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Old 24th January 2010, 05:31 PM   #11
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I thought the fundamentals of current drive in loudspeakers were well explored for decades now. If you want to order a custom designed driver for your current drive product I think a handful of manufacturers would welcome it so long as you can afford 50 pieces or so. It wouldn't be too surprising to end up noticing sales of their regular line continue indefinitely for parts used in highly competitive products, however. So much more care in use of the words "secret" and "reveal" by technical people would be refreshing in this age.
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Old 27th January 2010, 11:36 AM   #12
ETM is offline ETM  Finland
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Quote:
Originally Posted by Andrew Eckhardt View Post
I thought the fundamentals of current drive in loudspeakers were well explored for decades now. If you want to order a custom designed driver for your current drive product I think a handful of manufacturers would welcome it so long as you can afford 50 pieces or so. It wouldn't be too surprising to end up noticing sales of their regular line continue indefinitely for parts used in highly competitive products, however. So much more care in use of the words "secret" and "reveal" by technical people would be refreshing in this age.
Presumably you are referring to the two articles by Mills & Hawksford from 1989 where they presented their exceedingly complex, impractical amplification system with velocity feedback. Though the other article offered also some distortion analysis, in no way can these papers be regarded as an adequate or exhaustive study of the fundamentals of current drive or the flaws of voltage drive.

I think the issue is too pivotal to be reduced to a mere little kitchen table project. It must be revealed and proclaimed until every man has a real possibility to choose between the reasonable and foolish, the uncorrupted and corrupted ways of sound recreation.
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Old 27th January 2010, 12:04 PM   #13
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Chris - the author and the poster appear to be one and the same, thus this is a commercial advertisement...

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Old 27th January 2010, 01:05 PM   #14
kikey is offline kikey  Sweden
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Hi

Never mind the use of words.
I think current drive have great potential.
I think it is a shame there are so little info and talking about cdrive on the net.
I appreciate that the author bring up this subject.

When it comes to guitar amps, there are several commercial products that use circuits inspired by cdrive. I think Peavey have some patents.
A cdrive guitar amp article:
Lenard Audio - Education - Guitar Valve Amps.


Best Regards,
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Old 2nd March 2010, 04:07 PM   #15
ETM is offline ETM  Finland
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The book has been released with a new subtitle and ISBN. The content is essentially unchanged. The old version is discontinued. The new subtitle Eliminating Major Distortion and Interference Effects by the Physically Correct Operation Method is perhaps more descriptive and easier to face.

Click the image to open in full size.

Available at Amazon.com and other retailers.
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Old 2nd March 2010, 08:15 PM   #16
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Quote:
Originally Posted by ETM View Post
The old version is discontinued. The new subtitle [I]Eliminating Major Distortion and Interference Effe
Hopefully makes it a collector's item. I'm halfway thru with a current mode speaker gestating in my head (we already have a variable transconductance amplifier)

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Old 2nd March 2010, 08:54 PM   #17
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My copy arrived last week. I like your book and there are some upcoming projects in my mind.
Thanks!
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Old 3rd March 2010, 12:17 AM   #18
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I've been thinking about current vs voltage drive as well.

Perhaps hybrid voltage/current drive would work best for "status quo" speakers. Say, voltage drive at low end and current drive at high end. Of course this would be specific to the speaker.

Current drive also solves problems with instability encountered with long cables and capacitive loads (of course statics are more suited for voltage drive If I understand correctly, to avoid confusion). (On that note, driving a static with a current amp wouldn't be any better than driving a coilspeaker with a voltage amp...)

I also suspect there is full duality between voltage and current. People generally think that voltage comes first, and then current, and that current is dependent on voltage. This seems only half true however, when you consider a coil moving through a magnetic field. That is a current source rather than voltage source, right?

So expanding on this, modern circuit design has mostly focused around voltage-oriented design, seemingly because the duality of voltage and current is not fully realized. Theoretically, circuits could be designed the other way around, fed by current sources instead of voltage sources (although most common semiconductors might be biased on way or the other, which is important to note).

