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.
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