Hi,
The simple fact is you are never going to get cheap speakers that
are optimised for current dirive, the extra bits are too expensive
for current drive and any benefits irrelevant at the budget end.
The sheer grating arrogance of the OP Author insisting everyone
is clueless, and has been for the last 100 years is beyond the pale.
What problems are caused with current drive is clearly obvious.
What is right with it belongs in active speakers, and to a lesser
degree halfway house implementations which are neither pure
current drive or pure voltage drive, usually valve amplifiers
and relatively simple FR or FR + T speakers.
There is nothing wrong with current drive done properly,
and it has some clear advantages over voltage drive.
That has been well known for a very long time.
Pity its such a PITA to use generically, its big downfall.
rgds, sreten.
The simple fact is you are never going to get cheap speakers that
are optimised for current dirive, the extra bits are too expensive
for current drive and any benefits irrelevant at the budget end.
The sheer grating arrogance of the OP Author insisting everyone
is clueless, and has been for the last 100 years is beyond the pale.
What problems are caused with current drive is clearly obvious.
What is right with it belongs in active speakers, and to a lesser
degree halfway house implementations which are neither pure
current drive or pure voltage drive, usually valve amplifiers
and relatively simple FR or FR + T speakers.
There is nothing wrong with current drive done properly,
and it has some clear advantages over voltage drive.
That has been well known for a very long time.
Pity its such a PITA to use generically, its big downfall.
rgds, sreten.
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Best speaker for Current Drive
There is no doubt that Current Drive gives less distortion and compression. If you can measure acoustic distortion, you can confirm this is the case both for cheap & nasty as well as expensive Golden Pinnae moving coil drive units. Mills & Hawkesford has typical results.
But these advantages will be heard mainly in subs with dedicated special amps.
The type of 'normal' speaker most likely to show improved audible benefits are small high efficiency full-range drive units in big sealed boxes. These will have little bass and often poor HF too.
Current Drive boosts the bass cos the LF impedance peak and the HF cos the increase due to voice coil L .. ie Current Drive gives the speaker flatter frequency response & hence sounds better.
Nelson Pass has a very good article on this.[1]
The Current Drive full range speaker will also have less distortion, but this is likely a small or irrelevant factor in the 'better' sound.
Speaker distortion is complex and audible speaker distortion even more so.
In Double Blind Listening Tests, it is not uncommon to find a speaker with much higher measured distortion described as sounding LESS distorted than another with better measured distortion.
[1] Alas, this type of system is unlikely to do well in Double Blind Listening Tests compared to a modern conventional system of much less cost & size.
There is no doubt that Current Drive gives less distortion and compression. If you can measure acoustic distortion, you can confirm this is the case both for cheap & nasty as well as expensive Golden Pinnae moving coil drive units. Mills & Hawkesford has typical results.
But these advantages will be heard mainly in subs with dedicated special amps.
The type of 'normal' speaker most likely to show improved audible benefits are small high efficiency full-range drive units in big sealed boxes. These will have little bass and often poor HF too.
Current Drive boosts the bass cos the LF impedance peak and the HF cos the increase due to voice coil L .. ie Current Drive gives the speaker flatter frequency response & hence sounds better.
Nelson Pass has a very good article on this.[1]
The Current Drive full range speaker will also have less distortion, but this is likely a small or irrelevant factor in the 'better' sound.
Speaker distortion is complex and audible speaker distortion even more so.
In Double Blind Listening Tests, it is not uncommon to find a speaker with much higher measured distortion described as sounding LESS distorted than another with better measured distortion.
[1] Alas, this type of system is unlikely to do well in Double Blind Listening Tests compared to a modern conventional system of much less cost & size.
This thread has interesting contributions from Bruno Putzeys (amongst others). The Grimm LS1 has current drive on the lower end of the frequency spectrum of its bass/mid unit. Don't bother to click through to the third page though - the author of the book already mentioned turned up and that terminated all discussion 
Current-Driving of Loudspeakers
Current-Driving of Loudspeakers
All that is required to FULLY (!) understand is the equation of forced motion
B*l*i = ds²/dt² + ds/dt + s
That is enough to see why voltage drive is not the "natural" method to drive speakers.
