Purifi-based monitor - options

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IslandPink
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#91 Re: Purifi-based monitor - options

Post by IslandPink »

Nice !
I'm still trying to get my head around the detail of that paper to be honest, but I'm hoping it will sink in stage by stage.
What we're talking about is a manifestation of intermodulation distortion, am i correct ?

Progress here in last couple of days is verifying that an old Sony amp with 40 wpc will work OK into a 4R load without blowing up ( it's designed for 8 & 16 ). I now have a comparison to check if there's anything coming from the chip amp at the powers I'm using to play music and test.

edit - God, I just threw another page. It keeps happening - every time someone posts something interesting, i end up throwing a page. Curses !
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#92 Re: Purifi-based monitor - options

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What we're talking about is a manifestation of intermodulation distortion, am i correct ?
I don't think so. I think its the result of the non linear behavior of the moving mass under resonant conditions. But I may well be wrong.
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#93 Re: Purifi-based monitor - options

Post by Scottmoose »

It's both, since they're basically connected.

There are a couple of mechanisms of relevance here. A drive unit inherently produces distortion in the motor -no such thing as a distortion-free motor (alas). So issue no.1 is that if a driver has a major mode at, say, 6KHz, then it will ring like a bell at that frequency and amplify distortion at sub-multiples, so you'll see HD3 spike at 2KHz, HD4 at 1.5KHz, HD5 at 1.2KHz &c. If the driver's baseline distortion level isn't particularly distinguished, that amplified HD peak can become audible. You can also run into audible intermodulations produced by it, even if the HD peak isn't amplified sufficiently to be audible in & of itself. In both cases the fix is the same though: stamp out the bell mode with a high impedance notch, and it goes away.
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#94 Re: Purifi-based monitor - options

Post by Nick »

It's both, since they're basically connected.
In the sense that one caused the other yes, but if a caused b, that doesn't mean b causes a. AFAIK, Intermodulation is the result of non linear behavior when applied to two or more fundamental frequencies. What I thought you were talking about was harmonics being created from a single frequency which is not intermodulation.

Further to that, a system can have a resonant frequency without needing distortion. The LC networks we were talking about have resonant frequencies, but that does not it itself create harmonics.
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#95 Re: Purifi-based monitor - options

Post by Nick »

stamp out the bell mode with a high impedance notch, and it goes away.
That's the bit I don't understand, how does a high impedance notch stamp out anything, surely a high impedance will reduce the damping on the motor, so if (for example) we excite the bell mode you are talking about with a external impulse (say another driver emitting that frequency, or itself being driven at a harmonic of that frequency) the motor will be more likely to resonate at that frequency than if it was just damped by the non frequency dependent driving impedance.
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#96 Re: Purifi-based monitor - options

Post by chris661 »

Nice to see it's not me going around in these circles.


Intuitively, the "apply a short-circuit to give maximum electro-magnetic braking" makes sense.

However, I can sort-of see the argument for preventing current flow in the motor at the breakup frequency. That way, the motor can't produce any distortion at that frequency..?

It's more fuzzy for me.

What I do have, though, are some drivers that are ideal for testing. I guess a close-mic'd measurement of the bass driver to get the HD profile would be a good start.

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#97 Re: Purifi-based monitor - options

Post by IslandPink »

I'm in the same position as I was with current drive lowering distortion, at the time the Pass F2 came out. The data seems to be there, but I can't quite get my head around it.
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#98 Re: Purifi-based monitor - options

Post by Scottmoose »

Bruno covers it in the pdf linked to, but basically, distortion on a moving coil driver (through these relevant BWs) is dominated by the dynamic modulation of Le via cone motion and / or hysterisis effects in the motor surrounding the coil. This is one reason a well-designed (well designed ;) ) symmetric drive motor, where the front & back plates, pole-piece etc. are heavily copper-plated, inherently have lower HD levels than a 'basic' motor design. A change in inductance under dynamic loads, such as caused by a big uncontrolled bell-mode / resonance generates a [distortion] voltage source in series with the coil proportional to the magnitude of that change. By inserting a high impedance at that frequency, the less the current produced by that distortion voltage source, taking SPL & the distortion with it. As Bruno's measurements and the example I posted show: it works, unlike the converse.

There is an exception to that converse of course, which is that if your driver already has a sufficiently low baseline distortion & the unwanted mode is sufficiently far out in the stopband, then you can use a series notch (i.e. in shunt with the driver) for EQ / part of the acoustic transfer function if that's what will give you the low-pass rolloff slope you need cheerfully enough. Zaph for e.g. did so with a zero pole / bottomless LC series notch in shunt on his L18 project and avoided issues with resonance amplification. You have to be a mite careful in interpreting why that filter works though; the notch doesn't actually address the HD amplification as such because it doesn't need to: he crosses LR4 at 2KHz, the H1224 has a very good motor design, and (critically, and very impressively) a primary cone mode out at 7.5KHz [nearly], putting its 3rd harmonic at 2.5KHz: 500Hz above his reasonably steep crossover frequency.
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#99 Re: Purifi-based monitor - options

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A change in inductance under dynamic loads, such as caused by a big uncontrolled bell-mode / resonance generates a [distortion] voltage source in series with the coil proportional to the magnitude of that change. By inserting a high impedance at that frequency, the less the current produced by that distortion voltage source
Yes I can see that, but does restricting the current in the coil have any effect on actually controlling the resonance in the cone? It may reduce the distortion in the motor current, but do we care about that as what we worry about is the actual movement of the cone (and motor). I can see what you are describing being relevant in the case of the cone being driven at H1 by a current from the driving amplifier, and the effect that has in reducing the resonance in the cone and the generated harmonics, but I don't see how it will help the generation of that physical resonance by excitation at H2 or higher (or an impulse, just though of that, see later).

