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Purifi-6R
Copyright 2021 © Troels Gravesen

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DRIVERS    CROSSOVER     CABINET    WORKSHOP PICS     MEASUREMENTS     SPEAKER-KIT     CROSSOVER LAYOUT

It's been quite some time since I tested the Purifi PTT6.5-W04-01B driver, now PTT6.5X04-NFA-01. Read here.
So, time to set up a 2-way stand-mount and hear what this quite special driver can do.
In-depth here: https://purifi-audio.com/blog/
- and here: https://purifi-audio.com/wp-content/uploads/2019/12/PURIFI-AES-9607.pdf
Another driver that has been on the shelf for too long is the Viawave GRT-145W-8 ribbon tweeter. Tested here.

All the time I had these two drivers in mind for a small stand-mount and I started making a mockup in some test cabinets to make final measurements for simulation and determination of dZ. Even with the ribbon's waveguide and retracted ribbon, the Purifi driver turned out to be acoustically some ~15 mm behind the ribbon on a flat baffle at point of crossover as dZ is frequency dependent. Not a lot, but something to consider when we start simulation.

Based on these new measurements, I made two basic simulations for testing, one LR4 crossover and one LR2 crossover. From the LR4 crossover the difference in acoustic depth was easily overcome and produced at flat response measuring somewhere between the ribbon and the midwoofer. For the 2nd order filter the optimum listening height was closer to the midwoofer axis and could handled by tilting the speaker a little, some 3-4 deg. Rather than making a stepped baffle I chose the tilt, but eventually not a big deal. One thing is what we measure with the microphone at a given point, another what we actually hear.

Initially I made the 4th and 2nd order crossover for comparison. I finally decided on the 2nd order, maybe because I have used it extensively, but also because the 4th order, although very straight forward, had the familiar upper-mid projection, not too strong, but noticeable. In the end a matter of taste, but I favour shallow sloped crossover as I find more tonal fidelity here. 1st order is no-go here as we have to protect the tweeter and the Purifi, although well behaved, doesn't have the most smooth roll-off around. 

 

Playing this rare Musica Nude vinyl recording, which I played hundreds of times, immediately made it clear this mid-woofer is special. Resolution was the first superlative that came to mind. I also know that whenever we introduce a new driver, we'll always hear something we didn't hear before. But having a ruler flat mid- to lower-treble response made it clear it wasn't just a peak or whatever that might have brought some details to life, rather a genuine ability not to smear the signal it is fed.
Clarity and resolution all indicates low distortion. Accuton C173 also came to mind, but without its distinctive character to the sound. Now, the Purifi doesn't work alone, the clarity of the Viawave ribbon defines the timbre of any basic note. Without overtones we don't have the tonal characteristics of any instrument. So, what counts the most? We can't tell, but only conclude these two drivers make a great couple crossing over around 2.2 kHz thanks the wave-guided ribbon.  


Links:

Test Purifi PTT6.5W04/PTT6.5X04-NFA-01: http://www.troelsgravesen.dk/Purifi.htm

Test Viawave GRT-145W-8: http://www.troelsgravesen.dk/GRT-145W-8.htm

Purifi website: https://purifi-audio.com/

Basics:
2-driver speaker.
Dimensions: 24 x 24 x 45 cm, WxDxH.
System sensitivity: ~87 dB/2.8V/1 meter.
Impedance: 4-8 Ohms.
Power requirement: 20+ watts/channel.
Power handling: 100 watts.
Please also read: http://www.troelsgravesen.dk/power-handling.htm, and remember any burned driver is a misused driver.

