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Purifi SPK5 Speaker Review (Prototype)

amirm

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#1
This is a review of the Purifi SPK5 reference speaker design for their PTT6.5W04-01A 6.5 Inch Woofer. It was built and sent to me by the company, Celuaris (member @sgoldwin). The unit was shipped from UK and did not travel well. Just about everything that could come loose, did come loose. I had to take the back apart and reconnect everything and solder a few components in the Purifi crossover board. Final product from the company will not have these issues. Still, I think we can get a lot of valuable data from the design as is.

EDIT: An updated sample was tested that fixed the enclosure leakage. As such please refer to new measurements than the ones here: https://www.audiosciencereview.com/...ds/celuaris-spk5-purifi-review-speaker.20125/

Celuaris has built the speaker to Purifi spec and visually it looks to be so:

Celuaris Purifi SPK5 Review.jpg


The backside is butchered up by me opening it back together (screws were very stiff resulting in me stripping a bunch :) ).

Celuaris Purifi SPK5 PTT6.5W04-01A Woofer Review.jpg


The port is "S" shaped inside. There was barely enough room for it inside and since it was not glued together, it may not be in the optimal position as designed.

The crossover board is external and uses Purifi layout and build:

Celuaris Purifi SPK5 PTT6.5W04-01A Woofer Crossover Review.jpg


I did not appreciate the black silkscreen/solder mask one bit. It completely covered the traces making it very hard for me to figure out what went where. Why on earth a development board was taught to be better this way? Confusing the implementer can't be a good thing. I expect more from Purifi. Looks should be secondary to easy of use.

Note that the bent components is my fault. :) I had to flip it multiple times and they started to loose their straight orientation. Fortunately electrons don't care. :)

As you see, fancy capacitors and such are used throughout. Combined with high cost of the Purifi woofer ($365?), this would not be a cheap speaker if so dressed.

For my testing I had to put the crossover on top of the speaker separated by a 2 inch piece of foam. The Klippel NFS robotic scanner goes around the speaker and nothing can be separate from it or it will dangle. I don't think that made much of a difference in measurements but thought I mention it.

Note that this test is for experienced readers to analyze the design components here and less so about the finished speaker. Company plans to send me one of those later for proper review.

The measurements were made at about 61 degrees F which was comfortable to me but not if you are from a sunny place. :)

I used over 1000 measurement points which gave me less than 1% error for most of the spectrum with the exception of 2.5 to 8 kHz where the error crept up a bit more to around 2%.

Purifi SPK5 Measurements: Spinorama Frequency Response
As usual we start with anechoic response of the speaker plotted in multiple ways:

Celuaris Purifi SPK5 Woofer Spinorama CTA-2034 Frequency Response Measurements.png


Well, this is not that good. Starting from left, we have a dip in upper bass frequencies. Things get better until we start to approach the crossover frequency and now we face a broad dip from 1.2 kHz to some 7 kHz! There was talk of Purifi adjusting the crossover region, pushing it up to 3.5 kHz. Don't know if that is the cause but what is there is definitely not optimal. We then hit a resonance around 8 kHz and a few more peaks with elevated response from the tweeter.

I researched other measurements and while there is some agreement with this data, they vary fair bit so I thought I dig in to see if what we have is correct. Let's start with near-field response of various radiating sources:

Celuaris Purifi SPK5 Woofer Measurements drivers and port.png


I had to adjust the port level so don't take that as absolute. What is strange about the port is that it does not augment the low frequency response of the woofer as we normally see. It simply reinforces what it can produce as is. I could be OK with it but not that sharp resonance that follows between 300 and 400 Hz. That corresponds with a dip in our frequency response so perhaps this is out of phase and sucks out the energy in that region.

Moving to the next region, that crossover region doesn't look right to me. We have that long and gradual slope for the woofer before dropping like a rock. Yet the tweeter has a sharper slope. Can't imagine those summing correctly. They do agree with the electrical measurements of the crossover as provided by Purifi though:

Celuaris Purifi SPK5 Crossover Electrical Response.png


The red curve when applied to near flat response of the woofer by itself (as provided by Purifi) will result in what I have measured. So we are seeing the response as designed. Just doesn't sum well.

Next is the resonances in the tweeter. I looked up the specs for the Mundorf AMT AMT 21CM 2.1 tweeter and this is what they show:

Celuaris Purifi SPK5 Mundorf AMT 21CM 2.1 Tweeter Response.png


A very large vertical scale of 80 dB (as opposed to standard 50 dB) helps to hide the same peaks we see in our measurements as marked. Was there not better choices here?

