Voltage, current and speakers

 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 10649
Registered: May-04
.

From another forum:





phase angle (not quite sure what that is)



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In a resistor, current and voltage flow in tandem, ie max current flows at the same instant that the max voltage is applied to the resistor. ( This is the easiest situation for the amplifier )

However, in Inductors and capacitors, the current can lag ( flow after ) or preceed the applied voltage.

The phase angle is a measure of just how much the voltage is out of sync with the current flowing ( in this case, into the crossover and speakers )



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This sort of helps. I'm still a bit fuzzy about how voltage can be divorced from current. Current is just voltage with a load. So, once you introduce a load, I don't quite understand how there may be a discontinuity between voltage and current.





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Yes, it is an un-usual but well established concept. Power Factor errors are the bane of Power Companies and distributors worldwide.

Unfortunately basic electricity courses at the school level typically address Direct Current (DC) as available from a battery.

Alternating Current (AC) either at 50 Hz or ANY frequency behaves differently with capacitors and inductors, compared to resistors.

As a loose analogy, consider a stone on a table, ( the load ) connected to one end of a metal rod.

If the open end of the metal rod is pushed suddenly, the stone will move immediately.

The metal rod is similar to the Resistor.

If the metal rod is instead replaced by a spring.... a sudden push at one end of the spring will have a time lag before the push is transmitted to the other end of the spring, and to the stone.

The stone will move a touch later, after the push is applied.

The spring is like an inductor that delays the flow of current, after a voltage is applied....




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So the point of the artical was that just presenting the impedance vs frequency raw information really doesn't tell you the whole story of how difficult the load is to drive. But if you do some math, you can come up with a fuller picture of the difficulties driving the load when driven by a complex audio signal.




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Gold Member
Username: Frank_abela

Berkshire UK

Post Number: 2224
Registered: Sep-04
What a brilliant description! Thanks for that. I've always struggled with how to describe this...

Regards,
Frank.
 

Silver Member
Username: Stefanom

Vienna, VA United States

Post Number: 749
Registered: Apr-06
Interesting.

Is it in any way related to the derivative of the impedance curve, out of curiosity? Its been a while since I've studied electrical physics.
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 10652
Registered: May-04
.

Is what related to what?



Phase angle to impedance? Not directly would be my answer but that, I think, would generally be considered wrong by a mathemetician or an engineer.


A resistor (theoretically) introduces no lag nor lead in amperage vs. voltage.

A capacitor has no resistance nor inductance - to speak of - but causes voltage to lead current and results in a positive going phase angle.


An inductor has measured resistance and causes current to lead voltage or a negative going phase angle.


And a resistor has some inductance (in many cases, though noninductive resistors are typically employed in better audio circuits).



Both inductors and resistors change value when temperature rises as a result of voltage and current run through them. (Remember the voice coil of any dynamic driver [a Magnepan is a dynamic driver stretched and flattened] is considered an inductor and it's overall rise in resistance and inductance through temperature change should be figured into the final crossover values.)



However, from Wikipedia; "Impedance may refer to:

the ratio of the voltage phasor to the electric current phasor, as in
Electrical impedance, a measure of opposition to time-varying electric current in an electric circuit.
Characteristic impedance, a measure of opposition to electric current propagation in a transmission line.




The impedance of a speaker, which is being driven by AC voltage from the amplifier, is the sum of the parts that are required to make it work as a complete unit. While resistance and impedance are both expressed in Ohms, impedance is the total of the resistance, capcitance and inductance of the circuit and is considered to be either a resistive load or a reactive load depending on the resulting phase angle. The more components such as capacitors and inductors included in the crossover, the greater the resitance to current flow, therefore the more reactive the load, even though the overall impedance measured in Ohms might remain reasonably constant through manipulation of parallel and series wiring. The higher the reactance of the circuit, the more power from the amplifier is wasted, in most cases the more severe the phase angle and the more difficult to drive the system becomes by requiring higher levels of current (usually, this wouldn't exactly be true in the case of a highly capacitive load).



Either lower impedance or higher phase angle will require more current from the amplifier. Together both low impedance and greater phase angle will be very difficult for any amplifier to manage. You can have low impedance yet also have a benign phase angle and the demands made on the amplifier will be much less than should a higher overall impedance with a greater phase angle be present.



If your choice is between a resistive load - typically a single driver, full range speaker qualifies as such - and a highly reactive load - multiple drivers with complex crossovers or electrostatics which typically present a load similar to a capacitor (which they are though they typically require large inductors [transformers] in line to energize the panels) - the ease with which any amplifier deals with the kinder resistive load will be paid back in better sound quality.


