Current source amps and shorting rings

[joe carrow]

Hi everyone,

I think I understand the basic idea of how current feedback amplifiers have an advantage over voltage source amplifiers for dynamic loudspeakers- from what I've read on here, the force applied by the voice coil is proportional to current passing through the voice coil, not the voltage across it.

Also, it seems to me that one of the large issues a current source amp can address is a non-constant speaker impedance. The amplifier delivers the voltage required to push the desired current, so the current response is very good, even when the impedance rises or falls. That makes sense to me- voltage response looks crazy, but current is what's really proportional to force.

Another big issue that current source amps seem to address is that a coil moving in a magnetic field induces a current- the current source amplifier's current feedback compensates for that, and again- all is well.

A third problem in a driver, and I'm not sure how a current source amp addresses it, is that as the speaker is working hard,the magnetic field fluctuates.... it's not stiff enough to remain constant. I have read that a copper ring in the gap has a current induced in it when the magnetic field changes, and the induced current opposes the change in magnetic field- so the shorting ring has the effect of stiffening the magnetic field. I'm not sure if that has anything to do with current feedback!

So I guess what I'm really asking here is-

1) Is the main purpose of a shorting ring is to oppose modulation of the permanent magnetic field?

2) Can a current source amplifier compensate for this modulation?

and

3) Is there more or less benefit to a current source amplifier when a shorting ring is present?

I am asking all of this because there has been a lot of talk lately about distortion in speakers. The Dayton RS speakers have low distortion; adire's XBL line have low distortion; JBL and Eminence are among the pro brands with extensive measures in place to reduce distortion..

What I want to hear is that the effects of a distortion reducing amplification system will have an even better effect with speakers that were low distortion to begin with. Just curious.

I noticed that Nelson Pass's article about current source amps concentrated on full range drivers. (that was kind of the point of the article, I know!) What I never hear about is what the measured distortion in these drivers really looks like!

That's enough from me for tonight.

[Svante]

You miss one thing: The acceleration* of the cone is not proportional to the force applied to it. Near the resonance, the mechanical impedance is much lower than at higher and lower frequencies, which will lead to a peak in the excursion and response at the resonance. If you drive the speaker with a constant voltage, the raise in electrical impedance will balance this.

Loudspeaker drivers are essentially designed for constant voltage drive. The only positive effect of constant current drive is the elimination of compression effects from the resistance increase of the voice coil as it heats up, and the elimination of distortion due to nonlinearities of the voice coil inductance.

*Sound pressure is proportional to the cone acceleration, largely.

[EC8010]

Agreed, post-Thiele & Small loudspeakers (everything designed after the 70s) rely on constant voltage drive to set the roll-off of the high-pass filter at mechanical resonance. However, if one can arrange not to drive the loudspeaker down to its mechanical resonance, then current drive offers a reduction in distortion in addition to freedom from power compression. Hawksford reported a 20-30dB reduction in distortion in a JAES paper, and he wasn't the first.

The more significant issue with current drive is that one has to start from that point and design from there. Passive crossovers are conventionally designed to be sourced from constant voltage amplifiers, and loudspeakers need different equalisation when driven from current drive (due to voice coil inductance). Current drive needs to be connected directly to the loudspeaker, so that means it's much easier to implement with full-range loudspeakers.