Practical implementation of current drive (EQ etc.)

[capslock]

Over in solid state, there are currently two threads, one started by me discussing the topology of the amp, and another on damping factor that evolved into a little discussion between Charles and Marcel on how to do the EQ that I want to elaborate on.

If you don't want to use MFB, there are two ways to equalize the fundamental resonance. One could use some kind of EQ before the amp, ideally a Linkwitz pole shifter calculated for Qt=Qm (because Qe= infinity).
Advantage: at fs, where excursion is probably pretty large, the advantage of a more linear driving force is maintained.
Disadvantage: damping is mainly done virtually be feed-forward correction of the input signal, the only physical damping being the mechanical damping. We know the speaker is not an ideal harmonic oscillator, i.e. both the spring action and the mechanical damping are non-linear, and there is no electric damping to reduce these effects.

Resonant electrical circuit in parallel to speaker terminals,
disadvantage: lower efficiency, higher dependence of driving force on excursion
advantage(?): driver is essentially voltage controlled at resonance, mechanical nonlinearities get electrically damped (but then, the damping depends on excursion also...)


I am not sure about the numbers, i.e. Marcels claim that voltage is proportional to excursion around fs. Are there any real advantages to this second approach?

[capslock]

On a side note:

Current drives makes it very easy to drive multiple drivers from one amp: you can series-connect them. To the ratio of amp to driver impedance (which is ideally infinite), the drivers won't influence each other

[ThorstenL]

Konnichiwa,

Originally posted by capslock
If you don't want to use MFB, there are two ways to equalize the fundamental resonance.

You forgot the "best" possible approach.

Why on earth would any average intelligent thinking being attempt to electrically compensate a mechanical resonance!? Talk about placing the cart before the horse.

Or about fixing a problem after it already happened. Next thing we know we will be told negative loop feedback is also in itself a "good idea", rather than a pretty awfull band aid to cover even worse problems.

Sayonara

[capslock]

You'd need pretty massive mechanical damping, which is hard to implement in an existing driver.

Even if you could design a driver from scratch, it would probably be hard to get constant damping over temperature, excursion and frequency. Besides, you'd need a higher current for all frequencies. Even if you no longer have to worry about thermal compression, there is still the field modulation issue...

Regards,

Eric

[ThorstenL]

Konnichiwa,

Originally posted by capslock
You'd need pretty massive mechanical damping, which is hard to implement in an existing driver.

Yet trivial in a combination of enclosure AND driver.

Originally posted by capslock
Even if you could design a driver from scratch, it would probably be hard to get constant damping over temperature, excursion and frequency.

Using a continuos aluminum voicecoil former or a metalisation of suitable thickness on a non-conducting voicecoil former (or a shorted turn on the Voicecoil) come redily to mind as doing exactly that....

Originally posted by capslock
Besides, you'd need a higher current for all frequencies. Even if you no longer have to worry about thermal compression, there is still the field modulation issue...

Which is trivial to resolve if the driver uses a fieldcoil.

If we design a driver "from scratch" for current drive we can make this work quite easily. But even with existing drivers we can reduce the Qm drastically by using suitable methods, many of which can be found in patents from the 1940's and 1950's....

Sayonara