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Author Topic: Current-Driving of Loudspeakers  (Read 20304 times)

zmix

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Current-Driving of Loudspeakers
« on: July 21, 2010, 11:55:01 pm »

A new book has been published on the subject of using current to drive a loudspeaker, rather than voltage...
I came across a review in the July 2010 issue of "Voice Coil" magazine..

Click on the cover for a link to the site:

http://www.current-drive.info/files/currentdrive.ota.fi/kuvat/coverimage34_a.png


Quote:

From the back cover
Is it indeed possible that all audio power amplifiers of the world operate and have operated since their inception - on an all wrong principle? Is it also possible that the sonic aberration caused by this gross technical mistake has substantially impaired our experience with loudspeaker-produced sound, with implications even on the musical evolution of our civilization?
According to laws of physics, electric current is that which in a speaker driver effects diaphragm acceleration, which in turn produces sound pressure. Yet all power amplifiers strive, often tooth and nail, to control the voltage at the loudspeaker terminals, which only indirectly affects the current flowing in the voice coil.
Current-controlling loudspeakers is nevertheless not an all-new concept. Some of the benefits it brings, like decreasing of harmonic distortion and expanding of frequency range, have been known at least by specialists. However, there prevails great ignorance about the many grave interference mechanisms that traditional voltage drive inevitably creates and that are fully or partially eliminable by correctly operating current-drive. Also, current-drive does not necessitate active speakers any more than voltage drive does, but amazing improvement in sound quality is achievable by simple means. Based on properties of linear systems and investigation, many other flawed conceptions prevalent in the field of audio technology are also pointed out.
When writing this, virtually no full-range audio equipment operating on the current-principle has yet been seen on the market. One reason may be that controlling the resonance of the bass driver under current-drive has been considered difficult. To resolve this, new handy techniques and circuits are introduced, so that technical obstacles for the extensive utilization of the current-principle are cleared away. Also, it is shown that so-called electrical damping, that pertains to voltage drive, is in reality totally inoperative outside the fundamental resonance region.
The book has been written out with easy-to-understand language for both professionals and interested laymen, so the novel amplifier and loudspeaker realization concepts are made available to all. An ordinary circuit simulator can be used also for predicting the acoustic response, without need for special software. In the appendices is provided valuable basic knowledge about analogue systems.
Additional highlights
how to simulate the acoustic response, as well as electrical behavior, of speakers; in both frequency and time domains
how to model driver inductance accurately for circuit simulation
practical measurement techniques for the essential resonance parameters
why sound pressure follows diaphragm acceleration and not displacement as often imagined
why minimizing the moving mass actually does not help in reducing any distortions
why for any given frequency response (amplitude & phase) there can exist only one transient response (for a given input)
novel and practical circuit designs to implement the Linkwitz transform for bass
interesting phenomena regarding the stability of driver resonance
how to measure capacitors and inductors with high accuracy with only basic equipment and a reference resistor
what and how serious is compression distortion (due to compressors, limiters, etc.)
what group delay actually means and what it doesn't mean
ingenious complementary single-op-amp filter topologies for band boost and band attenuation

bruno putzeys

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Re: Current-Driving of Loudspeakers
« Reply #1 on: July 22, 2010, 02:41:12 am »

The author is making a somewhat one-sided point, but some aspects of loudspeaker performance do improve (sometimes dramatically) with current drive. I'm using a mixture in the LS1.

Because of its belligerent style I'm skeptical of whether this book will really separate wheat from chaff. OTOH, this article from 1989 does so very nicely:  http://www.essex.ac.uk/csee/research/audio_lab/malcolmspubdo cs/J12%20Distortion%20reduction%20MC%20current%20drive.pdf
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zmix

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Re: Current-Driving of Loudspeakers
« Reply #2 on: July 22, 2010, 09:25:53 am »

I agree.  Apparently the AES refused an advertisement he wanted to place in the JAES.

I have two sets of  nearfield monitors that I built to use current drive, I was inspired  by Nelson Pass' article on current drive, HERE.  

The reviewer in Voice Coil actually modified one channel of an amplifier and ran tests of the theory.

johnR

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Re: Current-Driving of Loudspeakers
« Reply #3 on: July 22, 2010, 11:25:21 am »

That quote from the book cover looks pretty arrogant. A valve output stage with no negative voltage feedback is a current source, so this is hardly new.