A relatively simple "analog computer" consisting of 2 integrators and 2 summators irons out all of the mechanical paramaters of the speaker in enclosure thus that one can achieve a linear relationship between driving current i and for ex. cone speed, and cone accelleration thus that the speaker is capable of reproducing "rectangle waves". However the price to be paid is large linear cone excursion and very high power handling capability of the speaker coil. This method does not correct the increase of real resistnace of the voice coil with temperature.
B*l*i = ds²/dt² + ds/dt + s
That is enough to see why voltage drive is not the "natural" method to drive speakers.
A relatively simple "analog computer" consisting of 2 integrators and 2 summators irons out all of the mechanical paramaters of the speaker in enclosure thus that one can achieve a linear relationship between driving current i and for ex. cone speed, and cone accelleration thus that the speaker is capable of reproducing "rectangle waves". However the price to be paid is large linear cone excursion and very high power handling capability of the speaker coil. This method does not correct the increase of real resistnace of the voice coil with temperature.
Which characterization in particular? Have you checked their website for an answer? They have an extensive section of literature, some very good technical papers.
edit: Here's a good overview. The derivation of the basic large signal equations is sorta messy, but the following sections are very clear and detailed on their methods.
http://www.klippel.de/uploads/media/Measurement_of_Large-Signal_Parameters_01.pdf
edit: Here's a good overview. The derivation of the basic large signal equations is sorta messy, but the following sections are very clear and detailed on their methods.
http://www.klippel.de/uploads/media/Measurement_of_Large-Signal_Parameters_01.pdf
I have also read the book of Esa Meriläinen. As a German, with limited knowledge of English, I did not really bother me at his expression. I did not understand all the mathematical models. But the basic messages of the book I was able to capture. The advantages which has a current drive can not be dismissed out of hand.
However, I felt that the components are prehistoric in the practical examples. Likewise, the board ..
Some developers use current drive in active speakers. Especially for mid and high frequencies, where you can cut the resonant frequency. For subwoofers, in fact a voltage drive seems to be easier (and better) to implement.
Maybe a current drive works with dipole subwoofers?
However, I felt that the components are prehistoric in the practical examples. Likewise, the board ..
Some developers use current drive in active speakers. Especially for mid and high frequencies, where you can cut the resonant frequency. For subwoofers, in fact a voltage drive seems to be easier (and better) to implement.
Maybe a current drive works with dipole subwoofers?
Thanks for the link. The paper did not answer my question. From an earlier paper I read, I concluded that they use a pulse to excite the speaker so that the speaker is not over stressed during testing. In that paper it talked about using the distortion products to calculate large signal parameters.
My problem with that type of stimulus is that the speaker's suspension will change radically over a rather long-ish time; as in many seconds.
The speaker that I posted in this thread, has its small signal properties shown in the list of T/S parameters. Fs is 32 Hz at rest. Fs becomes 17 Hz but not anytime soon enough for a pulse test to reveal. That 17 Fs occurs after 15 to 30 seconds with 3 amps RMS into its 12 ohms at resonance.
This is why the WTPro measures with steady state sinusoids. Many of our customers are using speakers at Xmax and want to know steady state behavior at Xmax.
Yes, their test system measures that as well. Damn, I wish I could afford one...
My plan is to have that running on a WTPro this year. Right now, with a voltage amp, its much too easy to blow a fuse measuring T/S parameters of a monster woofer at Xmax.
The WTPro is smidge more affordable than a Klippel.
I have a customer who makes extremely expensive speakers. He has told me that he is not satisfied with results obtained having his drivers characterized on a Klippel system.
To each their own.
To FULLY understand speakers, you need to complete the equation and work out what is the acoustic response (at some specified point or acoustic power or bla bla) to current, voltage or whatever. The cone motion is only a step along the way which may or not be important.
Rice & Kellog did this and it shows Voltage Drive is the 'natural' way to drive moving coil speakers. You can derive the response of such a beast from first principles and have it emulate whatever filter function you require.
My first sight of the correct derivation was Baxandall in Wireless World aug68 though it was some years before I fully understood it.
Other useful derivations are Neville Thiele's original paper which spawned the present TS parameter craze.
