Lets talk about transconductance amplifiers

[vacuphile]

Originally Posted by hahfran

From the mechanical-electrical analogy it is immediately obvious that electrodynamic speakers are current driven. Using a current source amplifier as drive requires a compensation of mass compliance ... i.e. of all mechanical properties of the speaker.
The attachment shows a circuit patented by AES for vented woofers, which is however
applicable to midrange and tweeter as well. The elements at the output of the power amp are the electrical representation of a vented woofer.
Thus current driving makes only sense for active speakers.
The attachment has a circuit of an inverting power amp.

Hi Hafran,

That seems to be a very creative solution for a basic problem with current drive. However, I see a problem with this approach.

The mechanical properties of any speaker are not linear, both at different power settings, and in time with aging etc. The compensation circuit would have to dynamically change when these driver parameters change, otherwise the compensation parameters will no longer match those of the driver.

Anyways, I would be interested to know if this setup has been implemented somewhere, and if results have been published,

vac

[indianajo]

Originally Posted by kipman725

I
It seems the best way of measuring driver position would be some kind of laser measurement device. My idea for this would to be measure the velocity of the cone via the Doppler shift on a modulating square wave on the laser. I don't know anything about lasers though or how fast photo detectors can go...

Putting the speaker (woofer anyway) in the feedback loop was done in 1972? L. W. Erath of Houston Texas did it in commercial speakers. I owned a pair of LWE III two way sealed speakers where some Jones plugs were installed in my dynakit ST70 amp to put the speaker sensor in the amp feedback loop. The speakers had impressive bass for 10" woofers, and not buzz, buzz, buzz either. Considering the technology breakthroughs in 1972, I imagine he was using aluminum reflective tape and lights and new analog sensors available then. Woofer excursion as about 1" on Tommy Boyce & Bobby Hart 45's. (ATCO 45's had very high velocity then).
One speaker finally died due to an open tweeter, not the woofer. Then my father threw them away while I was in the Army. Don't know what was on the back of the woofer.

[bear]

Iirc, and afaik, the LWE speakers used a resistor in series with the woofer to generate the feedback signal...

There is a thread on them here, back a few years...

You can build ur own LWE style feedback loop onto almost any speaker...

_-_-bear

[jan.didden]

Philips did it in the 70-ies with a piezo-electric element inside the dustcap. The voltage generated by the piezo was representing speaker acceleration, and they integrated that to get velocity and then again to get position.
This information was used in a feedback system to control the low end.

jan didden

[hahfran]

Originally Posted by vacuphile

Hi Hafran,

That seems to be a very creative solution for a basic problem with current drive. However, I see a problem with this approach.

The mechanical properties of any speaker are not linear, both at different power settings, and in time with aging etc. The compensation circuit would have to dynamically change when these driver parameters change, otherwise the compensation parameters will no longer match those of the driver.

Anyways, I would be interested to know if this setup has been implemented somewhere, and if results have been published,

vac

sure google speaker maker AES .
AES had this compensation patented and is about to implement the method
on midrange in a 3 way. The trick is as simple as possible, the output impedance of the driver mirrors the impedance of the speaker . This can of course only be accomplished with a transconductance amp.

[forr]

For long, I could not get any satisfying explanation for the reason why, for the same excursion at a given frequency, a current driven loudspeaker unit generates less distorsion than when voltage driven.

It now seems to me that it's related to the phenomenon known as instability of a driver :
[...]As the coil moves away from the central rest position, the bacl EMF is reduced due to the lowered Bl factor and hence more power [I should say "current"] is drawn from the amplifier (assuming a normal constant voltage source)
Martin Colloms "High Performance Loudspeakers"

Is a driver a device controlled by voltage or by current ?
Both points of view are valid.

Frequency response is usually flatter when voltage driven, and frequency response is the first thing we try to control. Whatever the source impedance, the same voltage at a given frequency across a driver will give the same output (neglecting temperature effects). This is the base for the design of passive crossovers and transforms, and then, the driver is considered as a voltage controlled device.

However, if we seek causes of non-linearities of a driver, it would be easier to observ its behaviour with a current source which is related to the force applied to the cone, and to consider it as a current controlled device.

An article of combination of a current controlled driver and acceleration MFB, I am not aware of an application :

Greiner, Travis et Sims :
"Loudspeaker Distortion Reduction
JAES Volume 32 Issue 12 pp. 956-963; December 1984
Nonlinear distortion and frequency response aberrations are the major weaknesses of low-frequency loudspeaker systems. A multiple-loop feedback system is presented that deals effectively with both of these problems. The feedback system, utilizing current and velocity feedback, also decouples the system Q factor from enclosure and driver parameters. A theoretical analysis is presented which has been the basis for several successful system designs. Extensive data on several practical systems have been taken to show the usefulness of this approach. Data for two of these systems are given here.
Authors: Greiner, R.A.; Sims, Jr., Travis M."

[kipman725]

I came up with an idea for a current amplifier with gain. I can see the obvious issue of device mismatch etc. simplicity is appealing.