The likely way that those distortion graphs are generated is by sweeping the driving frequency and filtering the fundamental and harmonics to give the H2 and H3 curves. But the situation I am considering would require analysis that looked for sub-harmonic distortion generated by the swept signal.

I wonder if a waterfall in response to a impulse would show if the situation I am describing actually happens.
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#100 Re: Purifi-based monitor - options

Post by IslandPink »

It's the bit on page 7, analysis of figure 5 that is intriguing me.
"The parallel notch filter response gives as expected around 10dB higher THD in the 1.65kHz area where the 3rd order harmonic frequency gets amplified by the SPL peak at 5kHz of the cone. The series notch is suppressing(sic) the peak by providing a much higher drive impedance to the driver at the peak frequency"
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#101 Re: Purifi-based monitor - options

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I think from what Scott is saying is that because the drive impedance is higher it restricts the motor from generating current in the voice coil at the frequency that is the third harmonic of the exciting frequency. And because the generation of current is reduced, the harmonic can not be generated as the mechanism of the harmonic generation is the non linearity of the motor moving in the magnetic field.

This to me seems to be putting the cart at least three steps in front of the horse, and what I don't understand is what of any effect it will have on the generation of the harmonic by the actual resonance of the cone itself.

But I guess if we are saying this is at the point where the cone behaves like a piston then there is no harmonics generated in the cone.

But given its being referred to as a bell resonance, it seems to me that the tone of an actual bell is far from single frequency and that involved zero coil or magnet or current.
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#102 Re: Purifi-based monitor - options

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May be the bell like sound of a penny dropping in the distance. The situation that is being described is such that the third harmonic (in this case) will not be directly excited because its above the xover frequency in use.
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#103 Re: Purifi-based monitor - options

Post by rowuk »

Maybe I am missing something, but my understanding of speaker harmonic distortion is that we only need to excite the fundamental. Then, just like with a musical instrument (or in this case a bell), all the harmonics are automatically excited. No crossover can change the behavior of the bell - only avoiding the fundamental can help.
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#104 Re: Purifi-based monitor - options

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rowuk wrote: Tue Aug 09, 2022 3:00 pm Maybe I am missing something, but my understanding of speaker harmonic distortion is that we only need to excite the fundamental. Then, just like with a musical instrument (or in this case a bell), all the harmonics are automatically excited. No crossover can change the behavior of the bell - only avoiding the fundamental can help.
Yes, while "we only need to", we don't have to, you can make a swing swing by only pushing every second swing, so we could excite a fundamental by exciting it at subdivisions of that frequency. So if (for example) the fundamental of a speaker mode is not excited by the fundamental if that frequency is restricted (for example by a crossover), we can still cause it to become excited by a subdivision of that frequency.
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#105 Re: Purifi-based monitor - options

Post by Scottmoose »

Sorry, not had any personal audio time for a few days.

Correct, it works both ways, which underlines the 'need' to deal with it in cases where the HD amplification is audible. It's not an obligation (!) although in a number of cases, you can suffer the consequences of not doing so -the Seas Thor project is a good example of that. We can't stop a mechanical resonance with a passive filter per se, since the mechanical resonance is a property of the cone (and its surround, coil etc.); however we can use the filter to achieve a situation where that mechanical resonance, and the consequences of it, are not significantly triggered in the first place -a driver is ultimately 'active' in that it's driven by an electrical signal, not a 'physical' one e.g. by being struck by something. In effect, we're manipulating that signal in such a way as to circumvent a given scenario; that's more or less what crossover design in some way, shape or form is all about. In this case, 'all' we're doing is establishing a situation where the cone motion at x resonant frequency (where its amplitude would suddenly go through the roof) is suppressed via the use of a high impedance 'blocking' notch, thereby reducing current draw at that point, which in turn then limits how much the distortion products at sub-multiples of the fundamental can be amplified (not just 3rd harmonic, it applies to all the harmonic sub-multiples, although relative to their averages the odd order are most significantly affected). The less current flow at that frequency, the less amplification of distortion at sub-multiples it can cause.

Example attached courtesy of HFC. This is an old 5in Seas Excel unit with a cast magnesium cone. You can see in the frequency response plot the main breakup / bell mode (see below) at roughly 8.5KHz. In the HD plot, track the spikes in distortion at sub-multiples of that frequency. HD2 goes at 4.25KHz, HD3 at 2.8KHz or thereabouts, HD4 at 2.1KHz [nearly], HD5 at 1.7KHz. This is of an unfiltered driver. If you applied said notch to the main 8.5KHz cone mode, those distortion peaks will largely disappear; they're not 'natural' or innate to the motor, but simply a result of that mechanical resonance modulating the coil inductance & generating a [distortion] signal.

Re 'Bell mode', the term is simply a traditional shorthand reference in loudspeaker design to radial modes generated at frequencies where circumference is sub-multiples of 1/2 wavelength within the cone. You also have concentric (in loudspeakers referring to bending / flex / TL) modes which tend to come into effect with cone designs that have, deliberately or otherwise, less emphasis on outright rigidity. Wideband drivers for e.g. are deliberately designed to use them as part of their response. 'Bell mode' was simply adopted as a convenient piece of brevity given the very broad connection between the resonant properties of pieces of metal in some broad variation of a curvilinear form. Basic sketch courtesy of the late Ted Jordan attached.
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