As a generalization, AC voltage drives the dynamic driver while current controls its motion.


http://search.yahoo.com/bin/search?fr=ybr_sbc&p=impedance


http://sound.westhost.com/site-map.htm




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Silver Member
Username: Stefanom

Vienna, VA United States

Post Number: 755
Registered: Apr-06
I'm just talking from a purely mathematical perspective. It just seems like in a lot of cases, when the impedance is rising, the phase angle is positive, and when the impedance is falling, the phase angle is negative, and a chunk of the peaks and troughs occur when the phase angle is 0.
 

Silver Member
Username: Stefanom

Vienna, VA United States

Post Number: 756
Registered: Apr-06
PS: Thanks for the info. It's nice to get some refreshers once in a while.
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 10654
Registered: May-04
.


Refreshers would be nice when they're correct.

OK, here's the moment you've all waited for, I screwed up. This is one reason I'm not an engineer or speaker designer and why I should have plenty of coffee on board before relying on my memory.


I posted: "A capacitor ... causes voltage to lead current and results in a positive going phase angle.


An inductor ... causes current to lead voltage or a negative going phase angle."


This reverses the properties of a cap and an inductor. Sorry. With the purely inductive circuit, the current lags behind the voltage and the result is a +90° shift. The capacitive circuit will reverse the voltage/current configuration and the result will -90° phase angle.



Mea culpa.





"It just seems like in a lot of cases, when the impedance is rising, the phase angle is positive, and when the impedance is falling, the phase angle is negative, and a chunk of the peaks and troughs occur when the phase angle is 0."


To be honest, I haven't noticed a trend toward + or - phase angle with rising or falling impedance, but I'm not a speaker designer so maybe I'm missing something here. Here are a few measurements done by Stereophile to look at for comparison.


http://stereophile.com/floorloudspeakers/607wilson/index4.html


http://stereophile.com/floorloudspeakers/706revel/index3.html


http://stereophile.com/floorloudspeakers/306monitor/index3.html


I'm not certain you're not seeing the results of the low frequency tuning (particularly) of a ported speaker when you suggest rising impedance relates to positive phase angle. Possibly someone with more technical knowledge than I can assist here.




As to peaks and troughs in impedance while at 0° phase angle, I don't know how to respond since most multi-driver speaker systems don't spend much time at 0°. The exception would be the largely resistive load of a planar driver such as the Magnepans.


http://stereophile.com/floorloudspeakers/303/index6.html
 

Silver Member
Username: Eramsey

South carolina United States

Post Number: 534
Registered: Feb-05
In an inductor the voltage leads the current,(E leads I). In a capacitor the voltage lags the current or I(A) leads E(V). The resistance(impedence)expressed as an equalivalent DCR) of just the voice coil of a driver(speaker) generally should be very low likely less than one Ohm. It is not until a driver is considered with it's total motor structure (voice coil,"spider" and other elements) or coupled with a crossover with it's resistive,inductive and capacitive elements that the impedence is raised to the proverbial but rather meaningless 4,6,or 8 Ohms. An inductor will create a phase shift(approximately 90deg) between the current and voltage in the positive half of a complete (360 deg.) A capacitor will on the other hand create an electrical phase shift(again 90 deg) in the negative range of a full cycle. True power(W) can only be made when the voltage and current are in phase with one another and applied to true (non-inductive) resisitance and both are either positive or negative. The total shift in phase for a crossover will depend almost entirely on the size and number of elements used and the crossover points if a multi-driver configuration, the ratings in Farads and voltage rating for a cap, and Henrys(L) and voltage rating for an inductor. It would seem to follow that the larger the elements,the steeper the crossover in dBl's per octave and crossover points(freq.) the greater the phase shift and thus the greater demand that the speaker will place on the amplifier. Also I have stated before and maintain that speaker sensitivity is also a consideration since this can directly effect amplifier effiency.
 

Silver Member
Username: Rsxman

Post Number: 289
Registered: Jul-05
wait....wait.....YEP

ive gone cross eyed in this one.
 

Gold Member
Username: Frank_abela

Berkshire UK

Post Number: 2226
Registered: Sep-04
OK, my brain just dribbled out of my right earhole and is lying in a sodden pool round my legs...

