Purifi-based monitor - options

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#1 Purifi-based monitor - options

Post by IslandPink »

This project is under way. Time to say something about it.
My friend Jon from work wants to upgrade his small home-studio, when he moves house.
Currently he has some Yamaha HS8's, which are a budget option but look flat according to their published curves, at least.
Yamaha_HS8.jpeg
Yamaha_HS8.jpeg (45.13 KiB) Viewed 2297 times
When I mentioned the new Purifi midbass drivers and got him to watch the interview with Lars Risbo


( as mentioned on the other thread ) , he got quite excited.
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#2 Re: Purifi-based monitor - options

Post by IslandPink »

One Steve's thread I spoke about a TL option for this driver, I'll post that later.
https://www.audio-talk.co.uk/phpBB3/vie ... f=4&t=8086

First I want to just show what the suggested ( Application Note ) SPK5 Bass Reflex 'bookshelf' option does -
https://purifi-audio.com/tech/

Being a Hornresp-jockey, I fed the details into there.
The design uses a long internal port tube, so I had to estimate a subtraction of the tube volume from the enclosure, but this wasn't too difficult. The only concern atm is that I haven't included any enclosure stuffing ( which is specified in detail ) . It seems to me there was an option to do this in Hornresp, but none of the entry boxes are showing this, on my set-up :
Purifi4_SPK5_SetUp_i.JPG
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#3 Re: Purifi-based monitor - options

Post by IslandPink »

I ran the thing on 'Combined Output' with a small delay on the port output to account for the distance from the rear port to the front of the cone :

SPL ( 2-pi ) :
Purifi4_SPK5_SPL_i.JPG
Purifi4_SPK5_SPL_i.JPG (50.22 KiB) Viewed 2294 times
Group Delay :
Purifi4_SPK5_GrD_i.JPG
Purifi4_SPK5_GrD_i.JPG (49.77 KiB) Viewed 2294 times
As expected, the group delay isn't great, but it's fairly low down in frequency.
As modeled here , it looks a bit under-damped, but maybe this would change in a favourable way with stuffing ?
I'm sure Scott or James would have an opinion.
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#4 Re: Purifi-based monitor - options

Post by chris661 »

Resulting performance will depend on the damping.

I tend to line the walls with an inch or so of recycled wool underlay, which usually sorts out internal standing waves, but leaves the Helmholtz Resonance more-or-less functional.
I've seen other designs where the cabinet volume is filled completely, which tends to kill any/all resonances, including any benefits of a port.

IMO, the best way to implement high-excursion drivers is to use passive radiators. No port resonances, no worries over vent velocity.

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#5 Re: Purifi-based monitor - options

Post by IslandPink »

I think there's some detail in the application note about the damping if I looked back. Good point about passive radiators. Purifi make one of their own, and I think I've seen a design somewhere for one with this driver. I have no experience of what that does for group delay, but I'm sure it cures some problems associated with port velocity.
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#6 Re: Purifi-based monitor - options

Post by JamesD »

In general I am not in favour of stuffing the volume but adding dampening to panels, as Chris, states, can be beneficial. Also, in general, I don't have a problem with port air velocity making noises at domestic volumes i.e. up to 105dB SPL in a domestic sized room but then I typically go for larger port diameters when I design ported enclosures and choose larger enclosures for high excursion drivers when using ther excursion capabilities - I also don't like to use vented boxes where Fc is below Fs i.e. the driver is being used in its unloaded condition when driven with a signal below Fc...

I had a quick look at the Purifi site and drivers - interesting and I wonder if they are worth the price compared to say the 18Sound or the B&C 6.5" mid woofers?

I've not liked the timbre of any of the passive radiator loaded bass systems that I have heard - although I haven't heard any modern designs...

WRT the problem of monitors for a desk sized DAW style system rather than a full control room system - I think the answer is wrong (to my way of thinking) I have designed many control rooms with Mid field and Near field monitoring and I do not like the conventional thinking - although I often have to deliver the conventional solution as that is what the Sound Supervisors in Professional Media expect... I believe (and have proven on the few occasions I have been able to build it) the better solution is a desk top (Near field) or Monitor Wall (Medium field) sealed speaker of 100-120Hz Hz to 25KHz pass band, clean up to 105 - 110dB SPL supplemented by a matching sub woofer 20Hz to 120Hz passband at 110dB SPL (under desk or under monitor rack) is far better for DAW use than a pair of small/medium ported boxes going down to 40-60Hz. In a medium to large control room I can be persuaded otherwise for something like the Harbeth Monitor 40 but they need free space around them to enable them to "breath in the acoustic" properly...

I need to think a little more on the problem before commenting further..

ciao

James
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#7 Re: Purifi-based monitor - options

Post by IslandPink »

Thanks James. If it helps I should add some more background to this.
Jon's studio is a fairly small affair - he did give me dimensions ( I have annoyingly failed to write this down ) but I don't think the room was quite as big as my front room.
Given that the mixing he does is a lot with acoustic instruments ( guitar, mandolin ) that he plays himself, plus some accompaniment, he doesn't play at loud volumes. I quizzed him on the max cones movements on the 8" drivers and he said he rarely sees them moving appreciably.