No such elegant relation exists between electrical current & acoustic response for current drive of dynamic units. (There IS one for current drive of electrostatic speakers but no present unit takes full advantage of it.) You ALWAYS have to fudge the response to get it flat with current drive of dynamic speakers.
Large cone excursion, square waves & high power handling are independent of current or voltage drive.
How do you model in SIM a speaker which produces distorted back emf? Then, use that model with amp SIM and Ls cable and various Zo from amps, gnfb, too. etc. Learn the best desiigns and affects of better speaker modeling loads on amps.
Who has done this... in all these zillion years?
Thx-RNMarsh
Who has done this... in all these zillion years?
Thx-RNMarsh
um - Bode Sensitivity analysis?
in sim add a parallel current or series Vsource to the load - plot the .AC at the amp output to see feedback reducing the amp's output Z, giving you a quick estimate # for effects of the injected loudspeaker error signals
or for real nonlinear sim use .TRAN - put in any function of any I,V in the sim you can write an equation for with behavioral I or V sources
look in the fft, or duplicate or step the sim to compare output(waveform or fft) w/0 the added output terms
[emphasis added to load Isource discussion]
in sim add a parallel current or series Vsource to the load - plot the .AC at the amp output to see feedback reducing the amp's output Z, giving you a quick estimate # for effects of the injected loudspeaker error signals
or for real nonlinear sim use .TRAN - put in any function of any I,V in the sim you can write an equation for with behavioral I or V sources
look in the fft, or duplicate or step the sim to compare output(waveform or fft) w/0 the added output terms
How about some more gain?
Graham’s Class A Imagineering articles spend considerable space discussing amplifier output impedance and interaction with “back EMF” from the complex loudspeaker load, with the possibility of nonlinear back EMF components tossed in as well
Suspicion over feedback control of the output impedance with standard dominant pole miller compensation is suggested as a source of poor performance with the complex/nonlinear speaker load
Cherry has addressed the same issues in “Feedback Sensitivity and Stability of Audio Power Amplifiers” and “ Output Resistance and Intermodulation Distortion of Feedback Amplifiers” JAES V30 #5 May, April 1982
Cherry’s prescription is more gain, specifically more hfe in the VAS and more loop gain to address output stage current and voltage nonlinearities respectively
The JLH output stage Graham uses naturally provides voltage gain, borrowing another of Hood’s ideas (“Gain Stage Investigations” Electronics World july 1998) I changed the Q2 driver transistor (~= VAS transistor) to a small signal mosfet for increased “hfe” at audio frequencies
2-Pole compensation that includes the output stage is implemented with Graham’s component values, slightly rearranged, (Cherry’s nested feedback around the output stage)
I’ve put my thread opening sim of Graham’s circuit together with my modded circuit in the following sim with a complex excitation, 3:1 2 and 20 KHz input at ~ ½ max swing let you look at intermod skirts as well as harmonics, a 1 A, 7 KHz current source in series with the speaker load shows output impedance and intermodulation interactions with the 7 KHz output disturbance that clearly show the advantage of increased gain
Increased loop gain at audio frequencies has improved 2 KHz harmonics ~ 30 dB and the 7 KHz output current induced signal is ~ 40 dB down with the mods in this sim
Speaker cable impedance will prevent the low level of the output impedance ( ~ 10 uOhms! ) from helping at the loudspeaker end but the amplifier’s distortion should show greater immunity to Graham’s “back EMF” concerns
[emphasis added to load Isource discussion]
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At the end of the last Millenium, Wolfgang Klippel did a lot of work on speaker distortion ... which I missed as I was deep in the throes of planning my departure from the Civilised World.
This century, I was amazed to find he had investigated in detail stuff which was really only known to a few major speaker makers.
This Millenium, he's proposed methods to deal with these distortions by modelling them and incorporating them in a 'feedback' loop.
Alas, it appears he hasn't figured out how to commercialise this so his papers only provide a tantalizing glimpse .. even more tantalizing for me as some of his stuff appear in the more advanced versions of my Powered Integrated Super Sub tech.
I think we are a long way from having a LTspice model of what he is doing. To be useful, it needs to make the link to acoustic response.
