Questions & Answers

How soon will you have audio equipment employing these principles, available to be purchased by the public?

I'm not a businessman and at the moment don't have plans or resources to set up a commercial production line. Instead, I'd rather see some existing or emerging manufactures awaken to the reality and the huge opportunities that the concept offers, possibly also making use of my circuits.

How are loudspeakers implemented for current-drive? Can it be used for full range drivers?

In general, loudspeakers have to be designed for current-drive, starting from the choice of drive units. Conventional speakers are seldom suitable as such. One design challenge is how the drivers can be kept in the current-drive mode when making passive frequency response shapings that are needed to compensate the rising response towards high frequencies due to the baffle step and horn effect of the cone. The bass resonance region also needs some treatment if the mechanical Q value of the system is not low enough.

The book shows how to accurately model the drive units and design these speakers using a circuit simulator, detailing two completed designs:

 - A two-way system with tightly tuned 1st-order crossover
 - A series mode 1.5-way system with passive compensation

Full-range drivers can also work, but I would not recommend large cones or very sensitive or exotic drivers. To get the best results, it is advisable (like usually) to avoid using drivers in their break-up mode region.

If one has to rely on existing speakers, they have to be closed (not reflex) and preferably two-way with 1st-order, and the response could be straightened by a graphic equalizer.

Almost as important as the corrent driving mode is also to fill the cabinet interior tightly by effective damping material to suppress the cabinet noise from leaking through the cone and the walls. This also helps much in keeping down the Q.

Can you make your circuits avoiding all negative feedback?

As power transistors are quite nonlinear and variable devices, distortion and other
problems can't be avoided in such design. However, I don't see any particular technical reason to avoid feedback, especially in current-drive.

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.

How can the speaker fundamental resonance be damped under current-drive?

The sonic superiority of current-drive shows up mostly in the middle and treble regions. Instead, at bass frequencies, where damping issues only have significance, the driving mode is not as important as elsewhere. Therefore, despite applying current-drive for the most part of the spectrum, we still have quite free hands to use various means of damping, also electrical, to treat the fundamental resonance region.

By and large, there is much misinformation in what is commonly conceived of speaker damping and the EMFs. Notably, the significance of electrical damping and so-called damping factor has been greatly overstated in the audio community and by the marketing departments. In the book, the subject is discussed and the myths exposed from an engineering standpoint, with appropriate equivalent circuits, underlying equations, magnitude/phase diagrams and real-world examples clearly presented; as opposed to the merely verbal, vague energy flow /voltage flow jargon and even wishful thinking commonly met considering these issues.

It is important to understand that damping and the Q value of the driver-enclosure combination have effect only near the resonant frequency. Instead, at all other frequencies, from about 200 Hz up for woofers, any driver damping doesn't have any effect at all. This can also be demonstrated by basic modelling with typical driver parameters.

The motional EMF of the driver can actually be called a back-EMF only in the resonance region, where this EMF voltage acts about in phase with the applied signal and therefore reduces the flow of current on voltage drive, thus effecting the damping. Instead, when frequency rises from the resonance region, the EMF voltage soon turns perpendicular to the resistive voltage and current and at the same time decreases in magnitude, going below the resistive component typically at some 150 Hz or so. Thus, in the whole mid-frequency region, the motional EMF no more damps or controls anything but acts merely as an uncontrolled interference source between driver voltage and current, doing nothing useful.

Electrical damping can in every aspect be substituted by mechanical damping with the same end result. What electrical damping exactly does is to exert to the moving system a mechanical counter-force that is at every moment directly proportional to the instantaneous velocity of the voice coil according to the equation F = (Bl)²v/R (=constant*v), where v is the velocity and R the voice coil resistance (plus other possible series resistances). There are no other effects produced by electrical damping than this velocity-proportional counter-force and the consequent reduction in the total Q. Just the same kind of force is also introduced by mechanical resistance, that can be determined by driver materials and structure and also adjusted by cabinet stuffing. Here, the force is simply F = bv, where b is the total mechanical resistance affecting the moving system.

Thus, there is nothing indispensable in electrical damping; and in principle, there cannot be any difference in the driver's resonance behavior, neither in frequency nor in time domains, whether the damping be accomplished by a low-impedance amplifier or mechanically.

On pure current-drive, the effective Q value is determined solely by the mechanical Q of the system. As all available speaker drivers are designed to work exclusively on voltage drive, their Q_m values are usually too high for current-operation as such. However, it would surely not take long to develop self-damping drivers if only some effort were put to it. Even now, there are rubbers that yield free-air Q_m values of around 1.5; and according to tests with cotton cloth enclosure stuffing, the final value can yet be considerably lowered from this.

Often it is not even necessary to reach to the 0.7 since with a slightly higher value, the mild boost that develops in the 100 Hz region can be used for benefit to compensate some part of the baffle step.

The damping can also be effected by active equalization with the same end result, and the book introduces several novel circuit ideas for this.

How much benefit can be had from using current-drive on headphones?

In electrodynamic headphones, the achieved benefits of current drive are ususally very minor compared with the improvement in loudspeaker operation. This is mainly because in the impedance of headphone transducers the relative proportion of the DC resistance is generally much higher than in speaker drivers, so the interfering current components produced by the electromotive forces are left rather small even on voltage drive.

A greater problem is generally constituted by the unevenness of frequency reproduction and its dependence on the ear canal shape. As with loudspeakers, the frequency response of headphones also exhibits certain changes when moving to current-drive. These changes may, depending on the case, also result in undesirable impressions.

How is speaker efficiency affected by current-drive?

The operating mode in itself does not affect efficiency; but at the resonant frequency (and only there), driver efficiency is about proportional to the resulting mechanical Q value. However, even at low Q_m values, that are suitable for current-drive, the limit of linear displacement is generally encountered much earlier than any power limitation.

Powers needed to drive the cone to the rated maximum excursion are actually surprisingly low. If we take as an example a middle-sized 8-inch woofer in a closed enclosure of 40 litres, with quite typical parameters: Bl = 9 N/A, R_c = 6 Ω, m = 0.025 kg, f₀ = 55 Hz, and mechanical Q = 2.9, the power it takes to move the cone at +/- 5 mm amplitude is at the resonant frequency(f₀) only 5 W, and only part of this is dissipated in the voice coil. With real audio signals, where the RMS value relative to peak values is considerably lower than in a sine wave, the average power needed to reach the +/- 5 mm limit mentioned is correspondingly yet lower.