Purifi shows the following measurements which were sadly gated and in-room and hence, lack the resolution to see the port problems:

Celuaris Purifi SPK5 Acoustic Response.png


Notice that the tweeter ringing is there. What is different is that they show same ringing in the crossover region which hides the wide dip that I measured. Disappointing that a company like Purifi would not bother getting a proper anechoic measurement. If they had, these problems may have been fixable.

Anyway, going back to our regular programming, here is the early window reflections:

Celuaris Purifi SPK5 Woofer Spinorama CTA-2034 Early Window Reflections Frequency Response Mea...png


It seems to be more "correct" which helps tame the on-axis response issues when we look at the combined result:

Celuaris Purifi SPK5 Woofer Spinorama CTA-2034 Predicted In-room Frequency Response Measurements.png


Horizontal beam width shows decent response until the narrow width tweeter starts to beam toward the end (gets directional):

Celuaris Purifi SPK5 Woofer Measurements horizontal beamwidth.png


And the same as contour plot:

Celuaris Purifi SPK5 Woofer Measurements horizontal directivity.png


Vertical dispersion says you better stay at tweeter level or slightly below:

Celuaris Purifi SPK5 Woofer Measurements Vertical directivity.png


Purifi SPK5 Distortion Measurements
The main claim to fame of the Purifi woofer is lower distortion. So let's look at our standard measurements:

Celuaris Purifi SPK5 Woofer Measurements relative THD distortion.png


Ignoring the bass for a moment, I am not happy to see the rising distortion in lower treble. Anyone want to guess as to the source?

Here is our 96 dBSPL @1 meter graph:

Celuaris Purifi SPK5 Woofer Measurements THD distortion.png


Was hoping to see nothing above 50 dBSPL but we have fair bit. I leave it up to you all to dig up past measurements and compare to other speakers with similar sized woofer.

The distortion amount is much worse than what Purifi shows from my quick look. Looking at contributions to it, I see a lot more of it from the port than the woofer. My reassembly of the speaker may have something to do with this as well. Not sure.

Here is the impedance graph:

Celuaris Purifi SPK5 Woofer Measurements Impedance and Phase.png


Our port issue/resonance is clearly apparent. Can't figure out the messiness around 30 to 40 Hz though. Anyone has a guess for that?

Since this is an ultra technical write up, I thought I include the waterfall/CSD plot as well:

Celuaris Purifi SPK5 Woofer Measurements Waterfall CSD.png


I have not tried to adjust thresholds and such. But we can see some of the same issues we have seen in other measurements.

Purifi SPK5 Speaker Listening Tests
I placed the speaker in my usual stand in my listening room with the crossover separated from it (my poor Revel Salon 2 speaker acted as a crossover stand!). I always start with female music tracks to see if the highs are produced well without being too bright or recessed. Well, here they were clearly recessed and not that nice. So the EQ tool came right out resulting in this set after fair bit of experimentation:

Celuaris Purifi SPK5 Equalization.png


The corrections follow the response errors. Lifting up the upper bass resulted in much needed bass energy as without it, there is just not a lot of it there. Broad EQ around 2500 Hz dealt with the crossover deficiency quite well, lifting the female vocals out and giving them the treatment they need: a bit brilliant and forward without it being too much. Actually, it was a too much at first without the higher frequency corrections. A PEQ dialed to lower the resonance at 8 kHz was very helpful as was the quick shelving filter above that.

At first I had the room mode dip active but as noted, there is not a lot of bass coming out of this speaker so I took it out. This helped and by this time, the response was very nice and high quality. Lots of detail without any bass notes becoming overbearing. It was a bit like getting rid of a room mode using EQ and the feeling of relief you get after that.

I played by "speaker killer tracks" and the SPK5 handled them quite well up to a point which was way ahead of what most small woofers can handle. It then followed with a bit of distortion. The woofer was going nuts as far as excursion by this time which would have really messed up the sound of other woofers. Push a bit more though and you are greeted with a static pop that implies the coil getting outside of the magnetic field.

There was a big miss though and that was any kind of physical bass response. Even at much elevated levels, you could barely feel the bass. You could hear it well and on techno tracks, celebrated it. But as a sensation which I crave, almost none was there. I wonder if this is due to the tuning of the poor which doesn't lower the response range.

Overall experience was a very light on its feet speaker that produced very satisfying sound with equalization. With out, it would not be my cup of tea.

Conclusions
Great to see innovations in woofer technology. Alas, the lesson of research in the last four decades was ignored in this development reference design: getting the tonality right. There could not be a more clear example here. I don't care how low the distortion is if you screw up the vocals. Or sound too bright. Fix these -- as I did with EQ -- and then the lower distortion of the bass (appears to ) come forward.