I did the maths for that kind of stuff about 25 years ago and, God, I'm glad that I haven't had to go back to it - fascinating, but beyond me. So time to make a fool of myself:

If capacitors have the opposite effect of resistors on phase angle, why hasn't someone used a highly capacitative winding in voice coils to reduce the phase angle to zero? I'm thinking of the equivalent of something like Townshend cables which are wide copper strips separated by a thin plastic making for a high capacitance interconnect and speaker cable. It might make the coil unfeasibly large of course, but the benefits should be immense.

Regards,
Frank.
 

Silver Member
Username: Stryvn

Post Number: 276
Registered: Dec-06
Hey, how bout them Brewers?
 

Silver Member
Username: Stryvn

Post Number: 277
Registered: Dec-06
Actually, I've filed it away on the puter in hopes that someday it'll all come together and make sense.
Thanks, Jan
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 10659
Registered: May-04
.

"If capacitors have the opposite effect of resistors on phase angle ... "


Better go back and read again, Frank. Resistors, in theory, have no effect on phase. Capacitors and inductors do the opposite of one another to the volts/amps thang. Your idea is somewhat odd to me since most amplfiiers do not care to drive a capacitive load and will blow up quite readily when faced with the task. Ask any owner of an original pair of Quads. I've never even understood how cable manufacturers get away with selling highly capcitive cables, they can be not much more than tone controls for overly bright solid state amplfiiers in my experience.




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Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 10660
Registered: May-04
.

ER - Thanks for the information. Explain, "the proverbial but rather meaningless 4,6,or 8 Ohms." Obviously the "Ohms" of the load is only a small portion of the load but I am not seeing that it is "meaningless". I understand that a four Ohm load that is very consistent is better than an eight Ohm load that wanders downward and upwards with high Q shifts or dramatic changes in total impedance. And that the phase angle is, in most cases, more important to the ability of an amplifier to drive the speaker than the "Ohms" alone would suggest. (Even more so than the "sensitivity" of the drivers or total system.) And that both the impedance and the phase at any one frequency will be the real problem an amplifier faces.


Are you saying it is meaningless to describe a speaker system as a nominal eight Ohm load or are you referring to a specific frequency at "X" Ohms as meaningless?






"The total shift in phase for a crossover will depend almost entirely on the size and number of elements ... "


"It would seem to follow that the larger the elements,the steeper the crossover in dBl's per octave ... "


Once again, explain. Is a cap not a cap? Is the phase shift not the same for any value of capacitor? I was under the impression that size did not determine the amount of phase shift since size is used to determine frequency of action. Smaller cap for higher frequencies and larger for lower. They both still give 90° phase shift at their respective frequency range.


And size does not determine the order of the crosover. The number of elements within the crossover will set the filter action. Size determines the frequency range where the element takes affect. A single inductor or capcitor is a first order filter no matter where it is placed in the frequency bandwidth and the rolloff will be six dB per octave, no more and no less.


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Silver Member
Username: Eramsey

South carolina United States

Post Number: 535
Registered: Feb-05
Hello Jan it's a bit late as I just got off work, but I'll try to explain what I meant. When I said that to rate a speaker at 4,6 or 8 Ohms is rather meaningless, I was just reaffirming what both you and I as well as other members have said that a typical speaker say rated a 8 Ohms will only actually see 8 Ohms for a fraction of it's total impedence curve. A cap is a cap, true but both DC and AC caps are used in crossover networks and both behave very differently in terms of the action current flow. I'm sure that your aware that in a passive speaker the only voltage it operates on is the voltage signal from the amplifier as supplied through the speaker output. When you have larger components in crossover networks larger caps and inductors your reactive power VARSc and VARSl increase accordingly and this can hurry, change and greatly effect the orginal signal. Actual part size will not determine the order of the crossover true and the number of elements within a filter will determine the filter action,also quite true. All filters operate on the principle of resonance. Now about "Q", high Q components will produce a narrow- band filter and low Q components will produce a wide-band filter. It would seem then that a minimalist design would definately be the best approach when designing a crossover network. You and I both know however that this is not always the convention with some speaker systems with multiple driver arrays having very complex crossover networks. This is why low budget amps and receivers typically have trouble with such a speaker, you have a low slew rate and throw in a low sensitivity for the speaker and you have a recipe for failure. You are correct in that generally the larger the driver the larger the components will be for it's particular filter. Perhaps we can discuss this at greater length sometime, of all electronic devices speakers are certainly one of the most fascinating for sure.
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 10663
Registered: May-04
.

"It would seem then that a minimalist design would definately be the best approach when designing a crossover network."



By "minimalist" I can only assume you are referring to a simple first order filter with -6dB action. I can't actually agree to that statement since there are many problems associated with the use of such minimum action filters. Given the correct circumstances, I find first order filters to be very acceptable as a low pass on the woofer side where mechanical roll out from the driver itself can be added to the filter's action. But they are, IMO, not at all well mated to most tweeters.


