Loudspeaker operation: Current drive better than voltage drive?

Here is something to think about this weekend: I thought that not very much new winds comes on HiFi field. But now I came by an interesting idea: current driven speakers instead of traditional voltage driven. EDN magazine has a nice two article series on this: Loudspeaker operation: The superiority of current drive over voltage drive and Loudspeaker operation: The superiority of current drive over voltage drive (Part 2). After reading those articles and checking some other sources, this current driving starts to make sense. Is it indeed possible that the audio power amplifiers of the world operate – and have operated since their inception – on a wrong principle?

Nelson Pass writes at Current Source Amplifiers and Sensitive / Full-Range Drivers that conventional wisdom holds that a pure voltage source amplifier is ideal for audio applications, and generally designers of loudspeakers work to that assumption. This belief has particularly been dominant since the development of high power solid-state amplifiers that began in the 1960’s. A small minority of audiophiles thinks otherwise, and these are often people using low wattage tube amplifiers with unusual looking speakers.

Loudspeaker operation: The superiority of current drive over voltage drive article is an overview of the destructive effects that voltage drive has on the performance of electrodynamic loudspeakers. Today, practically all available audio amplifier and loudspeaker equipment works on the voltage drive principle without significant exceptions (exhibiting low output impedance and thus strives to force the voltage across the load terminals to follow the applied signal) . The article says that in both technical aspects and listening experiences equally indicate that voltage drive is a poor choice if sound quality is to be given any worth. There cannot be found any scientifically valid reasons that justify the adoption of voltage as the control quantity – it is only due to the historical legacy originated almost a century ago.

Loudspeaker operation: The superiority of current drive over voltage drive (Part 2) continues the current driving theory with more equations. A more comprehensive treatment of the subject can be found in the book Current-Driving of Loudspeakers: Eliminating Major Distortion and Interference Effects by the Physically Correct Operation Method by Esa Meriläinen. The book is also available in Finnish. There are also other scientific papers on current drive.

Why has a whole field of technology of better loudspeaker performance been left practically unexplored? That’s that Esa Meriläinen is asking in his book Current-Driving of Loudspeakers Eliminating Major Distortion and Interference Effects by the Physically Correct Operation Method and http://www.current-drive.info/ web site.

Why have the basic laws of electrodynamics been disregarded in the design of loudspeaker operations? A frank exposure of the flaws of voltage drive and a thorough, groundbreaking guide to the physically reasonable way of operating electrodynamic loudspeakers – current-drive. The author argues that the sound quality of virtually all existing speaker systems has been severely impaired by the manifoldly indefinite electromotive forces induced in the voice coil (arising from both motion and inductance), that corrupt the flow of current; and what’s dramatic – the measurable evidence is plain and overwhelming. These inherent EMFs in themselves can never be suppressed by any kind of amplifier, but their detrimental effect on current and hence on sonic performance can be eliminated by adequate source impedance. Thus, the secret of valve amplifiers also becomes apparent.

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.

An amplifier that converts input voltage into output current is generally called a transconductor (or transconductance amplifier). There are also circuit examples like Transconductance Amplifier With Adjustable Resonance and Baffle Step Compensation.

Current driving will change the amplifiers, but it will also affect speaker design. In general, loudspeakers have to be designed for current-drive, starting from the choice of drive units. Conventional speakers are seldom suitable as such. There are also design examples: A two-way system with tightly tuned 1st-order crossover and  A series mode 1.5-way system with passive compensation. Full-range drivers can also work.

What about the negative feedback in amplifiers related to current drive? As power transistors are quite nonlinear and variable devices, distortion and other problems can’t be avoided in current amplifiers. In conventional voltage amplifiers, giving up negative feedback usually increases the output impedance of the amplifier remarkably. There is every reason to believe that it is mostly this increase in impedance and the consequent decrease in the EMF-derived interference currents that is behind the apparent sonic advantages in such an approach rather than the absence of the feedback in itself. Check also page Effects Of Source Impedance on Loudspeakers.