Useful links (Please read before writing!):
http://www.troelsgravesen.dk/tips.htm
http://www.troelsgravesen.dk/crossovers.htm
http://www.troelsgravesen.dk/LCR-RC.htm
http://www.troelsgravesen.dk/Inverted-Polarity.htm

FAQ (Please read before writing!):
You cannot change cabinet front panel dimensions and drivers' placement without needing a new crossover - and I cannot help.
You cannot use any other drivers with the crossover specified.
http://www.troelsgravesen.dk/crossovers.htm
http://www.troelsgravesen.dk/choices.htm

 


DRIVERS
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Purifi  PTT6.5X04-NFA-01 midbass driver. (The name was changed from PTT6.5-W04-01B to PTT6.5X04-NFA-01)
Read test here: http://www.troelsgravesen.dk/Purifi.htm


Viawave GRT-145W-8
Read test here: http://www.troelsgravesen.dk/GRT-145W-8.htm

 
Download specs here:  PTT6.5X04-NFA-01    GRT-145W-8


CROSSOVER
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The crossover features a standard LR2, 2nd order, crossover around 2.2 kHz. The GRT ribbon here has quite some energy in the 1-2 kHz range, thus R5+C2+L2 helps shaping the high-pass roll-off. This ribbon has a phenomenal sensitivity, up to around 100 dB compared to the Purifi having some 88 dB intrinsic sensitivity, hence a few resistors to bring it down to that level, R1/R4/R6.

The PTT6.5 low-pass section is pretty straight forward and R7/C3 helps shaping the low-pass roll-off. 


CABINET
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Cabinets were made from 20 mm Baltic birch. Use 19-22 mm depending on local availability and adjust dimensions to keep specified volume (16.4 litres). Stick to front panel dimensions and drivers' placement!!
The port is Ø68 x 160 mm and lined with felt material. See images below. This lowers the Fb (port tuning) from 50 Hz to 42 Hz and we have F3 = 37 Hz, quite impressive.


 


I wouldn't hesitate to recommend the Purifi 6R for studio monitoring if placed horizontally.
Turn the ribbons 90o and listen on mid-woofer axis.


Workshop pics
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Every speaker construction must start with a mockup to measure what the drivers do on the intended baffle and I had some ~15 liter test boxes and made a preliminary front panel, heavily chamfered to make 22 cm effective baffle width. Fortunately the frequency response of both ribbon and mid-woofer were so good, this turned out to be ideal for my final cabinets.


Fine-tuning of crossover in workshop set-up.

The final cabinets:


Left: All pieces for one cabinet. Right: Test assembly with tape. Note slots for the brace.
Rear panel made 5 pieces to minimise loss. Front panels have oak fillets.


Making the bracing.
And BTW: NO! You do not see a laminate of side panels. Just the rebate for the rear panel. Sorry, have this question all the time :-)
http://www.troelsgravesen.dk/tips.htm#MAKING_REBATES


Left: Rear panel in place.


Routing for the drivers and port. The Purifi driver is NOT flush-mounted. Route a 145 mm hole.
Chamfer the front panels to make 22 cm effective width.
To make the port fit without using glue, the hole must be 96.5 mm diameter :-)

 
Make sure to chamfer for the midbass to allow free ventilation.


Finally! Gluing cabs.

 

Left: I decided to add 4 mm bitumen pads and increased depth to 25 cm to compensate for loss of volume.
If you want to do this, add 2 pcs 50 x 50 cm 4 mm bituman pads to your order.
Glue to panels with floor tile adhesive:  http://www.troelsgravesen.dk/tips.htm#Glue_for_Bitumen_Pads
Right: Adding felt damping. Cut two sheets 20 x 54 cm to cover sides and top. No felt on rear panel. Here we place the crossovers and cover with acoustilux.


Left: Gluing the front panel. Right: First coat of lacquer.

 

 
Cut two pcs like seen on drawing for the port. Slide it nicely into the port and fasten with dots of Superfix - or similar. See image below.

 


Port seen from rear. Secure felt in port with a strip of Superfix.


Port seen from front.