We still worship ancient pseudo gurus whose designs are unstable into real speaker loads.
http://www.klippel.de/uploads/media/
Need an accuarte load to model with for amp design -
I bring this up at this time as we progress thru the amp -esp CFA OPS issues -because we still - after all these decades - only use passive loads of R, L and C at best.
We really need to be able to measure the relevant speaker parameters and from that make a nonlinear model which is at minimum be a generic test load for amp design. This is quicker than miles/kilometers of math formula and theoretical massaging of what you could/might/ought to get. Who has time for that? The ref work of Klipple is substantial but a quick read thru gives little help to the amp designer who needs to SIM with a realistic load. I wonder if he can be induced into helping us with an electrical model useful to amp designers?
So 'what' do we need to measure ('how' is not so hard with available test equipment). We just cannot continue to design amps without a real speaker load or SIM of one which shows the back emf generated et al. Then we will come closer to designing amps which actually correlate to listening or be more predictable in that regard.
Thx-RNMarsh
I bring this up at this time as we progress thru the amp -esp CFA OPS issues -because we still - after all these decades - only use passive loads of R, L and C at best.
We really need to be able to measure the relevant speaker parameters and from that make a nonlinear model which is at minimum be a generic test load for amp design. This is quicker than miles/kilometers of math formula and theoretical massaging of what you could/might/ought to get. Who has time for that? The ref work of Klipple is substantial but a quick read thru gives little help to the amp designer who needs to SIM with a realistic load. I wonder if he can be induced into helping us with an electrical model useful to amp designers?
So 'what' do we need to measure ('how' is not so hard with available test equipment). We just cannot continue to design amps without a real speaker load or SIM of one which shows the back emf generated et al. Then we will come closer to designing amps which actually correlate to listening or be more predictable in that regard.
Thx-RNMarsh
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again why the program to reprise all of the 70's-80's amp design debates?
the point of Sensitivity Analysis is to show the effect of any signal/distortion vs frequency of the added signal - it works very well for the typical conditions of operation of audio band and audio amps
more detailed analysis are only going to give pointless extra digits of resolution to the magnitudes shown in Sensitivity Analysis
reasonable audio power amps really don't often have audio band problems with loudspeaker loads short of I,V and power limits - nothing "subtle" going on here
high loop gain and good output stage design to Cordell, Self “textbook engineering” level renders "interface distortion, IMD" negligible
read the history, read Cordell, Self, Putzeys, read some "conventional engineering" texts - you'll save lots and lots of time
the point of Sensitivity Analysis is to show the effect of any signal/distortion vs frequency of the added signal - it works very well for the typical conditions of operation of audio band and audio amps
more detailed analysis are only going to give pointless extra digits of resolution to the magnitudes shown in Sensitivity Analysis
reasonable audio power amps really don't often have audio band problems with loudspeaker loads short of I,V and power limits - nothing "subtle" going on here
high loop gain and good output stage design to Cordell, Self “textbook engineering” level renders "interface distortion, IMD" negligible
read the history, read Cordell, Self, Putzeys, read some "conventional engineering" texts - you'll save lots and lots of time
The question of test loads for power amps doesn't really belong here but ...
There is no such thing as a 'standard' non-linear load. There's no such thing as a standard linear load either. But competent designers have always done more than measure on 8R. To mention 2 points
- Unconditional Stability with load as described in tpc-vs-tmc-vs-pure-cherry.html
- Performance on reactive loads to see if SOA protection is triggered in real life
The stupid signals & loads that Otala postulates NEVER occur. In fact there is only 1 situation where a speaker demands more current than what its minimum Impedance predicts. Klippel analyses this thoroughly and its nearly (??) a fault condition. You have to ask yourself what you want the amplifier to do in such a situation. Continuing to deliver zillion amps is NOT it.
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Its clear that a lower Zo from the amp helps the back emf situation as well as a triple EF and/or fets for isolation of the output/back-emf back towards the Vas.
When SIM amps into resistive loads without back emf from speaker have ppm THD, it would be good to know what the comparative THD increased levels are with a real speaker loads.
I'll bring it up in the CFA arena.
THx-RNmarsh
When SIM amps into resistive loads without back emf from speaker have ppm THD, it would be good to know what the comparative THD increased levels are with a real speaker loads.
I'll bring it up in the CFA arena.
THx-RNmarsh
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