As an example, Theil prefers first order crossovers but then uses multiple notch filters to smooth the driver's response irregularities and add baffle step compensation. The result has been a speaker that soaks up power as the amplifiers slugs its way through all the elements of the crossover ahead of the drivers. Theil has progressively gained the stature to have their drivers built to their specifications which has simplified many of the correction networks, but their speakers still represent one of the more difficult loads an amplfiier will face due primarily to the severe demands of the phase angle created by numerous inductors and capacitors. A few of their more recent speakers actually are not difficult loads if you look only at the impedance swing, which is fairly minimal by comparison with most speakers. But, add in the phase angle of the crossover's design and the speaker requires a very hefty and high current solid state amplifier in front of it to operate to its optimum. And, in many cases still, a solid state amplifier only adds to the "accuracy" problems of the Theil design. Personally, I find their sound to be relentless. In the past Theil advertisements proudly displayed a crossover board stuffed with twenty or more elements implying, "Look at all you get for your money". I suspect most Lowhter owners cringed (and chuckled) at the thought.

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Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 10664
Registered: May-04
.

I think also, it should be pointed out that a 90° shift in electrical phase does not imply the audio signal is thrown 90° out of phase. What happens to the AC signal from the speaker outputs is important in that work can only be accomplished when voltage and current are in phase and as they shift away from each other, less work is done. Therefore, the amplifier driving the high electrical phase angle speaker must work harder to get the same results compared to what a speaker with a lower electrical phase angle could accomplish. Wasted power and high current demands are the problems which result from high electrical phase angle.



What will make this more confusing is the fact that a first order crossover filter will result in a 90° phase shift in the audio signal's arrival at the driver. Each further filter order will progressively take the signal another 90° out of phase. But, even with a fourth order filter (-24dB) where the audio signal has been thrown out of phase 360°, the electrical phase shift will be less than 90°.



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Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 10942
Registered: May-04
.

"You get a phase angle when the load is "reactive" - that is
it has either an inductive or capacitive component.

The current for either a capacitor or an inductive won't
be in phase with the applied voltage. That's because
both capacitors and inductors store energy.

The capacitor stores charge, which gives rise to a current
when it is discharged.

The inductor stores energy in the form of a magnetic field,
that can then induce current flow when the voltage changes.

Because of the stored energy in both these devices, the
current doesn't have to flow in lock step with the voltage.

Dr. Gregory Greenman
Physicist"



++++++++++++++++++++++++++++++++++++++++++++



"The "phase" jargon is really very simple to conceptualize. Imagine a capacitor that gets connected on to a battery, for example an alkaline AA cell. If we were to measure things AT THE VERY INSTANT the capacitor poles are connected to the battery poles, then we would see that:

(a) the voltage across the capacitor plates is zero (because it hasn't charged yet), but

(b) the current starts flowing as though the battery were short-circuited, because the capacitor wants to suck all the charge it can.

Now as the current keeps flowing, charge starts to build up in the capacitor plates, and the current flow slows down accordingly.

If we measure again after things have settled down, the voltage across the capacitor plates has become the same as the battery voltage, and current flow is now zero. The capacitor has charged fully.

So, when the current was at its maximum, voltage was zero and now that voltage has reached maximum, current is zero.

If we wanted to phrase the above sequence of events in physics jargon, we would say that "across purely capacitive loads, voltage phase lags current phase by 90 degrees".

An inductor acts in the exact inverse way, because the magnetic field created by current flow wants to keep that current flow steady and "dislikes" current fluctuations.

There is nothing confusing about such a simplified, idealized situation. If, however, we try to imagine that the voltage is not just the 3 Volts DC an AA cell can produce, but an alternating mixture of all kinds of frequencies that tries to create current flow across a complex smorgasbord of real AND virtual resistances, capacitances, inductances and friggin' back-EMF's (such as a loudspeaker with multiple drivers and a crossover), now THAT'S when things can get really confusing Even from the point of view of our previously humble AA alkaline, which has now become the output stage of a power amplifier as it tries to make do with whatever cards its power supply has dealt it.

And this was the purpose of Keith Howard's excellent article: to simplify such a complex situation down to a simple x-y axis chart.

--------------------
RDoh "
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 11044
Registered: May-04
.



http://www.stereophile.com/reference/707heavy/
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 11045
Registered: May-04
.


http://www.symphonysound.com/articles/tubefriendly.html
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