- keantoken
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Old 3rd March 2010, 12:32 AM   #19
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Originally Posted by keantoken View Post
Perhaps hybrid voltage/current drive would work best for "status quo" speakers. Say, voltage drive at low end and current drive at high end.
Esa suggests that possibility in the book.

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Old 3rd March 2010, 03:31 AM   #20
anatech is offline anatech  Canada
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Hi Bear,
Yes, I realized that later. Your post really drove it home though.

Hi ETM,
Quote:
The book contains at least thee whole chapters that deal with designing the speaker in a totally new way. So why are you so humbly advising me on this?
What you have stated here seems to be in direct conflict with what I have seen directly while working with speakers over many years.

Quote:
Damping of the speaker is factually only a matter of shaping the frequency response at the low end, and there is a multitude of ways to accomplish this in current-drive as well as in voltage drive.
The only way to control the movement of the cone to damp a resonance is to dissipate the energy due to back emf. A dynamic brake if you will. If this option is not available, how would you arrest the motion of the cone?

I am worried about how you intend to control the movement of a speaker cone with a high impedance. From what I have done on my own, this will not happen. It happens that I use a high impedance source when measuring the loudspeaker to find out what the T/S parameters are. Yes, I know you don't use them with current drive. The fact remains that with a current source to drive the loudspeaker, the acoustic response does follow the impedance curve for that driver.

Quote:
However, this here is not a proper way to design crossover filters for current drive because the impedance seen by the drivers is not taken into account.
The same issues exist for a voltage source driving a speaker system. No difference between them for this point.

Quote:
There is full dualism between these two modes of operation.
If you mean that you can design a crossover network either way - then yes. However, I don't agree that driving a system with a pure (or close to pure) current source solves your problems. I think you trade one set for another set of troubles.

Quote:
Multi-amping with voltage drive is the worst method because the drivers see zero impedance at all frequencies and thus the spurious interference currents generated by the electromotive forces (both the motional EMF and inductive EMF) are at maximum for all frequencies.
Ideally, each driver may see a zero impedance across it's operating frequency and out in the stop band also. I could only wish! But here you've thrown in the term "spurious interference currents", but they are not spurious at all. Any back emf results from current flow as the loudspeaker motor acts as a generator whenever the coil moves through the magnetic field. These are not spurious, but rather something that is defined completely.

Quote:
the spurious interference currents generated by the electromotive forces (both the motional EMF and inductive EMF) are at maximum for all frequencies.
No, the current is created as a direct result of the voice coil moving through the magnetic field. The exact same amount of current is created whether you drive the speaker with a high or low impedance. It is completely defined by the number of "lines of magnetic force" that are cut as the coil moves through "x" distance with "y" velocity. So if you compare the back current for a given cone displacement within the same amount of time, you will generate identical opposing currents. The "back emf" is an unrelated quantity that depends solely on how much resistance this current flows through. The voltage is an artifact and we are concerned with the current no matter how we supply the power to the speaker system.

Quote:
Of course the design considerations differ somewhat, but even with existing types it is possible to reach amazing results. This is also addressed in the book.
Addressed how? A short summary will not damage your book sales.

Quote:
Such "compelling reason" may well have been that tubes are not able to deliver enough current to drive a loudspeaker directly.
They could have designed loudspeakers with a much higher impedance for one, and that would have lowered the turns ratio between primary and secondary. On top of this, the current flow in the secondary would be greatly reduced.

We currently do have a higher impedance sound distribution standard in place, used mostly in commercial or industrial applications. I am referring to the very common "70 volt line" seen in PA applications. Since this is a standard, it would have been very easy to adopt this standard for home use. They didn't, and they won't from what I can see. This represents a half way point between voltage and current sources. Many commercial amplifiers do not use a matching transformer, the line is driven directly by the output stage. I would think that this might be an entry point for your ideas. A voltage amplifier, these things ain't.

