Purifi-based monitor - options

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IslandPink
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#31 Re: Purifi-based monitor - options

Post by IslandPink »

It's a slightly bigger volume and a shorter port - but I like the effect of the wadding around the port interior. I will do a simulation first.
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#32 Re: Purifi-based monitor - options

Post by IslandPink »

Unfortunately he explains nothing about his choice of volume and port size/length.
I did a basic Hornresp set-up for it with unlined port, then changed the port diameter ( on the basis he'd put felt in there ) until I got the closest match to the measured impedance curve ( with lined port ). This came out about 20cm^2 area, which suggest the port acts as if it's about 5cm dia ( though more damped ).
If I sim that for SPL it looks like this -
PurTroels_SPL.JPG
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The group delay doesn't look too pretty.
So I'm not sure this convinces me to build it.
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#33 Re: Purifi-based monitor - options

Post by chris661 »

A peak before rolloff will tend to ring.

Make the port longer to drop the tuning. The result should be a smoother frequency response, and I'd also expect the group delay to improve and/or push issues down in frequency.

Another option would be to go sealed, and EQ the speaker + room result.

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#34 Re: Purifi-based monitor - options

Post by Ant »

Try an aperiodic slot rather than a port? God knows how you would model it though
Ten square inches per cubic foot of cab volume was about the best info i could come up with for sizing
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#35 Re: Purifi-based monitor - options

Post by chris661 »

IIRC, Hornresp can model it if you morph it into an offset-driver alignment.

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#36 Re: Purifi-based monitor - options

Post by IslandPink »

After a long gap... finally, some progress.
This is a temp cabinet run-up by a friend at work , it's just an 18mm MDF version of the Purifi 'serving suggestion' for the 6.5" driver.
I found a cardboard tube that was almost the right diameter for the port, mod with a bit of 25mm dowel glued down the inside to get the area more accurate. I have lightly stuffed the box insides with long-haired wool.
It's just the bass/mid working here, the tweeter is just blocking a hole !
Sounds very good in places, although I can hear the box contributing. The surprises at the moment are mostly in the bass. 'Behind the Lines' by Genesis sounds Ace, the bassline is so clear.
However it's also doing the trick of keeping the vocals totally clean despite whatever's happening in the mix.

Sorry about the colour, should have adjusted the camera for indoor light.
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#37 Re: Purifi-based monitor - options

Post by IslandPink »

The next test version is under way. Again I have support for the woodworking on this - Jon's friend Dave will run one up , using 18mm birch ply this time.
These are the plans, not quite to scale, but hopefully give the idea.
Volume is about 20% bigger, the port area is smaller and more of a slot vent. The tuning is lower, there should be about 10Hz more quality bass on this ( before the group delay gets big ) , at the expense of more air velocity in the port. I have a hunch it will be OK, so it's work building and listening.
I will put up some Hornresp data and sims later, when i get chance.
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#38 Re: Purifi-based monitor - options

Post by IslandPink »

I've got a crossover designed and working for the pair of drivers, currently in the MDF 1st cabinet ( Purifi application note ) .
Already lots to enjoy and sounding clear and fairly well integrated, dynamic.Treble is a bit high, based on first-guess Lpad.
Only 7 wpc channel here from my little chip amp, hoping for a bigger amp on loan soon.
Does help it's playing CD's through Nick's DAC.

Here a shot of the 'ripped backsides' as Iggy Pop would put it.

I will sketch a circuit when I get chance, roughly it's 1st-order low-pass on the Purifi which increases in slope as the driver rolls off ( gracefully ) ; then a 2nd-order HP on the **** tweeter, both around 2kHz atm.
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#39 Re: Purifi-based monitor - options

Post by Toppsy »

Slow progress Mark, but progress none the less. Your Colleague for whom you are designing and building these speakers must be a very patient man.

Sounds like you may need a little more attenuation on the tweeter. From the manufacturers specs there would seem around 6dB difference in efficiency between the Purifi and Satori. I have a spare Monacor adjustable L-Pad that we used on Nick's OB test mules. You're welcome to borrow these if it helps with the XO tweeking. Easy enough to send it in the post. Also with using 1st order on the Purifi there will be quite a lot of HF overlap with the tweeter. Which might be what you are hearing?

I note you are crossing over around 2K. Using the same tweeter we cross over around 1K on my Edingdales and was as you heard them at the last Owston gathering that prompted you to go with this tweeter for your build.

These gonna be ready for Owston?
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#40 Re: Purifi-based monitor - options

Post by IslandPink »

Toppsy wrote: Wed Jul 27, 2022 10:29 am 1. Slow progress Mark, but progress none the less. Your Colleague for whom you are designing and building these speakers must be a very patient man.

