Purifi-based monitor - options

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Nick
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#76 Re: Purifi-based monitor - options

Post by Nick »

Can I just say I don't understand what:
To put it another way: to increase the Q of a parallel notch in series with the driver, you increase (not decrease) the level of resistance in circuit.
Means. Does your Q in the above refer to the Q of the LC resonant element (what you are calling a notch), or does it refer to the Q of the LC and driver in series?

Assuming the R in a LCR you are talking about is in series with the L increasing the R will always decrease the filters Q from my understanding of such things.
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#77 Re: Purifi-based monitor - options

Post by IslandPink »

Nick wrote: Thu Aug 04, 2022 1:32 pm 1. You may be missing what I meant
2.It was the use of the word "proper" that I was questioning.
1. No , I wasn't :)
2. I suspected that, and I didn't want to get bogged down in semantics, it was the point about comparing two rather dissimilar notch filters of the two types, that I was questioning, in regard to the conclusions about distortion suppression.

By all means call the DCR 'R3' , I'm OK with that .
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#78 Re: Purifi-based monitor - options

Post by Scottmoose »

Except they aren't dissimilar other than the fact that series and parallel notches are inherently so when used for EQ / response-shaping. The object of the exercise in the pdf (see pp.4 - 5) was to create the same baseline frequency response (that's the baseline for comparison), via either a series or a parallel notch, and illustrate that only the former (in series with the driver) affects the distortion amplification.

This works because of how moving coil drivers function. The distortion is a product of the driver motor, & subject to the resonance amplification of a cone going into breakup -the greater the amplitude of that breakup, the greater the amplification. To prevent that, you need a high impedance notch at the cone breakup frequency, acting in effect to break circuit at that point for the coil current.
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#79 Re: Purifi-based monitor - options

Post by IslandPink »

Hoping to get back onto this later subject tomorrow once the electricians have been and passed my house as safe (!)
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#80 Re: Purifi-based monitor - options

Post by chris661 »

Scottmoose wrote: Thu Aug 04, 2022 9:00 pm Except they aren't dissimilar other than the fact that series and parallel notches are inherently so when used for EQ / response-shaping. The object of the exercise in the pdf (see pp.4 - 5) was to create the same baseline frequency response (that's the baseline for comparison), via either a series or a parallel notch, and illustrate that only the former (in series with the driver) affects the distortion amplification.

This works because of how moving coil drivers function. The distortion is a product of the driver motor, & subject to the resonance amplification of a cone going into breakup -the greater the amplitude of that breakup, the greater the amplification. To prevent that, you need a high impedance notch at the cone breakup frequency, acting in effect to break circuit at that point for the coil current.
Interesting. Thanks for the explanation, Scott.

I would've thought that shorting out the driver (hence providing maximum electro-magnetic braking) at a breakup frequency would be beneficial, but I can see your point: if the coil never receives the frequency in the first place, no distortion can be made.

Guess it's time to revisit my crossovers, then.

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#81 Re: Purifi-based monitor - options

Post by Nick »

if the coil never receives the frequency in the first place, no distortion can be made.
I wonder of the mode could still be excited by a harmonic or undertone of the original frequency?
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#82 Re: Purifi-based monitor - options

Post by IslandPink »

I see he talks about reduction in distortion at 1650Hz as a result of the notch at 5kHz, the sub-third harmonic, however this is not overtly an intermodulation test. I've not got my head around it all yet, there's a lot in there, very nice piece of work.
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#83 Re: Purifi-based monitor - options

Post by chris661 »

I suppose it could. With a cone with a strong resonance at 4.5kHz, a 1.5kHz input would have its 3rd harmonic amplified acoustically. I don't think a high driving impedance at 4.5kHz would rescue that.

Something that I would've liked to see in the Purifi paper is a higher drive level. Their "after" measurements have distortion levels close to the noise floor. In fact, the slightly higher SPL produced by their "parallel" notch ("parallel" because it's across the driver) might even account for the differences in HD curves.


Looks like some measurements are in order.

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#84 Re: Purifi-based monitor - options

Post by Nick »

I don't think a high driving impedance at 4.5kHz would rescue that.
Though (and just thinking out loud as its a interesting subject) maybe a parallel notch (across the driver) might by damping the motor at 4k5
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#85 Re: Purifi-based monitor - options

Post by IslandPink »

If I chuck the two 5kHz filters into my own circuit, they are not the same in effect, although both are quite deep.
These are Config #3 and config #4 -
Purifi_paper_series_notch.JPG
Purifi_paper_series_notch.JPG (160.69 KiB) Viewed 1718 times
Purifi_paper_parallel_notch.JPG
Purifi_paper_parallel_notch.JPG (170.71 KiB) Viewed 1718 times
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#86 Re: Purifi-based monitor - options

Post by Nick »

I think you need the voltage source to be on series with the loudspeaker to model the effect the pdf is talking about.

I also think they will only look the same of you consider voltage in one case and current in the other.
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#87 Re: Purifi-based monitor - options

Post by IslandPink »

I was just trying to verify if the notches were having the same effect on amplitude at or around 5kHz.
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#88 Re: Purifi-based monitor - options

Post by Nick »

Well, the two resonant frequencies will not be exactly the same as 0.22 * 4.2 != 0.15 * 6.2
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#89 Re: Purifi-based monitor - options

Post by IslandPink »

That too.
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#90 Re: Purifi-based monitor - options

Post by Scottmoose »

Since pictures speak a thousand words, a little demon- :twisted: stration may be of use.

I should probably preface by pointing out that this is far from a 'new' idea; a few of us have been doing this for years, mostly with metal cone midbass drivers optimised for pistonic behaviour over a modest BW at the price of very high Q bell-modes above that.

It doesn't have to be just for bass drivers though; it can apply to tweeters as well, particularly metal dome types. Most aluminium / magnesium / titanium dome varieties in the 1in size bracket tend to have a major bell mode somewhere between roughly 26KHz - 30KHz. The distortion amplification lower down can be both visible in measurements, and audible, depending on how good the motor & baseline distortion level is. The attached animated gif shows the HD2 and HD3 of a 1in Seas aluminium / magnesium alloy dome in a speaker project first created about 15 years or so ago, with and without a high impedance notch on the 26KHz mode.
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Tweeter HD with & without notch.gif
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