Thinking out loud, I wonder how much bass energy we loose by lowering distortion. Harmonics of low frequencies perceptually would raise the total energy in that region. I wonder if this, together with port issue, was behind lack of sufficient impact in bass response.

I am left thinking if a 6.5 inch driver is the right choice for this technology. Maybe an 8 inch driver would have been a better showcase so that we could have bass and low distortion. Alternatively maybe other designs would do better. I don't know. What I do know is that this was not a good attempt by Purifi to showcase their speaker.

Let's hope some issues here are due to difficulty I had in putting this speaker in working condition. I welcome detailed analysis from you all though on how it could be improved if the problems are real.

Overall I cannot recommend the Purifi SPK5 without equalization.

------------
As always, questions, comments, recommendations, etc. are welcome.

Appreciate any donations using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
 

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MZKM

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#4
Oddly, it doesn't have the peak at the crossover region (~3200Hz), that the NRC & Purifi document shows:

NRC
0-15-30


45-60-70



Purifi document
30 degrees
Screen Shot 2020-11-23 at 5.26.18 PM.png


_____
Distortion is also different

NRC
101dB (96dB @2m)



I wonder if this can be attributed to the shipping issues.
 
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CtheArgie

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#5
Fascinating! A couple of observations, if I may. It appears this may not be the ideal tweeter for this combination. Also, the crossover frequency may not work very well between these two. I have also read that a port is very difficult to implement with this woofer.

It would be ideal to then check this performance with the kit/speaker built by @March Audio.

Lastly, I thought that @mitchco had reviewed (measured?) this kit and had a different response to it.

I am very glad that they sent this to you and that you measured. I wonder what their interpretation or possible changes will be.
 

tmtomh

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#7
Clearly not fully baked. Perhaps the final product will addresses or ameliorate some of these issues.
 

MZKM

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#8
Mhm, not good at all. I hope @March Audio has done a better job. Port design seems to be tricky, their dual PM maybe helps.
This is a design made by Purifi to show that their 6.5” woofer can go up to ~3500Hz. It’s optimal crossover is ~1500Hz and lower, which almost no AMT/ribbon and most 1” tweeters can’t handle; @March Audio is using a BleiSMa 1.3” tweeter that can handle a 1500Hz crossover.
 

MZKM

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#9
Mundorf AMT AMT 21CM 2.1 tweeter and this is what they show:



A very large vertical scale of 80 dB (as opposed to standard 50 dB) helps to hide the same peaks we see in our measurements as marked.
HiFiCompass measurements:
https://hificompass.com/en/speakers/measurements/mundorf/mundorf-amt21cm21


At ~$1000/pair, I have little clue why Purifi chose this AMT.

Price: Maybe to show that their woofers mate with tweeters that are even more expensive?
AMT: To show it mating with a driver type that commonly needs a high crossover.
 
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amirm

amirm

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Thread Starter #13
Looks like something's wrong with port or enclosure seal (probably both). It's supposed to be tuned at ~32Hz:
That may be my doing :). I see the same resonance dip in Soundstage measurements posted above:

1606174556708.png
 

MarsianC#

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#14
however I wonder about distortion
With steep crossovers no problem. KH80 DSP? Ez. My Event Opal? Ez².
As said, there so point in crossing that low that high with this midwoofer. A combination with Seas' DXT would be worth a shot.
 

thewas

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#16
Another example that just good/expensive components/drivers are not enough to make a good total as many audiophiles believe.
Although it must be also said that these Purifi reference designs are according to them just a demonstration of their drivers and not a fully sophisticated loudspeaker design which can be obviously witnessed.
 

andreasmaaan

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#18
Waveguides will help with SPL and directivity matching; however I wonder about distortion.
Waveguides tend to reduce distortion by loading the driver, thereby increasing its efficiency and hence reducing the voice coil displacement required to reach a given SPL.

Having said that, the kinds of shallow, wide-beamwidth, conical waveguides often used in home audio don't tend to provide very significant loading, and hence the distortion benefits are not generally very great.
 

wje

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#19
Question: How come the crossover board has 2 speakon connectors, but only 1 speakon connector is provided at the back of the cabinet for the connection?
 

MZKM

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#20
That may be my doing :). I see the same resonance dip in Soundstage measurements posted above:

View attachment 95393
Yeah, your impedance plot shows both peaks, but shifted higher in frequency (the saddle in between is also higher in your plot):

From their document:
1BC10121-6E55-47CB-81D5-DC3A2E69096C.jpeg
 
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