Point 75i
Copyright 2005 © Troels Gravesen
CROSS-
OVER
MEASURE-
MENTS
   

SEAS L22RN4X-P + W15CY001 + Expolinear NDRL81


Acapella SE and Point75i aside.

During the last 1½ year I've kept contact with Martin who bought my Point75A speakers and from time to time we have been discussing a possible upgrade in order to get better bass performance and a little more warmth and weight in lower midrange. Analysing the response from the W15 and 18W/8535 drivers in LspCAD made it clear that some improvement might be possible. Fitting a larger bass driver into the smaller Point75 cabinet (compared to the Acapella LWJ) would take a special driver if the low-end response shouldn't be sacrificed. An 8" Vifa polypropylene driver was tested, but due to low membrane weight and a huge magnet the F3 would rise some 6-8 Hz and this made a noticeable reduction in bass performance. The SEAS 8" alu drivers had been on the shelf since the Acapella LWJ upgrade and these drivers have a 43 grams cone weight, a reasonable Qt of 0.32 and would fit nicely into the approx. 28 litres volume giving a F3 of 36 Hz. Quite impressive. These L22NR4X/P drivers can be used in relatively small cabinets giving an impressive low-end response. The trade-off is efficiency, but the 18W/8535 never was an efficient driver and some improvement in efficiency might be possible after all. So, it was decided to replace the Scan-Speak drivers with the SEAS alu driver and as can be seen from the picture below, the driver holes needed some routing to get the new drivers in place.
Initially I thought that I only needed to make a new bass low-pass section for the SEAS (how naive!), but things developed and it appeared that an overall modified crossover would make it possible to increase performance significantly - and numerous hours went into modelling and testing a whole range of options. Crossover version 19 was where it finally stopped and I'm pleased to say that I like these speakers so much that I'm considering building a pair of Point75Ai for myself. I've always been fascinated by the challenge of making small speakers sounding big, and this is one of the kind.

Should anyone consider the upgrade, most of the old crossover components can be reused by unwinding the coils to new values, except for a new 1.0 mH coil needed in the mid-section. The overall phase-tracking between drivers has been improved and the NDRL upper response has been flattened a bit to give an overall smoother treble. Attenuation of the mid and tweeter has been reduced to give a rise in efficiency of 1-1½ dB. The vent and port-tuning was left as is, but in a possible new incarnation I would lower the bottom panel like the Acapella SE to gain another ~2 litre volume and I would use a larger vent diameter - 60 mm ID - and a length of 190 mm get a vent tuning of approx. 33 Hz.
I've really enjoyed having the Point75As back and I couldn't resist trying the JP3 and G2Si ribbons also, and it appeared that both the Fountek JP3 and the Aurum Cantus G2Si can readily substitute the NDRL/ATD ribbon. These drivers have similar frequency response and apparently also the same transformer unit "driving" the ribbons (impedance). No basic changes to the crossover, only attenuation had to be adjusted. The JP3 and G2Si require a 4R7-5R6 input series resistor to match the W15 mid-driver. Simple as that. Please don't ask which sounds the best. The NDRL has a wider horizontal distribution if this may be an issue. But I would use the Fountek JP3 - half price compared to the NDRL, at least here in Denmark.
Comments from owner at bottom of page (only in Danish).


Enlarging driver hole to fit with L22RN4X/P.

The Crossover


Modified crossover for Point75i.
Red = real resistors, "empty" resitors = internal resistance of coils.
Note: inverted polarity of tweeter.

Components for a pair of crossovers:

2 x 0.18 mH, 0.9 mm wire, 0.23 ohm, tweeter, I-0606
6 x 0.15 mH, 0.9 mm wire, 0.21 ohm, tweeter, I-0604
4 x 1.0 mH, 1.4 mm wire, 0.28 ohm, mid, I-0060
2 x 1.5 mH, 0.9 mm wire, 0.76 ohm, mid, I-0628 (this coil has intentionally high resistance)
2 x 2.7 mH, trafo, 1.2 mm wire, 0.12 ohm, bass, mo-125420

6 x 4.7 uF, pp, tweeter and mid, mcap-00470
4 x 6.8 uF, pp, tweeter, mcap-00680
4 x 10 uF, pp, mid, mcap-001000
2 x 33 uF, polyester, bass, I-1451
2 x 22 uF, pp, mcap-002200
2 x 100 uF, electrolytic, bass, I-1085

2 x 10 ohm, 5 W MOX, tweeter, I-1944
2 x 3.3* ohm, 5 W MOX, tweeter, I-1932
2 x 4.7 ohm, 5 W MOX, tweeter, I-1936
2 x 1 ohm, 5 W MOX, tweeter, I-1920
2 x 4.7 ohm, 10 W MOX, mid, I-1982 (must be 10W series resistor for mid)

* 2 x 4.7-5.6 ohm for Fountek JP3 or Aurum Cantus G2Si (only input series resistor (R-1021)

