Purifi-based monitor - options

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Nick
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#121 Re: Purifi-based monitor - options

Post by Nick »

Yes. But of I understand what Scott was suggesting is that the voice coil in the magnetic field can generate harmonics which is why providing a high impedance with a series notch will prevent current flow.
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#122 Re: Purifi-based monitor - options

Post by chris661 »

Yes.

Which means the harmonics will disappear. The cone won't produce harmonics on its own - it needs to be excited at the right frequency by the voice coil. If the voice coil can't pass current at the relevant frequency, there'll be no output, regardless of how the voice coil current is being excited (distortion in the motor or signal from the amplifier).

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#123 Re: Purifi-based monitor - options

Post by Nick »

OK, I will admit I was thinking the harmonics was being generated by the moving system, but as you say it will be stopped by the restriction of current at that frequencies.

But I don't understand why a moving mass like the cone cant also produces harmonics any less than a vibrating guitar string can't have different modes of oscillation? This is a separate question though so don't feel the need to answer in the context of the notch filter.
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#124 Re: Purifi-based monitor - options

Post by Nick »

And I will repeat my question, why doesn't this notch filter idea get used with tweeters? I would have thought they would have the same problem with high frequency resonances (above audible frequencies) causing images lower down the frequency range (where they would be audible).
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#125 Re: Purifi-based monitor - options

Post by IslandPink »

It could be used, yes. The problem might be getting components that are transparent enough to make it worthwhile, although the saving grace is that the capacitor, which is likely the worst , is mainly conducting above the notch frequency, ie. ultrasonic.

Here is the response and harmonics for this tweeter. There could be a case for notching the 25kHz peak, although it's not obvious that there are noticeable distortion products at the f/H values . For another (cheaper) tweeter there might be a better case for it.
The main thing to watch out for when you are affecting the main HF operating band is that you don't have significant phase shift in the 10 to 15kHz region, otherwise you will kill the top octave.
Tweeter_Compass_measured.JPG
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TweeterDist_Compass_measured.JPG
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#126 Re: Purifi-based monitor - options

Post by Scottmoose »

Nick wrote: Mon Aug 29, 2022 12:49 pm And I will repeat my question, why doesn't this notch filter idea get used with tweeters? I would have thought they would have the same problem with high frequency resonances (above audible frequencies) causing images lower down the frequency range (where they would be audible).
Sorry, not had much audio time this last fortnight.

It does get used actually -just not very often. The animated distortion plot I posted a couple of weeks back for e.g. was of an old Seas metal dome tweeter with / without its ~26KHz ultrasonic bell mode notched out.

Why isn't it used regularly? Short version is that a lot of people, including many extremely able designers, simply don't know you can reduce resonant amplified distortion products in this way, whether with bass, midrange or HF units. That's not to claim any non-existent profundity in this quarter -it's simply a slightly obscure field of crossover & driver design / engineering. A handful in DIY do it; in the commercial field IIRC Avalon and Magico have done so, particularly when using the ceramic inverse dome Accuton tweeters, since the main mode is usually still a bit low and their motors not always class-leading. Possibly Tidal / Vimberg but I'm not 100% on that.

This is purely arbitary, but assuming a reasonably good motor, I generally find that if an ultrasonic mode is > 35KHz, it's usually not worth addressing. 30KHz - 35KHz -maybe. 20KHz - 30KHz? Yes, generally worth the trouble.
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#127 Re: Purifi-based monitor - options

Post by Scottmoose »

Probably the 'classic' example of where it's worth doing.

Seas H1212, aka the 27TBFC/G. Aluminium magnesium alloy dome. Been around about 20 years, really good tweeter for the money; distortion performance was superior to a lot of more expensive tweeters when it was released (and still is). Not perfect though. You can see the 27KHz ultrasonic breakup mode, and you can track it in the HD lower down. This was audible for some people, sufficiently so to put them off speakers using it. HD3 spiking ~9KHz, HD4 at 6.75KHz, HD5 at 5.4KHz. Notch that breakup mode out, those HD spikes vanish & it goes from being very good for the money to just very good, end of.
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#128 Re: Purifi-based monitor - options

Post by IslandPink »

Nice !
That is a big spike, though, compared to what I'm using.
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#129 Re: Purifi-based monitor - options

Post by Scottmoose »

'We're using' ;) -remember, this is what's in the Edingdale GT.

No, I didn't use any ultrasonic notches on the Be Satori tweeter when I designed the GT filter, primarily because its ultrasonic breakup isn't concentrated into a single high Q mode but spread out over a wider bandwidth, which makes implementing more problematic, and since it has a very good motor & distortion at realistic levels, any minor advantages weren't worth the trouble.
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#130 Re: Purifi-based monitor - options

Post by IslandPink »

Just a quick comment here, been too busy to do a proper report, as I'm dismantling the front room somewhat, to help the electricians do silly stuff on Friday.
i got the Mk.2 cabinet on Friday and got the drivers, wires and stuffing sorted out by Sunday to listen. CD player is working now so was able to test with a few standard discs.
It was worth the gamble with the reduced area slot port ( tuning lower ) . The bass is more extended and more natural, and chuffing isn't a problem at even fairly loud levels ( for me.. ) which will be more than Jonny is using.
Some of the bass, especially Jaco on 'Don Juan's reckless daughter' is as good as I've ever heard it. This drive unit seems to reveal differences in volume/dynamics better than anything so far.

Here a couple of pics of some preps , more in a few days -
KIF_0966.JPG
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KIF_0968.JPG
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#131 Re: Purifi-based monitor - options

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View of Mk2 from front during testing. Some crossover parts spilling out to the rear.
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#132 Re: Purifi-based monitor - options

Post by IslandPink »

SPL ( average of 5 with speaker/microphone in slightly different locations )
I can see that the tweeter level ( from 2 kHz ) is still 1 or 2dB too high, so will do some calcs on that and correct. Also the notch for 3.5k on the woofer is not doing enough.
Pur_Mk2_SPL_Avg5_7Oct.jpg
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Distortion , nothing stands out, but I'm surprised the numbers aren't lower for the tweeter, makes me suspicious of the measurement/calc :
Pur_Mk2_distortion_7Oct.jpg
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#133 Re: Purifi-based monitor - options

Post by IslandPink »

Here's another distortion plot, for measurement 3 or 4. It looks quite a bit different, which suggests that the room modes are having a big effect. Not sure how that can be, especially since the tests are at 1m distance.
Pur_Mk2_distortion_ii_7Oct.jpg
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#134 Re: Purifi-based monitor - options

Post by IslandPink »

Here's SPL for the woofer with crossover, no tweeter.
Pur_Mk2_SPLWoof_Avg3_7Oct.jpg
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#135 Re: Purifi-based monitor - options

Post by IslandPink »

I was doing more on Sunday but ended up chasing my tail a bit by taking down the 3.5k peak but losing more at 2 to 2.5kHz.
Here is a re-worked LP for the Purifi that uses a lower primary LP inductor, and a slightly narrower notch. Interesting also that R3 in the zobel can be used (dropping down in value) to fettle the slope on the 4kHz+ side.
Puri_LP_10Oct22_ii.JPG
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One other thing I noticed just now is that my problems around 2 to 2.5kHz may be partly due to the tweeter which has a bit of dip there. I can still come down a bit lower on that HP filter, so may do this next, if the overall FR needs more fettling.

Hopefully this will all be listenable at Owston....
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