MEASUREMENTS
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A few comments on MEASUREMENTS before you start interpreting the readings below.
First of all, if we think measurements will tell us how a speaker sounds, we're wrong. The perception of sound is way too subjective to be reflected in any measurements we can perform. A loudspeaker system is meant to give us a satisfying idea of an acoustic event and for some people a pair of 5 USD ear-plugs are enough, others spend 200 kUSD on a truly full-range pair of speakers - and the latter may not be happier than the former.
Measurements may give us an idea of tonal balance of a system, i.e. too much or too little energy in certain areas, although dispersion characteristics play a vital role here. A two-way 7+1 and a three-way 7+4+1 may display similar horizontal dispersion, yet sound very different. Measurements may tell us about bass extension if far-field measurements are merged with near-field measurements. In addition to this, ports may contribute to bass extension. Most of we diy'ers do not have access to an anechoic room for full-range measurements from 20-20000 Hz.  
What cannot be seen is what kind of bass performance we get in a given room. Bass performance is highly dependent on in-room placement of your speaker and the same speaker can be boomy in one place and lean in another. Actual SPL level at 1 meter distance and 2.8V input is useful for en estimate of system sensitivity and combined with the impedance profile may give an idea of how powerful an amplifier is needed to drive the speaker to adequate levels.
What measurements do not tell is the very sound of the speaker unless displaying serious linear distortion. The level of transparency, the ability to resolve micro-details, the "speed" of the bass, etc., cannot be derived from these data. Distortion measurements rarely tell much unless seriously bad, and most modern drivers display low distortion within their specified operating range. 
Many people put way too much into these graphs and my comments here are only meant as warning against over-interpretation. There are more to good sound than what can be extracted from a few graphs. Every graph needs interpretation in terms of what it means sonically and how it impacts our choice of mating drivers, cabinet and crossover design.
What measurements certainly do not tell is the sonic signature of the speaker, because speaker cones made from polypropylene, aluminum, Kevlar, paper, glass fiber, carbon fiber, magnesium, ceramics or even diamonds all have their way of adding spices to the stew. Nor do measurements tell what impact the quality of the crossover components add to the sound, from state of the art components to the cheapest of coils and caps, they all measure the same if values are correct, yet sound very different.


Response of drivers on actual baffle with no crossover. PTT6.5X04-NFA-01 driver merged with near-field response at 250 Hz.
As can be seen, the ribbon requires quite some attenuation.

 


Impedance of PTT6.5X04-NFA-01 driver in free air (red), in cabinet with un-lined port (green) and felt lined port (orange).
With the felt lined port we have a port tuning of ~40 Hz.


SPL of Viawave ribbon w/wo crossover.

 


SPL of Purifi driver w/wo crossover, merged at 200 Hz with nearfield response.


Final system impedance. Minimum 3.8 Ohm @ 187 Hz.


Vertical dispersion. Quite good. Optimum flat response at a listening height somewhere between the two drivers.


Horizontal dispersion at 0, 10, 20, 30 and 40 deg. off-axis.

 


SPEAKER-KIT
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All kit and component prices may be subject to change and are always to be confirmed by Jantzen Audio Denmark.

Download Complete Kit Sale Presentations:

All technical questions to troels.gravesen@hotmail.com

All questions regarding purchase of kits, please mail Jantzen Audio at contact@jantzen-audio.com


CROSSOVER-LAYOUT
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Layout for tweeter. Place tweeter crossover on rear panel in upper part of cabinet.
Connect input to either R1a or R1b depending on preferred tweeter level.

Check this out:

http://www.troelsgravesen.dk/tips.htm#CONSTRUCTION_OF_CROSSOVERS

 


Layout for bass. Place board on rear panel above speaker binding posts.

 

 

SPEAKER WIRING

 


Tweeter crossover.


Bass crossover.


Details about R7 and C3.


Right: Soldering input wires to the crossover board.


Left: Mount the bass crossover on rear panel up against the brace.
Right: Mount the tweeter crossover in upper section with coils upwards.


Connecting the drivers. The ribbon inverted polarity is done on the crossover board. See wiring.

 


Take one piece 50 x 50 cm acoustilux and cut in 20 and 30 cm width.
Fold the 30 cm cut, fold and place above tweeter crossover and up against top panel.
Fold the 20 cm cut, fold and place above bass crossover.