Some  related links:

http://www.enjoythemusic.com/magazine/manufacture/0212/

http://www.customanalogue.com/tlcl_speaker.htm

http://repforums.prosoundweb.com/index.php?topic=33953.0

http://www.ultrasound-hifi.com/Us_wh_1/FR/Amplificatori/pilot_corrente_en.html

 

2 Comments

  1. Tomi Engdahl says:

    There seems to be at least one commercially made current mode HIFI amplifier:

    The First Watt Model F1
    Current Drive Power Amplifier
    And Introducing A Pair Of Matching DIY Loudspeakers:
    The BassZilla Leopard And Panther
    http://www.enjoythemusic.com/magazine/equipment/1204/firstwattf1.htm

    Also unusual is the amplifier itself, the brainchild of Nelson Pass, and as far as I know, the only current-drive design currently available to the consumer. Before you get the idea the F1 is in fact an ordinary consumer product, take heed of the following excerpts from the F1 owner’s manual, penned by none other than Nelson himself.

    While I have a nice business (Pass Labs) to tend in the daytime, sometimes I come home and can’t resist making something. First Watt is a kitchen table operation – the projects that are not appropriate for my day job.

    F1 is Nelson’s first “kitchen table” design in the First Watt series – a 10 wpc, stereo transconductance power amplifier; a fancy name for an active current source. The F1, unlike a voltage-controlled amplifier, attempts to sink a constant current into the loudspeaker – irrespective of variations in the load impedance. There is only one gain stage, operating in pure “Class A” mode: 100 watts per channel are dissipated to deliver a precious 10 watts to the speaker. The amplifier runs hot to the touch and requires plenty of ventilation. And, of course, no feedback is used to reduce distortion. Despite the lack of feedback, this is probably the quietest amplifier money can buy, with a typical figure of about 100 pico-watts noise (pico-watt is one trillionth of a watt).

    “This is a tinkerer’s amplifier, and experimental tool. If you just want to be a consumer, then buy something else, otherwise be prepared to patiently experiment with your system to get the best performance.”

    Mills and Hawksford coined the term current drive in 1989, in two essential papers published in the Journal of the Audio Engineering Society, dealing with Current Drive Technology.

    Current drive offers a major alternative to the speaker-amplifier interface. A transconductance amplifier is operated as a current source with a high source impedance relative to that of the speaker. The output impedance must also be linear and frequency independent. An ideal current source would possess an output impedance so large as to be considered infinite relative to that of the speaker. The F1′s 80-ohm impedance is certainly adequate in this regard. As a result, as Nelson puts it: “the amplifier delivers a precise current to the voice coil of the loudspeaker driver, ignoring the series impedance elements in the circuit, including the wire, the inductance of the voice coil, the resistance of the voice coil versus temperature – all that stuff.”

    Current drive offers the potential for dramatic reduction in moving-coil speaker distortion due to power compression and other voice-coil non-linearities. However, there are two basic practical problems to overcome. On the amplifier design side, it turns out that it is difficult to design a high-power current source.

    Consider that a woofer’s resonance region is no longer current limited and sinks the same current as the frequency band above resonance. That makes for a huge boost in bass response around the resonance region.

    The bottom line is that using drivers of differing nominal impedance but identical sensitivity ratings will result in output mismatches. Finally, conventional parallel type crossover networks do not work well with current sources.

    Other article on same topic:
    http://www.6moons.com/audioreviews/firstwatt/firstwatt.html

    Reply
  2. Tomi Engdahl says:

    Loudspeaker operation: The superiority of current drive over voltage drive
    http://www.edn.com/design/consumer/4423155/Loudspeaker-operation–The-superiority-of-current-drive-over-voltage-drive

    This is an overview of the destructive effects that voltage drive has on the performance of electrodynamic loudspeakers. A more comprehensive treatment of the subject can be found in the book Current-Driving of Loudspeakers: Eliminating Major Distortion and Interference Effects by the Physically Correct Operation Method by Esa Meriläinen.

    Today, practically all available audio amplifier and loudspeaker equipment works on the voltage drive principle without significant exceptions. This means that the power amplifier acts as a voltage source exhibiting low output impedance and thus strives to force the voltage across the load terminals to follow the applied signal without any regard to what the current through the load will be.

    However, both technical aspects and listening experiences equally indicate that voltage drive is a poor choice if sound quality is to be given any worth. The fundamental reason is that the vague electromotive forces (EMF) that are generated by both the motion of the voice coil and its inductance seriously impair the critical voltage-to-current conversion, which in the voltage drive principle is left as the job of the loudspeaker.

    http://www.current-drive.info/

    Reply

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