This project is partly an excuse to hear the Purifi drivers, they do seem to be something very special. If you watch the vid with Lars linked above, it seems he has solved two major problems in cone speakers - the edge surround resonance in the upper midrange, and the variation of BL with excursion ( oh, plus pole-piece hysteresis ) so I really want to hear what they can do on vocals and vocals plus bass- they measure very low on intermod.
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#8 Re: Purifi-based monitor - options

Post by JamesD »

These drivers do look interesting! I would love to hear them - what is obvious from the stuff on the internet is that these have a real chance to be special - would love to hear them against a pair of AERs :-)

In terms of a desk monitor for the DAW - I think the 4" looks the most promising - specifically the PTT4.0X08-NFC-01 - as it has a Qts of 0.37 and is flat to 5KHz - so suitable for a sealed box of 6.5 to 7 litres - 0.5Qtc and 86Hz F3 with a nice gentle slope. Sensitivity isn't high so the amplifier would need a little welly... more on that later... The 4" is the best suited for a sealed box cabinet with benefits to the sound sub 300Hz...and I'm not pushing the bottom end... and its Nearfield monitoring so no need to push it...

Tweeter choice is interesting - staying with the Scandinavian driver theme - use the TW29RN-B-8 (Fabric dome - affordable) or TW29BN-B-8 (Beryllium dome - expensive a bit more delicate and more insight) crossover between 3-5KHz...

Scandinavian Sub woofer under the desk. SB29-NRX75-6 in a 100-120 litre box for a 24Hz ish F3 and use a 0.33R series resistor to yield a Qtc of 0.41 (compromise but still good) - SBB4 alignment - This should be a mono sub driven from L+R as its designed for under the desk placement. Drive it with a 50-100W class D amp (Chinese kit and a Meanwell SMPS with a switching/rf filter on the output) Separate amp on the sub woofer so it can be turned down - maybe right down if it becomes annoying in the house...

All cabinets Scandinavian void less plywood - 15-18mm

Crossover components from Jansen Audio for a completely Scandi monitor setup :-)

Main amp is interesting as this is not a sensitive system but for normal level monitoring (say normally max 85dB SPL) then 5-10Watts minimum is fine but I would probably recommend a good PP amp for this application so start with a minimum of a 2A3 PP or EL34 triode PP. Just to ensure the sound is even going between his usual recordings and, say, Metallica or Mahler Symphony No.8 or a big Opera with large chorus...
Also good would be a First Watt PP amplifier and at a sensible price a good quality chip amp - one of the recommended DIYAUDIO GBs would be fine...

Where can we hear the drivers???

ciao

James
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#9 Re: Purifi-based monitor - options

Post by IslandPink »

Let me catch up a bit ! ... first things ... tweeters will be 'very nice'. They are decided, i will tell you via PM.
Secondly - I am trying to steer Jon towards not necessarily being limited to small desktop cabinets. I have an intriguing Tl ( MLTL ) design that was shown on the other thread, that would get more bass with less G.D. from the 6.5"-ers.
ps. I really want to hear the 6.5" ones :)
More soon..
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#10 Re: Purifi-based monitor - options

Post by IslandPink »

Some progress on this today.
I've just been talking to Colin, it looks like we have at least two pairs of the 4-ohm ±10mm 6.5" units going to be ordered. Scott and Colin will be launching a project of their own.
If there's anyone else out there who is tempted, please let us know, as there would be a further discount from Purifi if we can order 6 units+
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#11 Re: Purifi-based monitor - options

Post by IslandPink »

Back down to one pair, but undaunted, Jon has ordered them ( direct from Purifi ).
Not without a minor glitch when it seemed that the UK had been removed from the potential destinations, on their checkout page.
Now back, and ETA at Jon's is next Monday. I will try to get them and my 30L lest box , very soon after.
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#12 Re: Purifi-based monitor - options

Post by IslandPink »

Drivers are now 'in da house' ( Jon's ) and the test enclosure will follow soon. I hope to grab these and do some listening next week.
Here's a minor revision of the first MLTL design I had. This for the 4 ohm driver.
As Scott pointed out, a little loose in terms of loading on the driver , at ~30Hz, but that may not be a problem with these. At 25W ( 10v rms ) , it's about 6 or 7mm excursion... if you have some loud 30Hz ....
PuriMLTL_4ohm_revised_Design.JPG
PuriMLTL_4ohm_revised_Design.JPG (62.51 KiB) Viewed 1511 times
SPL using about 0.3 ohm driving impedance -
PuriMLTL_4ohm_revised_SPL.JPG
PuriMLTL_4ohm_revised_SPL.JPG (52.18 KiB) Viewed 1511 times
Group delay is only 5ms at 40Hz, less than a third of what the ported box delivers.
PuriMLTL_4ohm_revised_GD.JPG
PuriMLTL_4ohm_revised_GD.JPG (49.86 KiB) Viewed 1511 times
Last edited by IslandPink on Tue Jun 08, 2021 9:46 pm, edited 1 time in total.
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#13 Re: Purifi-based monitor - options

Post by IslandPink »

Driver displacement for 1W :
PuriMLTL_4ohm_revised_DDisp.JPG
PuriMLTL_4ohm_revised_DDisp.JPG (50.07 KiB) Viewed 1510 times
Sorry , forgot to show the scheme as a picture :
PuriMLTL_4ohm_revised_Scheme.JPG
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#14 Re: Purifi-based monitor - options

Post by chris661 »

Mark,
I'm a bit concerned about the port diameter - what's the air velocity when the bass gets going?

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#15 Re: Purifi-based monitor - options

Post by IslandPink »

Good point.
I was thinking of a slot port, like the ones I built for the 103e's.
Checking that, it's pretty slim - 16 x 0.45
On the BR box by Purifi it's 25 cm^2
Looks like I can calculate the port velocity at least on the BR, using a tool on Hornresp.
I'll report back in next day or two.

I'm inclined to do a prototype of the MLTL before committing to it.
I have another MLTL design I was looking at too.
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