Woofer Distortion 101

[speaker dave]

Apart from lowering the Qt, negative resistance has been said to have good effect on linearity in the resonance region (Jeff Macaulay, "MFB Reply" in Letters, Electronics World, January 2003, p43).

I'd like to see measurements of that. I've gone the other way with high impedance approaching current drive and when you equalized to give the same response the distortion was about the same for low frequencies.

Excursion changes dramatically with these changes so you have to be careful with comparisons.

Ace Bass was another interesting hybrid feedback scheme for subwoofers. It used feedback to convince the woofer that it had different mass.

David

 

[forr]

I'd like to see measurements of that. I've gone the other way with high impedance approaching current drive and when you equalized to give the same response the distortion was about the same for low frequencies.

My thinking is that controlling the current with either negative or high impedance should be beneficial from the movement linearity point of view. You're more experienced than me and your advice is wellcome but I hope to be able to test these schemes one day, mostly to convince myself that sophisticated techniques, intellectually appealing, are not worth the complication.

 

[jneutron]

Yes, it drives me a little bonky when someone puts a number on woofer Le and thinks it is a precise value.

Most in depth studies think coil inductance as a loosely coupled transformer. Dick Small did some research for us at KEF and found a frequency dependent resistor inductor combo best tracked a measured curve for the widest range. Otherwise you can do an impedance fit to a cascade of 2 or 3 L parallel with R sections and get a decent fit for a decade or so.

As you say it is a matter of level and greatly varies with woofer position as the iron pole piece moves in or out of the coil. Under cut core pole can reduce the Le variation with excursion. I have seen cases so bad that, with high level pink noise, you can hear the treble come and go with excursion.

I think the Klippel web site covers the nonlinearity aspect fairly well and Linkwitz goes into flux modulation effects and Le effects in some depth. All T/S parameters are meant to be thought of as small signal effects and will vary with level. Fs varies with level very strongly, which throws off Qt and other dependent parameters.

David

It's so very amazing the commonality between this and my work. Even though R in my case is zero, we have to consider the effective R to find a quenching super when the currents are changing and the metals and walls of the cryostat are dissipating. ITER has such a problem, when the plasma initiates, the secondary current hits 15 million amps, and it will alter the Ls/Rs of the primary coil by coupled flux..they have to find 100 millivolts out of 29 kilovolts and 45 kiloamperes. You can imagine the level they will have to make the model to see that low in that environment.

I'll look up your recommendations, I suspect they will be of significant use.

Thank you.

jn