments on his view?
I've read the full discussion (very interesting, thanks to all) and I've been surprised having not found any mention to Stanley White's powtron concept.
A loudspeaker can be controlled at low impendance (voltage) or high impedance (current) but the real phisical quantity that must be forced to follow the signal is the electrical Power, ie the product of current and voltage.
Stanley White Powtron amplifier, described in his site (I do not remember, google gives it shortly) works on this concept, having two feedback loop, one for current and one for voltage applied in a clever manner on the input phase inverter stage.
Any comments on his view?
Re: power feedback
Konnichiwa,
Why. It's intermediate between Current feed and voltage feed, with the disadvanted of both and the advantages of non.
More worth mentioning would be a variety of "adjustable impedance/damping" amplifiers which allow the outputimpedance to be adjusted from very high (current source) via intermediate (aka "powertron") values to zero Impedance and finally to NEGATIVE impedance.
I experiemnted with that in the 80's. Interesting and very eludicating. You arrive at a simple conclusion though.
1) If you need to manage mechanical resonances in the driver/box combo in the interface voltage drive is essential (eg instead of dealing with a mechanical problem directly by mechanical means you try to compensate by electrical means).
2) If you do not need to manage mechanical resonances in the interface pure current drive is best by far.
3) Negative impedance seems a good idea at first, but teh sonic results suck big time.
That is patently untrue. It is trivial to demonstrate that direct proportinality between signal and cone motion in speaker drivers exists ONLY between current and signal. The Voltage and Power are best viewed as "incidental" and "parasitary".
If you had one voicecoil made from a supraconductor (eg one that had practically no direct current resistance at all) and one made from a high resistance wire of equal weight with identical numbers of turns and gemoetry and you fed the resulting speakers, which are identical in all thing excet one having a high resistance voice coil and the other a "no resistance" voicecoil, with a pure nearly ideal current source you would observe the following:
1) Both speakers produce EXACTLY the same acoustic output, within experimental/manufacturing error.
2) One Speaker operates practically "powerless" as it is a dead short, the other operates at very high power, relatively (we hope the voicecoil does not melt).
Hence current in the voicecoil is the control parameter, not voltage or power.
Sayonara
Konnichiwa,
plovati said:
Why. It's intermediate between Current feed and voltage feed, with the disadvanted of both and the advantages of non.
More worth mentioning would be a variety of "adjustable impedance/damping" amplifiers which allow the outputimpedance to be adjusted from very high (current source) via intermediate (aka "powertron") values to zero Impedance and finally to NEGATIVE impedance.
I experiemnted with that in the 80's. Interesting and very eludicating. You arrive at a simple conclusion though.
1) If you need to manage mechanical resonances in the driver/box combo in the interface voltage drive is essential (eg instead of dealing with a mechanical problem directly by mechanical means you try to compensate by electrical means).
2) If you do not need to manage mechanical resonances in the interface pure current drive is best by far.
3) Negative impedance seems a good idea at first, but teh sonic results suck big time.
plovati said:
That is patently untrue. It is trivial to demonstrate that direct proportinality between signal and cone motion in speaker drivers exists ONLY between current and signal. The Voltage and Power are best viewed as "incidental" and "parasitary".
If you had one voicecoil made from a supraconductor (eg one that had practically no direct current resistance at all) and one made from a high resistance wire of equal weight with identical numbers of turns and gemoetry and you fed the resulting speakers, which are identical in all thing excet one having a high resistance voice coil and the other a "no resistance" voicecoil, with a pure nearly ideal current source you would observe the following:
1) Both speakers produce EXACTLY the same acoustic output, within experimental/manufacturing error.
2) One Speaker operates practically "powerless" as it is a dead short, the other operates at very high power, relatively (we hope the voicecoil does not melt).
Hence current in the voicecoil is the control parameter, not voltage or power.
Sayonara
Hi Thorsten,
so Yours experimentation leads You to the conclusion that current driving is best until tha loudspeakers approaches resonances and the impedance starts to rise.
In this case voltage driving is necessary to damps the cone movement.
So, apart from White's pictorial description, the combined voltage/current feedback controls current when impedance is low, at mid frequency, because the predominant feedback signal is due to current and controls voltage at the resonance when impedance rises and feedback is almost entirely due to voltage component of output signal.
This, in my naif view, seems the perfect source that You described. What I'm missing?
so Yours experimentation leads You to the conclusion that current driving is best until tha loudspeakers approaches resonances and the impedance starts to rise.
In this case voltage driving is necessary to damps the cone movement.
So, apart from White's pictorial description, the combined voltage/current feedback controls current when impedance is low, at mid frequency, because the predominant feedback signal is due to current and controls voltage at the resonance when impedance rises and feedback is almost entirely due to voltage component of output signal.
This, in my naif view, seems the perfect source that You described. What I'm missing?
Konnichiwa,
No.
They lead me to the conclusion that mechanical problems are best handeled mechanically, so a moving coil driver should in fact have a Qm that is the same as our current drivers Qt (and constant with excursion) and should be driven by a modulated cuurent source for best performance.
Alternatively conventional drivers can be used with enclosure systems that add the needed damping mechanically, in other words a mechanical solution to mechanical problems.
Voltage drive is NOT needed to damp the cone. That is putting the cart before the horse, putting the horse in bakwards too and trying to drive the whole contraption around an obstace course.
That is however not what happens. White's system does not does anthing of the like. It does aim to make the power applied to the speaker constant, but merely makes it MORE constant than in case of voltage or current drive. In other words it produces a fairly high but constant output impedance which however is much lower than a true current source.
The main benefit of doing this the "powertron" way and not by adding a suitable value series resistor is that the high internal resistance of the amplifier is a "virtual" resistance and thus dissipates no power.
Sayonara
plovati said:
No.
They lead me to the conclusion that mechanical problems are best handeled mechanically, so a moving coil driver should in fact have a Qm that is the same as our current drivers Qt (and constant with excursion) and should be driven by a modulated cuurent source for best performance.
Alternatively conventional drivers can be used with enclosure systems that add the needed damping mechanically, in other words a mechanical solution to mechanical problems.
plovati said:
Voltage drive is NOT needed to damp the cone. That is putting the cart before the horse, putting the horse in bakwards too and trying to drive the whole contraption around an obstace course.
plovati said:
That is however not what happens. White's system does not does anthing of the like. It does aim to make the power applied to the speaker constant, but merely makes it MORE constant than in case of voltage or current drive. In other words it produces a fairly high but constant output impedance which however is much lower than a true current source.
The main benefit of doing this the "powertron" way and not by adding a suitable value series resistor is that the high internal resistance of the amplifier is a "virtual" resistance and thus dissipates no power.
Sayonara