In the early '90s (if not earlier) it became common for solid-state guitar amps to use exactly the same technique shown in the diagram on the book cover to reduce damping factor. I've seen articles revisiting the subject of current drive in the JAES in recent years too.

Doubt if I'll be buying the book.
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Geoff Emerick de Fake

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Re: Current-Driving of Loudspeakers
« Reply #4 on: July 26, 2010, 08:30:41 am »

There are some problems with current-drive:
The voltage delivered to the speaker varies proportionally to the impedance, so if your speaker/cabinet combination has a large peak at resonance, it will be amplified.
For the same reason, in a multiway system, you will need to linearise the impedance of all sections (a lot of Zobel's and conjugates).
Most loudspeakers are optimised by their manufacturers for voltage drive. Unless you have access to custom-built or OEM speakers, you won't be in a postion to really optimise your design.
I'm not convinced there are many advantages with current-drive.
The most common claim "electric current is that which in a speaker driver effects diaphragm acceleration, which in turn produces sound pressure." is only partially true.
It's the F = B.i.l product that moves the diaphragm, and the induction is not constant, neither the l (length of voice-coil wire immersed in the magnetic field).
For this reason, the effect of counter EMF (and a reasonably low Rdc), is beneficial to the linearisation process of the pressure vs. voltage curve.
Anyway, the current optimisation consists in largely overhung voice-coils, with about 80% of the energy wasted in heat; this is a consequence of the desire to linearise the frequency response of the electrodynamic transducer, which is inherently non-linear.
Voltage translates in voice-coil speed; as a result, the natural response would be a rising one at 6dB/octave, but actually it is the case for the compliance-controlled range, up to a point where it becomes force-controlled and flat, until it reaches the mass-controlled region at -6dB/oct.
Combine this with the cabinet's response and the logical conclusion is that electronic EQ (and/or servo) is the way to go.
Current-drive fixes one aspect of these non-linearities, while enhancing other.
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ssltech

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Re: Current-Driving of Loudspeakers
« Reply #5 on: July 26, 2010, 09:08:29 am »

Brilliant.

Current drive may be great for reverb tanks, but that's a great primer regarding the 'pluses and minuses'.

Thanks.

Keith
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MDM (maxdimario) wrote on Fri, 16 November 2007 21:36

I have the feeling that I have more experience in my little finger than you do in your whole body about audio electronics..

Jim Williams

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Re: Current-Driving of Loudspeakers
« Reply #6 on: July 26, 2010, 11:59:42 am »

ssltech wrote on Mon, 26 July 2010 06:08

Brilliant.

Current drive may be great for reverb tanks, but that's a great primer regarding the 'pluses and minuses'.

Thanks.

Keith


Beat me to it, but I've been jeeping in the high Sierras. I designed some reverb tank circuits many years ago using current drive for the reverb driver coils. Accutronics made custom 3 spring tanks with reversed polarity recovery coils on one tank. I would use 2 tanks per channel. The drive coils were placed in the driver circuits feedback loop with one coil reversed. That cancelled the fundemental frequency and eliminated the "sprong" from low frequency fundementals. No more surf music, it sounded more like a plate.
The recovery coils were wired in series as one was reverse polarity magnetically. That created a hum cancelling pickup immune from hum pickup. With low noise recovery preamps, the s/n ratio was quite good, rivaling plate reverbs.

That was in the very early 1980's though. Got the Lexicon 224 and I stopped using springs...
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zmix

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Re: Current-Driving of Loudspeakers
« Reply #7 on: July 26, 2010, 02:43:27 pm »

I breadboarded a Current drive Headphone amp this morning... with very interesting results..

The first problem I encountered was the fact that most headphones share a common ground between the drivers.  I obviated this by working with a modified Howland Current Pump.

I used a simple 5532, 8 -  7k5 1% resistors, 2 50Ω resistors for the current sense and a pair of 470Ω across the outputs ( to prevent open circuit runaway current). All subject to change, of course!

I've so far only tested it using my Fostex T40RP and T50RP headphones, but the difference in sound is both interesting and fantastic..

