Flux modulation

[454Casull]

We know that flux modulation is the name given to the change in permeability due to current through the voice coil (roughly speaking), and that there are (at least) two significant measurable effects - non-constant Le(i) and non-constant BL(i). The first comes from varying permeability of the iron through and around the voice coil. The second from the varying flux density (permeability -> field strength -> flux density) acting on the coil.

Can we not negate the effect of Le(i) by powering the driver with a current source (doesn't care about inductance) as opposed to a voltage source? Or is my understanding flawed?

[Archmage]

Current Driven Loudspeakers and Tranconductance Amplifiers

point me to some DIY designs and theory after you're done with that

[Archmage]

Ok...

Now I've noticed that YOU have posted in that thread years ago...

[gedlee]

Originally posted by 454Casull
We know that flux modulation is the name given to the change in permeability due to current through the voice coil (roughly speaking), and that there are (at least) two significant measurable effects - non-constant Le(i) and non-constant BL(i). The first comes from varying permeability of the iron through and around the voice coil. The second from the varying flux density (permeability -> field strength -> flux density) acting on the coil.

Can we not negate the effect of Le(i) by powering the driver with a current source (doesn't care about inductance) as opposed to a voltage source? Or is my understanding flawed?

Yes, constant current does minimize the change of Le(i) but not as much the change of Bl(i).

[454Casull]

Originally Posted by gedlee

Yes, constant current does minimize the change of Le(i) but not as much the change of Bl(i).

To clarify, are you saying that constant current has a significant effect in attenuating BL(i)-related distortion? I thought that was strictly in the domain of the motor, not of the drive?

[speaker dave]

Some measurements here. Current drive (50 ohm source) reduced flux mod distortion about 10dB.

raising driver Qts - you can't tuna fish

In the domain of the motor? The way to look at it is the magnetic circuit nonlinearity creates a nonlinear load. If you have a low source impedance driving the nonlinear load you can have low distortion voltage but distorted current. With a current source you can have low distortion current but distorted voltage. The later is preferable because force on the cone is related to current.

David S.

[gedlee]

Originally Posted by 454Casull

To clarify, are you saying that constant current has a significant effect in attenuating BL(i)-related distortion? I thought that was strictly in the domain of the motor, not of the drive?

Actually looking back, I think that was a typo and I meant to use BL(x), not BL(i). They both exist and the motor design determines which is the greater. Constant current would have an effect on BL(i), but if this is smaller than BL(x) (as it almost always is) then its effect would be negligable. Le(i) is a major concern because it occurs in a frequency range where x is negligable and hence it is the dominate nonlinearity in a driver in that range. BL(i) can also be a factor in this range as well, but generally not at lower frequencies.

[speaker dave]

Whether BL(x) is greater or smaller depends on frequency range. At low frequencies excursion is high and nonlinear BL vs. displacement dominates. At higher frequencies the excursion is negligible and hysteresis is the greater factor as shown in my curves. The transition in my measurements is a little above 100 Hz.

Now when you say Le(i) and BL(i) which one would be commonly called "flux modulation" (driving the magnetic opperating point around the hysteresis loop of the magnetic material)? I wouldn't think of that as Le related. There is an L vs. x term, commonly viewed as a "solenoidal force", but that is a 2nd order nonlinearity.

Note that the drop in 3rd harmonic from current drive is very comparable to the improvements that flux damping rings typically give.

Regards,
David S.

[454Casull]

I'm still trying to wrap my head around this.

Right from the outset it is obvious that a current source eliminates effects stemming from variance in Le, therefore Le(i) and Le(x) as sources of distortion are negated. If this is true, then dominant source of distortion for high frequencies would be BL(i)? Kms(x) and BL(x) will change very little, whereas BL(i) may show variance as B can be affected by voice coil current; this is what you are referring to as hysteresis? I would argue that it's not the proper terminology as the field strength never gets low enough as to show an actual "loop" effect, but I digress...

In other words, for low excursions (BL(x) and Kms(x) are constant), the most effective ways to reduce distortion are to:

1) use a high output impedance (and EQ out impedance-related nonlinearities later as necessary)
2) reduce variance in BL(i) by:
2a) adding shorting rings/sleeves to reduce the change in B (how does this keep B from changing? by reducing the coil-opposed induced voltage?)
2b) pushing the magnetic iron around the voice coil gap closer to saturation so as to reduce the change in B
2c) by increasing the electrical resistivity of the "iron" to reduce change in B (see 2a)
2d) select a magnetic iron which has flat B vs H in region of interest, to eliminate change in B

It would make sense that 1) and 2a) result in the same effect, as you note.

What am I missing?

EDIT: So basically, a perfect driver would have a spider like the Dayton RSS subs or the Scan-Speak Illuminator 18WU (see Klippel data at DIYMA), a current source amp, the magnetic structure made from FINEMET FT-3H, and utilize XBL^2 in the gap.

[speaker dave]

In my experience current drive our high source impedance only seems to have significant impact on hysteresis distortion. Please see my curves in the linked thread above.

There seem to be a lot of terms here and I'm not sure we are all on the same page (hence my question to Earl), so let me explain what my understanding is.

I assume Bl(x) means the drop in Bl as the woofer moves from the mid point. B field peaks in the middle of the magnetic circuit gap, the coil has equally spaced turns so a moving integration for the distance equivalent to coil height will give a BL that varies with excursion. Constant current drive seems to have no effect on this.

Le(x) would be inductance vs. coil centering. Usually the coil inductance goes up and the coil goes in and more core pole iron is in the coil. This is the definition of a solenoid and so current tends to draw the coil into the core pole. This is a cause of second harmonic distortion and in fact a woofer that is fully demagnitized will respond purely with 2nd harmonic output due to this force. As far as I know current drive makes no difference here.

Le(i) I'm not clear on. Actually I dont think Le varys with drive. Is this a term for hysteresis distortion?

By hysteresis distortion I mean the hysteresis of the magnetic material. Ferrite material gives rise to an operating point of the magnet circuit that is pushed around a minor loop by the drive of the voice coil and signal adding and subtracting from the DC magnetic field. This leads to 3rd harmonic distortion that is reduced by constant current drive. (Again, see my curves.) This occurs most at mid frequencies where inductance is minimal. "Flux modulation rings" provide a shorted turn that resists the variation in operating point and gives a similar reduction in 3rd harmonic. Neodymium and Alnico magnets suffer less from this effect. Pushing the structure to saturation, as you say, is also a cure.

Kms or suspension nonlinearity is an LF effect and is not, to my knowledge, impacted by source impedance. Note that my curves show a little indeterminant change in LF 2nd harmonic distortion and zero change for LF 3rd harmonic as source Z is changed.

As to what you are missing, I think we are in general agreement, so I wouldn't know.

David S