Quote:
At least with modern amplifier technology, there is not any compelling reason, or any valid reason whatsoever, to operate a speaker by voltage.
Sure there is, otherwise we would not be doing this to begin with. Solid state devices, operated in a feedback loop, are exactly what is commonly done, and this does present as a low impedance.

So, can you expand on what these reasons might be? You made a comment without any statement that might back your word up. I am being open minded about this, I am looking for reasons why what you are saying might be true.

Quote:
Nobody has ever presented such a (scientifically valid) reason, and nobody ever will because such a reason simply does not exist. (And reasons relating only to frequency response shaping are not valid reasons.)
That is all well and fine for you to say, but can you elaborate at all on this? Whatever you are seeing is not blindingly obvious to me. I know why the industry and current state of the art is with voltage drive. You are in opposition to this idea, but that is all you have said so far. Throw us a bone here please.

Quote:
We have namely been deceived - not so much with intent but by negligence or ignorance anyway - by providing to our use solely audio power equipment that disregards the realities of physics, and by establishing, as the backing of the practice, yet odd myths that do not stand up any closer examination.
That is a mighty strong statement to be making for beginners. All our current technology does in fact agree with physics, otherwise nothing would work. I have spent the better part of my life involved in audio, one of my common comments are that what we do is simply a small corner of physics. You are going to have to come up with a strong argument to back up what you have said here. Chapter 4 can be read by those who bought your book, I'll accept anyone's comments that can be argued while also not running afoul of the laws of physics.

Quote:
Most regrettable in the present practice is that the manifoldly indefinite electromotive forces of the loudspeaker are allowed to freely mingle with the reproduced signal."
Huh? What exactly are you trying to say here, the wording confuses me.
Firstly, forget EMF. We can only talk about current flow, back EMF being the result of that current interacting with the impedances in the entire system.

Back to current flow. As I mentioned earlier, every time the voice coil moves in any magnetic field, it will generate an opposing current to that current which caused the motion to begin with. If that force was applied mechanically, the current will create an opposing force back through that mechanical system. That's why the shaft of a generator is harder to turn when you draw more current out of it. This is also why we short out the terminals to a meter movement for shipping, the magnetic brake provides protection from bending the pointer due to mechanical shock. Lab quality instruments drive the meter from a low impedance source to damp the pointer so it settles to a new position quickly. We do the same thing to loudspeaker coils (with the output impedance of the amplifier) for the same reasons. To accurately control the position of the cone at all times. That means that the energy transfered to the air is a more accurate representation of the electrical signal causing the motion.

I can't see how you will do this with a current source type amplifier.

Quote:
In short, we have these two choices: Either the voltage is controlled whereby the current becomes severely corrupted (as is summarized here) or then the current is controlled whereby the voltage becomes severely corrupted.
I agree with the choices, but not with the assumptions you have attached to the choices. In fact, what I have read on your web site seems to contain many conclusions that are not supported in any way.

I really have to take exception to the terms you use, "becomes severely corrupted", for example. It is true that if you control one parameter, then the other (say ... current) is not directly controlled. But that does not mean it is "corrupted". In fact, current flow in this case is modified by the driver itself in predictable ways, and in accordance with the accepted laws of physics. Another point you make is the fact that a speaker also operates as a microphone. This is absolutely correct, and explains how a simple intercom system works. However, the magnitude of this property has been greatly exaggerated in your text. I have worked with transconductance amplifiers before (Acoustat Trans-Nova anyone?, Hafler Trans-Nova?), and I am familiar with how they both operate and sound with conventional speaker systems.

Also, current drive does not cure poor connections, or mitigate the problems caused by bad connections. I know because I worked a great deal with telecommunications systems. When you have a bad connection, the current loop a regular telephone works with still acts up. It is true that a diode junction has less of an effect on a signal if it's riding on direct current that never runs into cutoff or saturation.

Esa, can you explain the science behind some of your comments made here? I'm always ready to learn something new.

Best, Chris
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