The T50RP use a printed copper ribbon driver, and it have a really interesting quality to the sound.  The overall tonality is somewhat like a ribbon mic, all of the details are there, but completely un-hyped. When comparing these to a conventional high end headphone they are initially unimpressive, even 'small' sounding. After wearing them for a while they reveal  a surprising amount of detail.  When driving these from the modified Howland Current Pump, the somewhat cramped tonality stretches out completely... the result is a smooth, open, effortless quality to the reproduction, with a remarkably smooth and extended frequency response..... fascinating, really!




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Geoff Emerick de Fake

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Re: Current-Driving of Loudspeakers
« Reply #8 on: July 26, 2010, 02:59:31 pm »

Headphones are very different animals than loudspeakers (although operating on the same principle and sharing a number of design aspects).
The impedance of headphones is much more constant than that of loudspeakers and their linearity is much better because their efficiency is not as important.
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bruno putzeys

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Re: Current-Driving of Loudspeakers
« Reply #9 on: July 27, 2010, 03:32:18 am »

@Geoff The point is that driving a loudspeaker unit with current and using other methods to control the response (motional feedback or plain EQ) results in a reduction of the distortion contribution of the motor.
I don't think anyone would seriously advocate driving an ordinary loudspeaker with a current source without further mods. In fact even a speaker with full impedance correction would no longer classify as current driven because the zobel networks now determine the electrical impedance seen by the voice coil and it is quite low for most of the frequency range. This is in direct contradiction to Meril
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Geoff Emerick de Fake

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Re: Current-Driving of Loudspeakers
« Reply #10 on: July 27, 2010, 05:57:58 pm »

bruno putzeys wrote on Tue, 27 July 2010 02:32

@Geoff The point is that driving a loudspeaker unit with current and using other methods to control the response (motional feedback or plain EQ) results in a reduction of the distortion contribution of the motor.
This is a common belief, based on a simplistic approach, that may work if the induction was constant. In fact, current drive has no value in linearising the effects of induction variation when the voice-coil moves in the gap. In voltage-drive mode, the effect of counter EMF counteracts the effect of induction variation.
When the coil gets to a zone where the induction is lower, the counter EMF decreases, thus the current increases. So B decreases but current increases. Since F = B I l, it tends to compensate.
Quote:

 I don't think anyone would seriously advocate driving an ordinary loudspeaker with a current source without further mods.
A number of audiophools would.  Razz
Quote:

 In fact even a speaker with full impedance correction would no longer classify as current driven because the zobel networks now determine the electrical impedance seen by the voice coil and it is quite low for most of the frequency range. This is in direct contradiction to Meril
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bruno putzeys

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Re: Current-Driving of Loudspeakers
« Reply #11 on: July 28, 2010, 02:52:19 am »

Well that's the problem with trying to find one answer, ie. "yes" or "no". I find little advantage in using current drive for long excursions (ie. low frequencies), but if you have current drive higher up, distortion easily drops between 10 and 20dB. I tried it and it works. So for the woofer in the LS1 I'm using normal low impedance at bass frequencies and high impedances higher up. So if you ask "is it better to use current drive for woofers yes or no" I should answer, Japanese style, mu.
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John Roberts {JR}

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Re: Current-Driving of Loudspeakers
« Reply #12 on: July 28, 2010, 03:51:45 pm »

bruno putzeys wrote on Wed, 28 July 2010 01:52

 So if you ask "is it better to use current drive for woofers yes or no" I should answer, Japanese style, mu.

It was hard enough learning that yes doesn't mean yes,, now we need to understand zen to grok no...

Laughing  Laughing

JR

PS: interesting culture. I prefer more direct answers that don't reflect on the questioner for asking. While some questions and questioners do deserve it.  
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bruno putzeys

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Re: Current-Driving of Loudspeakers
« Reply #13 on: July 29, 2010, 03:17:32 am »

Ah but it's pretty western to think that "question does not allow definite yes/no answer" reflects on the questioner. It doesn't. People embed false dichotomies in their questions all the time. Everyone does it. Daily. To have a third member outside yes and no is, IMHO, a powerful tool to think about such things.
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Barry Hufker

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Re: Current-Driving of Loudspeakers
« Reply #14 on: July 29, 2010, 07:58:23 am »

"Moot" comes somewhat close to the idea, although also implies open to debate.

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