2. Sounds like you may need a little more attenuation on the tweeter. From the manufacturers specs there would seem around 6dB difference in efficiency between the Purifi and Satori. I have a spare Monacor adjustable L-Pad that we used on Nick's OB test mules. You're welcome to borrow these if it helps with the XO tweeking.

3. Easy enough to send it in the post. Also with using 1st order on the Purifi there will be quite a lot of HF overlap with the tweeter. Which might be what you are hearing?

4. I note you are crossing over around 2K. Using the same tweeter we cross over around 1K on my Edingdales and was as you heard them at the last Owston gathering that prompted you to go with this tweeter for your build.

5. These gonna be ready for Owston?
1. No need to rub it in ! Luckily Jon has been moving house. Helps because it wasn't the most stable eight months for me !

2. Yes, I calc-ed an Lpad at 2.2R into 3.3R across the tweeter and couldn't find a 2.2R in the favoured BI flat ones, just 2R0, but I just need to look at this and see what the values would be for an extra dB. After that I'll measure. I'll see how I go, might not need the monacor Lpad, but thanks for that offer. I have a big stock of resistors.

3. From listening, I can hear there's a bit of a peak around 2 to 3k as well, the vocals are over forward, so the overlap can be adjusted too. However it's not far off, the integration between them is really nice already.

4. I know and we discussed it. I should really try it, after getting a good set-up with my preferred style. I am just aware of there being a lot of extra bits needed. Presumably it needs a 4th-order high-pass on the tweeter to go this low ? What would Scott suggest ( type ) for this , and would the woofer need something like a 3rd-order LP to go with it ? I haven't done much crossover work in the 1kHz region, so I might need to loan some coils ( I'll prob be OK for caps ) in order to do it.
ps. did you send me an example circuit ? - will check.

5. 'These' may not be, since I'm doing the early work and trials on two bass loadings using only one channel. I will be able to bring the Mk.2 box, from birch ply ( if it works better ) plus a more refined version of a crossover. That will be worth hearing.

ps. more listening last night and running-in, and this is already great. The quality of the drivers is obvious. Looking at the list of up-coming upgrades and adjustments, I think these are going to be very exciting as a stereo pair.
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#41 Re: Purifi-based monitor - options

Post by Toppsy »

All positive Mark and no sarcasm was intended. Perhaps I could have worded my comment on the slow development of these speakers a little better.

I have a few air core and iron core inductors left over from previous or abandoned projects. You are welcome to use them for the final tuning if I have suitable values. Let me know what you need and I'll check my stock.

Have you checked the current prices of BB birch ply? Made my eyes water when I asked my local supplier! If you have to buy the plywood new and cost is no object, fine. However if you have to buy the wood for the final build I doubt with those small boxes you will hear any gain in sonics using furniture grade chipboard over BB ply of the same thickness. Believe me I have been there. A few years back I built 2 identical small speakers one in ply the other Chipboard and veneered both. Couldn't hear any difference between the two. Folk are sceptical of this and so insist on BB birch ply cos the web says so. Just saying.
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#42 Re: Purifi-based monitor - options

Post by vinylnvalves »

I would agree on small boxes birch ply or chipboard won’t make much difference. You can always line the cabinets with 3M damping tape ( 435 is probably most suitable) if you want to kill midrange vibrations.
Have you made any measurements, to understand if the HF is too hot, or are you relying on your ears?
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#43 Re: Purifi-based monitor - options

Post by IslandPink »

I trust my ears enough these days to do a first revision without measurements, Steve. I'll be digging out the measuring kit to have a look at it all, once I've had a listen.

Re. the wood and cost, I'm lucky on this, Jon is getting a whizzo cabinet maker from work, Dave, to do all the woodwork for prototypes and final. I'm the technical lead, and when I asked for 18mm birch on the Mk.2, they went and got some :)
Jon is well aware what high-quality monitors in the pro world cost, so he's somewhat relaxed about what I'm doing ( for free ) for him.
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#44 Re: Purifi-based monitor - options

Post by vinylnvalves »

I don’t trust my ears anymore :( . If I tune by ear it would sound like audionote equipment, with a midrange hump that’s sounds great initially, but becomes tiresome with extensive listening.
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#45 Re: Purifi-based monitor - options

Post by IslandPink »

.. but then you could re-tune it to how you really wanted it ? ...

Here's the crossover today, but it will change.
There's some harshness in the vocal harmonics, and i can see without testing that the woofer low-pass needs some more thinking because of the ramp-up of the Purifi from 1.5kHz to 3.5kHz.
I had success with a 1st-order LP plus a soft notch filter to tailor the response of the FE208Ez, which is a bit similar. I might try that. I'll get some tests done first at the weekend.
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