I've been looking at some Fostex fullrange datasheets, and one spec. In particular always jumps out to me. Their very low X-Max. Well under 1mm. Even their drivers specifically mentioned by Fostex as designed for bass-reflex application, such as the FF series, seem to have very low X-Max. Markaudio equivalent drivers spec. X-Max of several millimeters. Nearly all of the Fostex DIY speakers which I saw on YouTube are BR. I only saw one back-horn speaker. I'll assume that X-Max hasn't been a problem based on the success of Fostex project builds.
So, my question is, why has this not been a problem in practice? Must Fostex drivers be played at a low loudness? Something must give, so what am I missing?
So, my question is, why has this not been a problem in practice? Must Fostex drivers be played at a low loudness? Something must give, so what am I missing?
The FExx6 & FExx8 series really really want to be in a back-loaded horn, and driven bt a highish Rout amplifier (typical SET, bridged ACA…)
FFxx5wk are much more versatile, working well in many horns, but also TLs and reflexes.
The 2 brands have a completely different philosophy.
The Fostex push towards the low xMax, high sensitivity, MA are unusual in their high xMax. If one pushes really hard near xMax cones can be turned inside out (they all come with arrstors now so that does not happen). The Fostx have limited xMax but the excursion beyond that is considerable.
I never paid much attention to xMax, just listened to them and see how they performed.
dave
dave
FFxx5wk are much more versatile, working well in many horns, but also TLs and reflexes.
The 2 brands have a completely different philosophy.
The Fostex push towards the low xMax, high sensitivity, MA are unusual in their high xMax. If one pushes really hard near xMax cones can be turned inside out (they all come with arrstors now so that does not happen). The Fostx have limited xMax but the excursion beyond that is considerable.
I never paid much attention to xMax, just listened to them and see how they performed.
dave
dave
IIRC, the Fostex motors are more like even-hung. ie, magnetic gap height and voice coil height are pretty similar.
The motor force falls off pretty gradually, though, so you can get away with quite a bit. FE126, for example, can manage a few mm peak-to-peak travel without sounding objectionable.
FWIW, I moved away from Fostex drivers for other reasons. Typically found their upper bandwidth too peaky to be considered useful wide-band units. YMMV.
Chris
The motor force falls off pretty gradually, though, so you can get away with quite a bit. FE126, for example, can manage a few mm peak-to-peak travel without sounding objectionable.
FWIW, I moved away from Fostex drivers for other reasons. Typically found their upper bandwidth too peaky to be considered useful wide-band units. YMMV.
Chris
Just so. Fostex primarily use underhung motors, so the nominal rated Xmax is relatively low, however, the mechanical limit of suspension travel (Xmech) is significantly larger, and they tend to overload relatively progressively. Which doesn't mean that's how they're best used, but it's not a lightswitch between 'good / bad'.
Unlike Xmech, which is a physical property, Xmax is purely a guideline; potentially a useful one, but no more. There is no single method for creating a figure for it: I gave up counting at a dozen, all of which give slightly different values if applied to the same drive unit. Different manufacturers tend to use different methods, rarely say which, and unless the data sheet is quite comprehensive, you won't be able to work it out either.
Unlike Xmech, which is a physical property, Xmax is purely a guideline; potentially a useful one, but no more. There is no single method for creating a figure for it: I gave up counting at a dozen, all of which give slightly different values if applied to the same drive unit. Different manufacturers tend to use different methods, rarely say which, and unless the data sheet is quite comprehensive, you won't be able to work it out either.
I notice that hifi drivers tend to either have long coils and short gap or the other way around. So at any one time only part of the coil or gap is used. When it goes out, it goes out pretty fast.
Full range drivers and pro drivers have long coils and long gaps. So their coils and magnets are almost 100% utilized, giving the motor higher force and higher efficiency.
So when the coil starts coming out of the gap, it moves a bit at a time, soo you could say it has a soft clipping built in. Some manufacturers like Faitalpro add in 1/3 of the coil length to it. Which makes sense because the coil is so long, it only comes out 1/3 and the rest is still immerse in the magnetic field.
Oon
Full range drivers and pro drivers have long coils and long gaps. So their coils and magnets are almost 100% utilized, giving the motor higher force and higher efficiency.
So when the coil starts coming out of the gap, it moves a bit at a time, soo you could say it has a soft clipping built in. Some manufacturers like Faitalpro add in 1/3 of the coil length to it. Which makes sense because the coil is so long, it only comes out 1/3 and the rest is still immerse in the magnetic field.
Oon
Almost all fullrange drivers are underhung (coil shorter than gap), reason being that in an overhung design (coil longer than gap) the coil windings hanging outside the gap lead to high inductance which kills high frequency performance.
A design which is close to even-hung (coil same length as gap) gives maximum efficiency however will lead to higher non-linear distortion at lower levels. The distortion however will theoretically be stronger in the lower order components (2nd, 3rd) rather than higher order (5th and above) compared to an equivalent design with a large difference in coil/gap lengths. Even hung drivers are prone to strong intermodulation distortion since as soon as they start moving a different amount of coil hangs out of the gap, so the inductance becomes strongly modulated by the cone movement leading to high frequency components being amplitude modulated by low frequencies which dominate excursion.
There is no reason that an underhung design with a lot of xmax and a strong magnet can't have the same efficiency as an even-hung design, and give lower distortion of all types up to painful SPL levels. In hifi applications efficiency is hardly ever a problem, and usually neither is spending $$ on a huge magnet. You only need 90-95dB for home listening so even if your driver is a dismal 80dB/1Watt/1metre you'll still only need 10-20Watts to achieve a loud listening level. Amplifier power is cheap. For that reason efficiency is only a concern to me when trying to level match multiple drivers for a passive design.
That said, is all theoretical as poorly designed drivers will produce audible levels of all orders of distortion regardless of their coil-gap arrangement. It is also possible, with careful design, to make an overhung driver with usable xmax and negligible inductance.
A design which is close to even-hung (coil same length as gap) gives maximum efficiency however will lead to higher non-linear distortion at lower levels. The distortion however will theoretically be stronger in the lower order components (2nd, 3rd) rather than higher order (5th and above) compared to an equivalent design with a large difference in coil/gap lengths. Even hung drivers are prone to strong intermodulation distortion since as soon as they start moving a different amount of coil hangs out of the gap, so the inductance becomes strongly modulated by the cone movement leading to high frequency components being amplitude modulated by low frequencies which dominate excursion.
There is no reason that an underhung design with a lot of xmax and a strong magnet can't have the same efficiency as an even-hung design, and give lower distortion of all types up to painful SPL levels. In hifi applications efficiency is hardly ever a problem, and usually neither is spending $$ on a huge magnet. You only need 90-95dB for home listening so even if your driver is a dismal 80dB/1Watt/1metre you'll still only need 10-20Watts to achieve a loud listening level. Amplifier power is cheap. For that reason efficiency is only a concern to me when trying to level match multiple drivers for a passive design.
That said, is all theoretical as poorly designed drivers will produce audible levels of all orders of distortion regardless of their coil-gap arrangement. It is also possible, with careful design, to make an overhung driver with usable xmax and negligible inductance.
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