Getting coils with the proper internal resistance is always a problem and I receive many mails asking questions about this.
The inductance (mH) tolerance is usually very low and I have bought coils from many sources and never found them to vary more than +/- 1-2%. Very good.
The resistance of the coils can from my experience vary +/- 10-20% without causing serious trouble, i.e.:
The 0.18 and 0.15 mH coils may vary from 0.1-0.3 ohm internal resistance without affecting performance.
The 1.0 mH coils may vary from 0.2 to 0.4 ohm.
The 1.5 mH coil may vary from 0.4 to 1.0.
The 2.7 mH coil may vary from 0.1 to 0.3 ohm. Don't go higher than 0.3 ohm or you will loose efficiency and bass performance.
Obviously the capacitors and resistors should be exactly as specified, but as they are all standard values this shouldn't cause trouble - although some strange capacitors values are seen in the US and a few other places.

Measurements:


Nearfield bass spliced with ½m midrange+tweeter. Point of crossover between bass and
midrange is around 400-450 Hz.


SPL response from midrange and ribbon, here from 800-22000 Hz. Blue = minimum phase.


Impedance of Point75i, here both speakers.


Point75i step response with marked peaks from tweeter, midrange and bass.


Red = NDRL tweeter with 3R3 series resistor.
Blue = JP3 ribbon with 5R6 series resistor.
As usual the tweeter level should be fine-tuned upon final set-up,
so better have extra resistors ready, e.g. 3R9/4R7/5R6/6R8.


Martin:
Som lovet kommer der her en lidt længere beskrivelse af mine oplevelser med P75'erne efter upgraden.   Det første man umiddelbart hører er at enhederne er meget bedre integreret, hvilket skubber højtaleren op i en helt anden klasse. Lydbilledet kommer derfor også til at hænge bedre sammen, hvor det tidligere i visse passager var tydeligt at 8535 og W15 enhederne ikke var helt optimalt integreret. Ligeledes betyder denne integration også at lydbilledet står meget mere klart/roligt og instrumenterne er nemmere at adskille og der opnås en bedre "3-D" oplevelse. P75i giver simpelthen en mere fuldent oplevelse fordi den formår at spille musik fra top til bund. Nu siger jeg den spiller musik, og det er nok den bedste beskrivelse af P75i - den spiller simpelthen musik. Bassen er betydeligt mere virkelig, da den føles som der er mere magt bag den, men om den går helt så dybt ved jeg faktisk ikke, men hvis ikke så er det i hvert fald så tæt på at det er ubetydeligt - jeg tror faktisk den går lidt mindre dybt, men at der så er mere fylde i den, hvorfor det ikke bemærkes. Ligeledes har mellemtonen fået mere varme og overskud især i nedre mellemtone samtidig og lidt mere "attack" højere oppe. Diskanten er også blevet forbedret da den står mere rolig og er mindre skinger. Alt i alt er højtaleren blevet en bedre allround højtaler.Hvorfor endte den egentlig med at komme til at hedde P75i?

Troels:
Tak for din beskrivelse. Jeg synes du rammer meget tæt på mine egne oplevelser. Det med dybbassen er svært fordi 8535 havde et 2-3 dB løft mellem 50 og 100 Hz og det kan snyde enormt meget.
Nærfeltmålinger viser at 18W/8535 falder ca. 4 dB mere fra 100 til 50 Hz sammenlignet med L22.
Navn?: "i" for improved. En "SE" er en special edition eller en helt ny konstruktion, hvor jeg mere opfatter dette som en up-grade, så derfor. Og så skulle den også bare hedde noget.

Martin:
Ja jeg vil ikke udelukke det er de 2-3 dB der gør at jeg tror de går lidt mindre dybt, men som sagt er det ikke noget jeg finder mærkbart/problematisk for som sagt, hvis de reelt går mindre lavt så er det minimalt. Jeg ser faktisk det som en fordel at der ikke er det løft på 2-3 dB, da de passer betydeligt bedre til min stue, hvor bassen før forvrængede meget ved visse frekvenser.
En ting jeg glemte er at min forstærker (Musical Fidelity X-A100R) umiddelbart har lidt nemmere ved at trække P75i, da den ikke skal skrues så meget op for den samme lydstyrke, men det ender jo med at jeg alligevel skruer højere op end før da højtaleren bedre formår at spille højt. Ligeledes bliver forstærkeren knapt så varm som tidligere.

Troels:
Point75A var baseret på delefilteret fra ProAc Future .5 og fulgte for bassens vedkommende forlægget ret slavisk, hvilket indebar havde en svagt nedadhældende frekvensgang fra 100 Hz op mod delefrekvensen, hvor W11CY001 tog over. Det gav nok for lidt varme i nedre mellemtone (160-320 Hz), noget der også kendetegner Response 2.5 konstruktionen.
Jeg har et simuleret forslag til en ændret 18W/8535 lowpass sektion. Det forudsætter dog de nuværende ændringer til mellemtone og diskant. Hvis nogen skulle være interesseret i at eksperimentere, så send en mail.